The investigated sites were Parque de los Cenotes, Cenote Cauich, La Hondada and a fourth aguada-type cenote in Cancun, in the northern part of the Yucatán Peninsula. Parque de los Cenotes was classified as a mixture of cavern and aguada types, Cenote Cauich as a grotto or pitcher-type cenote, and the other two sites as aguada-type cenotes. These water bodies occupy an ecological context that has received less attention than cenotes associated with tourism, rural areas or dense jungle.

Some urban cenotes lie in public parks, between busy avenues or on private property, and the article notes that some contain litter, discarded tires and electrical waste. The researchers selected four sites to examine species richness, community structure and composition, with a particular focus on phytoplankton and protists. These groups had remained incompletely characterized in cenotes in the Mexican Caribbean, where the broader freshwater and marine biodiversity of the region is also poorly studied.

The design was observational rather than experimental: it described what was present at four locations across two seasonal periods, without manipulating environmental conditions or testing the effects of specific pollutants. The authors described microbial diversity as a potentially useful indicator of ecosystem condition, while also emphasizing that identifying organisms suitable as indicators of eutrophication or environmental impact remains difficult without more comprehensive data. At each sampling station, researchers collected two 10-liter water samples.

Material larger than 150 micrometres was removed with a plankton net, and the retained material was suspended in 300 milliliters of water collected from the same cenote and stored in sterile 500-milliliter screw-cap bottles. One sample was fixed with 4% Lugol solution, while the other was analyzed fresh, using a modified method. Samples were kept at room temperature for no more than 24 hours, passed through a 45-micrometre membrane, and the material retained by each membrane was resuspended in 1 milliliter of distilled water for observation as soon as possible.

Optical microscopes equipped with 10-, 40- and 100-fold objectives were used to identify and document organisms, and photographs were recorded with a camera. Taxonomic assignments were based on established identification keys and reference resources. Abundance was calculated from the organisms counted in the concentrated preparation and the volumes used for concentration, counting and sampling, with results expressed as organisms per liter or milliliter.

Inventory completeness was assessed with species-accumulation curves and the ACE Mean and Chao 1 Mean estimators, which use singleton and doubleton records to estimate how thoroughly the samples represent the communities. Rank-abundance curves were used to examine community structure, including the relative representation of dominant and rare taxa. Within-site diversity was summarized with the exponential Shannon-Wiener index, which expresses the effective number of species.

Ward’s method was used to construct a cluster analysis for assessing similarity among the four communities. The researchers also calculated the percentage dissimilarity in diversity between pairs of sites. Together, these methods assessed not only which organisms were detected, but also how evenly the records were distributed and how much the four aquatic communities differed from one another.

Across the four cenotes, the survey registered six phyla, four subphyla, 10 classes, eight subclasses, 15 orders, 15 families and 18 genera. The inventory contained 17 identified species and four forms that could not be determined beyond the species level. Eight of the 17 identified species were phytoplankton, approximately 47% of the identified total, while nine were protists, approximately 53%.

The named organisms belonged to Euglenophyta, Heterokontophyta, Bacillariophyta, Cyanobacteriota, Ciliophora and Amoebozoa, including euglenoids, diatoms, cyanobacteria, ciliates and amoeboid protists. The study contributed 11 new records for the Yucatán Peninsula. These included the named species Euglena mutabilis, Lepocinclis acus, Phacus orbicularis, Coscinodiscus radiatus, Oscillatoria limosa, Arcella gibbosa, Amoeba radiosa and Mayorella vespertilioides, together with genus-level records for Vorticella, Aspidisca and Coleps.

The authors noted that most of the protist records except Paramecium aurelia were also reported for the first time in the Mexican Caribbean. Species-accumulation estimates indicated relatively high completeness, with values generally near 85% and an estimated 100% for La Hondada. That estimate does not mean every microscopic organism in the water was captured, because completeness is influenced by sampling design, timing, environmental conditions and habitat complexity.

During the rainy season, researchers identified 12 species in Parque de los Cenotes, eight in La Hondada and seven in the fourth cenote, but no species at Cenote Cauich during that period. In the dry season, four species were recorded at Parque de los Cenotes, two each at Cenote Cauich and La Hondada, and one at the fourth site. The seasonal contrast was consistent with the observation of greater protist abundance during the rainy season, especially at Parque de los Cenotes.

Diversity was highest at Parque de los Cenotes, with a reported Shannon-based value of 2.3, followed by La Hondada at 1.7, the fourth cenote at 1.6 and Cenote Cauich at 0.6. Across the sites, the value was 3.2 during the rainy season and 2.5 during the dry season. Rank-abundance curves indicated moderate evenness at La Hondada and Cenote Cauich, high evenness at Parque de los Cenotes and relatively low evenness at the fourth cenote.

No organism dominated all four communities, although Euglena mutabilis, Radiocystis geminata and Aspidisca sp. were prominent at Parque de los Cenotes and Cenote Cauich. Cluster analysis revealed low species similarity among the cenotes, suggesting that each contained a substantial share of taxa not shared with the other sampled sites. The authors related this pattern partly to the broad range of food requirements among protists, while noting that differences in nutrient concentrations, temperature and other physical parameters may also be relevant; however, the study did not directly test which environmental variables produced the community differences.

The scale of the investigation remains a major limitation. It covered four urban cenotes, sampled during two seasonal periods and relied on microscopy and taxonomic keys for identification, so the results should be treated as a snapshot rather than a comprehensive account of all cenotes in Cancun or the wider peninsula. The authors called for systematic monitoring, effective management and conservation, public policies supported by technical and hydrological expertise, and participation by society in protecting these vulnerable freshwater ecosystems.

Subject of Research: Phytoplankton and protist diversity in four urban cenotes in Cancun, Mexico

Article Title: Aquatic microdiversity from urban cenotes in Cancun, Quintana Roo, Mexico

Article References: Díaz-Hernández, J. A., Ugalde-Silva, P., Berriozabal-Islas, C., Novelo, A., Hernández-Uc, J., Arana-May, A., Pech-Patrón, S. D., Nava-Jiménez, I. A., & Borbolla-Vázquez, J. (2023). Aquatic microdiversity from urban cenotes in Cancun, Quintana Roo, Mexico. Subterranean Biology, 46, 129-145. https://doi.org/10.3897/subtbiol.46.108082

Image Credits: AI Generated

DOI: 10.3897/subtbiol.46.108082

Keywords: urban cenotes, Cancun, Yucatán Peninsula, phytoplankton, protist diversity, microbial communities, karst aquifers, seasonal diversity, aquatic microdiversity, water ecosystems, Aquatic, microdiversity

The study, published in Frontiers in Marine Science, draws on water-quality records collected between 2001 and 2024 at 13 fixed monitoring stations along Kuwait’s coastline, six inside Kuwait Bay and seven distributed along the open Gulf coast to the south. Chlorophyll-a, the pigment that gives phytoplankton their green color, is widely used as a proxy for phytoplankton biomass and as a practical indicator of the trophic status of coastal waters. Because phytoplankton respond rapidly to changes in nutrient availability, light, temperature and water-column stability, long-term chlorophyll-a records are among the most effective tools for detecting eutrophication and distinguishing gradual ecological trends from abrupt shifts in baseline conditions.

The numbers are striking. Between the pre-construction period of 2001–2012 and the construction years of 2013–2017, mean annual chlorophyll-a increased 2.9-fold at the Bay sites, rising from 1.29 to 3.7 milligrams per cubic meter, and 2.7-fold at the Gulf sites, rising from 1.16 to 3.08 milligrams per cubic meter. Annual means peaked in 2016, reaching 7.6 milligrams per cubic meter inside the Bay and 5.0 in the adjacent Gulf waters. Concentrations during 2021–2024 declined from those construction-era highs, but at 2.31 and 1.92 milligrams per cubic meter respectively they remained well above the pre-shift baseline. The system, in other words, did not simply return to normal once the construction cranes left.

Statistically, the pattern is better described as an abrupt regime shift than a steady climb. The researchers applied the non-parametric Mann–Kendall test and Sen’s slope estimator to annual mean chlorophyll-a in each region. Over the full 2001–2024 record, chlorophyll-a increased by 0.061 milligrams per cubic meter per year at the Bay sites and 0.051 at the Gulf sites, but the monotonic trend was statistically significant only inside Kuwait Bay, where the Mann–Kendall tau of 0.38 carried a p-value of 0.025. The Pettitt change-point test, a non-parametric method designed to detect a single abrupt shift in the mean of a time series, pinpointed a significant change at the 2012–2013 boundary in both regions. Crucially, when the pre- and post-change segments were analyzed separately, neither showed a significant internal trend. The positive full-series slopes therefore reflected a step-like transition to a new, higher baseline rather than continuous year-by-year accumulation.

What makes the finding genuinely puzzling is that the usual suspect, nutrient enrichment, does not line up with the timing of the shift. Multiple regression models fitted to log-transformed chlorophyll-a and seven co-measured environmental variables, including water temperature, salinity, dissolved oxygen, nitrate, nitrite, phosphate and total suspended solids, explained only 9.5 percent of chlorophyll-a variability at the Bay sites and 11.9 percent at the Gulf sites. Exploratory Pettitt tests applied to each environmental variable found no significant shift at the 2012–2013 chlorophyll-a boundary after correction for multiple testing. The abrupt ecological change, the authors conclude, cannot be explained by a corresponding increase in concurrent dissolved nutrient concentrations.

This nutrient–biomass decoupling is less paradoxical than it first appears, and it echoes patterns documented in coastal systems worldwide. Chlorophyll-a reflects accumulated phytoplankton biomass, whereas a water sample measures only the dissolved nutrient pool remaining after transport, biological uptake and internal recycling. During a shelf-sea spring bloom in the Celtic Sea, for example, rising chlorophyll-a was accompanied by rapid nitrate drawdown over days, illustrating how phytoplankton growth can deplete the very nutrients that fuel it. Monthly sampling may simply miss such short-lived dynamics. Internal nutrient recycling can also sustain production without producing elevated nitrate or phosphate readings, and longer water residence time can allow biomass to accumulate before it is exported from a coastal system. The negative association between nitrate and chlorophyll-a retained in the Gulf-site mixed-effects analysis is consistent with biological drawdown during bloom periods, and the positive dissolved-oxygen association likely reflects photosynthetic oxygen production by the phytoplankton themselves rather than oxygen driving biomass.

The temporal correspondence with the causeway is where the story becomes genuinely provocative. The Sheikh Jaber Al-Ahmad Causeway, a 36-kilometer crossing of Kuwait Bay built largely between 2013 and 2018 and opened in 2019, is among the longest marine crossings in the world. Independent field observations and hydrodynamic modelling show that the structure and associated land reclamation measurably altered the bay’s circulation. Surface-current speeds within 5 kilometers of the causeway declined by roughly 16 percent in summer and 12 percent in spring, an effect attributed to the densely arranged piers acting as a physical barrier to water exchange. Subsequent modelling found that the bay-wide average water residence time increased by 1.29 days, or 3.49 percent, while local effects ranged from a 66-day reduction to a 56-day increase depending on location. Complete renewal of Kuwait Bay, which already renews slowly, was estimated to require 150 to 320 days. Chlorophyll-a reached its highest recorded levels in 2015–2016, during the most intensive phase of construction.

The authors are careful to state that temporal correspondence does not establish causation, and they did not identify an independently documented abrupt regional hydrographic or climatic event at the 2012–2013 boundary. Longer-term hydrographic change provides relevant context but not a discrete trigger: the estimated freshwater contribution at a northern Gulf station declined from 25.6–42.5 percent in 1997 to 0.8–4.6 percent by 2012–2013, accompanied by a salinity increase of roughly six units between 2000 and 2013, a progressive decadal trend rather than a sudden event. Comparable engineering studies support the proposed mechanism, however. In Doha Bay, Qatar, land reclamation increased mean water residence time by 3–6 days between 2000 and 2020, tripling in some areas, while at Korea’s 33.9-kilometer Saemangeum seawall chlorophyll-a was consistently higher inside the enclosure and particularly elevated when sluice-gate operation restricted water exchange. Restricted exchange, these studies suggest, can promote phytoplankton biomass accumulation by extending the time nutrients and cells are retained within a semi-enclosed system.

The spatial pattern adds another layer of intrigue. The shift was detected not only inside Kuwait Bay but also at Gulf stations distributed along roughly 80 kilometers of open coastline, some within 14 to 27 kilometers of the Bay, indicating a coastal-scale response rather than a purely local one. This divergence from the wider Gulf is notable: satellite ocean-color analyses indicate that chlorophyll-a across the Gulf as a whole declined by approximately 16 percent between 2003 and 2021, with interannual variability governed largely by climate oscillations such as the Indian Ocean Dipole and the El Niño–Southern Oscillation. Kuwait’s coastal waters bucked that regional decline, pointing to local and subregional processes, including chronic nutrient inputs from urban and industrial sources and changes in circulation and retention, acting alongside broader climatic forcing. Because the Bay and adjacent Gulf waters form a hydrodynamically connected system, changes affecting circulation and biological retention need not remain confined within the Bay.

The implications reach well beyond Kuwait. The study demonstrates that routine water-quality monitoring can detect abrupt ecological change but cannot, on its own, disentangle the relative contributions of nutrient loading, biological processing and hydrodynamic modification. The authors argue that chlorophyll-a time series should be paired with phytoplankton community observations, continuous high-frequency measurements of chlorophyll-a, dissolved oxygen and temperature, event-based nutrient sampling, and direct measurements of currents and water exchange supported by hydrodynamic modelling. For future coastal engineering projects, monitoring should begin before construction and continue through and after it, with appropriate comparison sites, so that project-associated effects can be separated from broader regional variability. In one of the world’s most environmentally extreme seas, where summer temperatures reach 37 degrees Celsius and salinity exceeds 40 practical salinity units, the message is clear: when humans reshape the physics of a coastal sea, the biology may answer with an abrupt, persistent and difficult-to-reverse shift that no nutrient measurement alone could have predicted.

Subject of Research: Marine Science

Article Title: Long-term chlorophyll-a dynamics reveal abrupt baseline shifts in coastal waters of the northwestern Arabian Gulf

Article References: Abdulhussain, A. H., Alosaimi, W., & Ben-Hasan, A. (2026). Long-term chlorophyll-a dynamics reveal abrupt baseline shifts in coastal waters of the northwestern Arabian Gulf. Frontiers in Marine Science, 13, Article 1949281. https://doi.org/10.3389/fmars.2026.1949281

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1949281

Keywords: chlorophyll-a, Kuwait Bay, Arabian Gulf, phytoplankton, causeway, coastal engineering, regime shift, marine monitoring, Long-term, dynamics, reveal, abrupt

The research, led by a team examining the relationship between the lakes’ ecological condition and the trail network threading through their catchments, set out to answer a deceptively simple question: does the degradation of hiking trails—widening, deepening, and the proliferation of informal paths—actually influence the trophic status of the water? The answer, based on Spearman’s rank correlation analysis, is that natural abiotic factors remain the primary drivers of the lakes’ eutrophication, while recreational pressure exerts a secondary, but detectable, influence. The distinction matters enormously for how the site is managed in the years ahead.

The study’s conceptual backbone is the DPSIR model—Driving Forces, Pressure, State, Impact, Responses—developed by the European Environment Agency. This framework allowed the researchers to systematically separate natural processes from anthropogenic pressures, hypothesising that while eutrophication in the lakes occurs naturally as part of their glacial-lake evolution, it is markedly intensified by recreational activities that degrade soil and vegetation in the watersheds. The team grounded its approach in recreational ecology, a field that examines how outdoor leisure activities affect vegetation, soil, wildlife, and water. Direct impacts on water bodies include increased turbidity, higher nutrient influx, and the introduction of exotic species, while indirect effects manifest as shifts in species composition and increased algal growth.

Five of the seven lakes—Bubreka, Bliznaka, Trilistnika, Ribno, and Dolno—were included in the analysis, representing 71.4% of the total population. Salzata and Okoto were excluded due to gaps in available ecological status data. The researchers compiled morphometric and hydro-climatic characteristics from decades of Bulgarian hydrological records, ecological status assessments based on Biological Quality Elements including phytoplankton, macrophytes, macrozoobenthos, and zooplankton communities, and trail characteristics from a 2020 field survey measuring width, maximum depth, and a derived cross-sectional area indicator of degradation at 17 points within the lakes’ catchments. Trail density and dispersion were derived from a GIS-based inventory of the entire trail network in the Seven Rila Lakes Cirque.

The statistical picture is complicated by the small sample size of just five lakes, which limits the study’s statistical power and the generalisability of its conclusions. Most correlations did not reach conventional levels of statistical significance, and confidence intervals computed using Fisher’s transformation were frequently broad. Nevertheless, the patterns that emerged are instructive. Abiotic factors—altitude, catchment area, lake depth, water volume, and warm-season temperatures—showed strong relationships with macrophyte assessments (rho = 0.9 with mean air temperature), macrozoobenthos evaluations (rho = 0.9 with lake surface area), and zooplankton community structure indices such as Pielou’s evenness (rho = 1.0 with altitude, p = 0.02—the sole statistically significant result in the study).

When the researchers turned to trail characteristics, a different set of relationships surfaced. Phytoplankton biomass correlated strongly with average trail dispersion (rho = 0.825), zooplankton abundance showed a very strong correlation with trail density (rho = 0.8), and zooplankton biomass correlated strongly with average maximum trail depth (rho = 0.8). The Shannon-Weaver biodiversity index for zooplankton correlated particularly strongly with average trail dispersion (rho = 0.975, p = 0.1, with a narrow confidence interval entirely within the positive range). Meanwhile, macrophyte assessments—often cited in prior literature as the primary indicator of recreation-induced eutrophication in oligotrophic mountain lakes—correlated poorly with trail indicators, challenging earlier conclusions about their relevance.

The mechanism the researchers propose is one of dispersed visitor pressure. Rather than concentrating foot traffic on formal trails, the unregulated visitor flow in the Seven Rila Lakes Cirque has generated nearly 35 kilometres of informal trail networks across approximately 4.5 square kilometres, roughly half of which show moderate to extreme degradation. These informal paths are more susceptible to erosion than maintained trails, leading to vegetation destruction and soil exposure. Runoff from these degraded surfaces carries eroded materials, mineral salts, and nutrients into the lakes, subtly altering water alkalinity and physicochemical parameters. Even minor changes in these parameters can affect phytoplankton and zooplankton communities, which are highly sensitive to water chemistry, ultimately influencing the trophic status of the lakes.

This interpretation aligns with previous research on the Seven Rila Lakes. Earlier studies demonstrated that nutrient concentrations depend on soil cover in the drainage basins, and that increasing nutrient influx over recent decades has contributed to a shift from oligotrophic toward mesotrophic conditions in some lakes. Sediment records reveal the cumulative effects of atmospheric pollution and reduced snow cover over the past 150 years, while diatom and zooplankton communities have been shown to respond to even small changes in water chemistry and temperature. The new study’s findings also echo broader recreational ecology research showing that informal trails degrade faster than formal ones, causing disproportionate vegetation loss and soil erosion.

From a management perspective, the study underscores the need for regular monitoring of both the ecological status of the lakes and the condition of the trails in their catchments, ideally at least once every five years. The researchers recommend daily monitoring of visitor numbers and behaviour, reduction of the trail network to a minimum, technical measures to minimise erosion in sections with intensive incision, and measures to limit the diffusion of visitor flow and regulate visitor numbers where necessary. Pre-visit information outlining rules of conduct and potential negative impacts, alongside the development of an ecotourism product highlighting the lakes’ natural heritage values, could further reduce pressure while fostering human-nature synergies.

The study represents the first targeted attempt to explain the mechanism by which recreation negatively impacts the water ecosystems of the Seven Rila Lakes. While it lacks definitive conclusiveness due to its small sample size and the absence of comparable methodological studies in the scientific literature, it corroborates long-standing concerns about eutrophication processes induced by recreational activities. Natural factors play the leading role, but the uncontrolled diffusion of visitor flow—generating informal paths and increasing spatial tourism pressure on the catchment areas—leads to adverse changes in phytoplankton and zooplankton communities. The researchers call for expanded datasets and broader-scale research to refine these findings, alongside visitor management measures to preserve the natural heritage of this iconic Bulgarian destination for generations to come.

Subject of Research: Environmental Science

Article Title: Hiking trail impacts on eutrophication of the Seven Rila Lakes, Rila National Park, Bulgaria

Article References: Mitova, R. (2025). Hiking trail impacts on eutrophication of the Seven Rila Lakes, Rila National Park, Bulgaria. One Ecosystem, 10, Article e139690. https://doi.org/10.3897/oneeco.10.e139690

Image Credits: AI Generated

DOI: 10.3897/oneeco.10.e139690

Keywords: eutrophication, Seven Rila Lakes, hiking trails, recreational ecology, Rila National Park, alpine lakes, zooplankton, phytoplankton, trail erosion, Bulgaria, Hiking, trail

Discostella asterocostata was originally described from China in 1985 under the name Cyclotella asterocostata, based on scanning electron microscopic examination of samples collected in the 1970s. The species was later transferred to the genus Discostella in 2004, after researchers recognised that the position of its marginal fultoportulae, the strutted processes sitting between the valve costae, distinguished it from related centric diatoms. Over the following decades, the species turned up in Japan, South Korea and the Russian Far East. Then, in a publication in 2021, it was reported from North America, where re-examination of archived plankton samples revealed it had already been present in Beaver Lake, Arkansas and the Wabash River, Indiana since 2012. Sediment core analyses from several American lakes pushed its known presence on that continent back to at least the mid-1980s.

Europe, by contrast, remained a conspicuous gap. Freshwater centric diatoms have been intensively documented on the continent for decades through national floras, monitoring programmes and decades of electron microscopic surveys of the Danube phytoplankton. The species’ very distinctive morphology, a ring-shaped pattern in the central area of the valve, makes it difficult to miss under the microscope. The researchers therefore argue that its detection in the Mura River in 2016 roughly coincides with its first appearance in Europe, rather than representing an earlier overlooked record.

The new study draws on routine biomonitoring samples collected between 2016 and 2022 at eight sites along the Mura, Drava and Danube Rivers in Croatia, and monthly phytoplankton samples from thirteen sites along the Hungarian Danube and two of its side arms between May 2021 and April 2022. In the Croatian samples, the species was identified through light and scanning electron microscopy, appearing in plankton from April to September at all eight sites. Its contribution to total phytoplankton biomass was always modest, ranging from 0.2 to 3.2 percent within the class Mediophyceae, but its consistent presence across years and sites suggested it had already become a permanent member of the community.

The Hungarian story is more revealing. In the first scanning electron microscopic survey of 2021 samples, only a single valve of Discostella asterocostata was found, in a sample from the Ráckevei-Soroksári Danube side arm in July. The species was absent from the Diat.barcode database version used in the initial metabarcoding analysis, so the researchers could not identify it molecularly at first. When updated barcode sequences from China became available, the team re-analysed their previously unclassified Discostella sequences and found a near-perfect match, with uncorrected p-distances below 0.01 and at most one or two base pair differences across the 263-base-pair rbcL region used for metabarcoding.

The re-analysis transformed the picture. Molecular detection revealed the species throughout the entire Hungarian section of the Danube by 2021 to 2022, with higher relative read numbers during the warm water period and greatest abundance in the Ráckevei-Soroksári Danube. Even more strikingly, re-analysis of unclassified sequences from earlier phytobenthos surveys of 328 Hungarian surface water samples collected in 2019 and 2020 identified the species in five oxbow lakes along the southern Hungarian Danube, with the earliest molecular detections in September 2019. Two unassigned amplicon sequence variants from the 4th Joint Danube Survey, also collected in 2019, proved identical to the researchers’ sequences, providing independent confirmation that the species was already present in the region six years before its first microscopic record in Hungarian plankton.

Morphometric comparison added another layer to the evidence. The researchers measured eight valve features, including five diameter measurements, the count of marginal strutted processes and the number of outer and central striae, on 35 Croatian and 10 Hungarian valves, then compared the results with published morphometric data from Chinese and American populations. Principal component analysis showed that the first principal component, accounting for 91.8 percent of the variance in the dataset, was strongly correlated with cell diameter. More importantly, the populations from Croatia, Hungary, the United States and Asia completely overlapped on the second and third principal component axes, indicating that the four populations are morphologically indistinguishable. Phylogenetic analysis of the rbcL sequences placed all Discostella asterocostata variants in a monophyletic group well separated from other Discostella species, reinforcing the conclusion that the European populations have not diverged from their Asian ancestors.

The ecological preferences of the species offer a clue to its future in Europe. Spearman correlation analysis of Croatian samples showed a significant positive correlation between the species’ biomass and water temperature, and significant negative correlations with total organic carbon, nitrogen forms and chloride. The Hungarian molecular data confirmed these patterns, with relative read numbers positively correlated with temperature and negatively correlated with total organic carbon, total nitrogen and nitrate. The species also showed a negative correlation with discharge, suggesting a preference for slow-flowing or stagnant waters. In short, Discostella asterocostata favours warmer waters with lower nutrient loads, a profile that differs from the dominant centric diatoms of very large European rivers and that aligns uncomfortably well with the trajectory of climate warming.

How the species reached Europe remains uncertain, but the researchers consider several plausible vectors. In North America, invasive Asian carp, particularly silver carp and bighead carp, have been suggested as carriers, since their filtering apparatus can collect algae larger than 10 micrometres from the water and some algae may survive digestion and be excreted alive in faeces. Asian carps were introduced to Hungary from East Asia as early as 1963, but the researchers consider it unlikely that they explain the species’ appearance there, given the timing. The pattern of spread, with the species appearing in floodplain and protected oxbow lakes and in the Ráckevei-Soroksári Danube before it was detected in the main channel of the Hungarian Danube, suggests that waterbirds may have played a role as dispersal vectors, carrying viable cells between waterbodies. Ship ballast water and fish transport water remain additional possibilities that cannot be excluded.

For now, the species has not shown invasive behaviour in European rivers. It has never been the dominant centric diatom in any phytoplankton sample, and its rapid spread, though indisputable, has not been accompanied by the high abundances that would trigger ecological alarm. But its clear temperature-related seasonal dynamics, its preference for warm, nutrient-poor and slow-flowing waters, and its expansion pattern across the Danube catchment all support the hypothesis that ongoing warming of freshwater systems could promote its further dispersal and increased abundance. The study’s authors emphasise that integrating traditional microscopy with high-throughput molecular techniques such as metabarcoding is essential for the early detection of non-indigenous diatom species, particularly when they occur at low densities that routine counts of 400 to 500 valves can easily miss. As rising water temperatures and enhanced dispersal pathways continue to reshape aquatic ecosystems worldwide, the quiet colonisation of the Danube by a tiny Asian diatom may prove an early warning of changes still to come.

Subject of Research: First European record of the non-indigenous diatom Discostella asterocostata in the Danube River

Article Title: From Asia through America to Europe: Molecular and morphological evidence for a non-indigenous diatom species, Discostella asterocostata (B.Q.Lin, S.Q.Xie & S.X.Cai) Houk & Klee in Hungary and Croatia

Article References: Stanković, I., Shemesh, S., Hanžek, N., Šušnjara, M., Gligora Udovič, M., Kiss, K. T., Trábert, Z., Udvari, Z., Bíró, T., Vadkerti, E., & Ács, É. (2025). From Asia through America to Europe: Molecular and morphological evidence for a non-indigenous diatom species, Discostella asterocostata (B.Q.Lin, S.Q.Xie & S.X.Cai) Houk & Klee in Hungary and Croatia. Metabarcoding and Metagenomics, 9, Article e154667. https://doi.org/10.3897/mbmg.9.154667

Image Credits: AI Generated

DOI: 10.3897/mbmg.9.154667

Keywords: diatoms, invasive species, Danube River, metabarcoding, climate change, freshwater ecology, phytoplankton, non-indigenous species, Asia, America, Europe, Molecular

The research team surveyed 24 small, shallow wetlands across nine Aegean islands, including Andros, Tinos, Mykonos, Naxos, Astypalaia, Rodos, Kos, and Chios, all located within a radius of about 150 kilometres in the eastern Mediterranean biodiversity hotspot. These wetlands ranged from freshwater ponds and marshes to brackish dams and saline cisterns, spanning surface areas from under ten square metres to more than ten thousand, and elevations from sea level to over 200 metres. Their intrinsic mesocosm-like character, combined with varying degrees of human influence and biogeographic isolation, provided an ideal natural experimental setup for disentangling the processes that govern community assembly.

Phytoplankton, the microscopic photosynthetic organisms at the base of aquatic food webs, served as the study’s focal group. Because these organisms respond rapidly to environmental disturbance, their diversity and community composition provide a sensitive indicator of human-mediated change. During the warm periods of 2022 and 2023, the researchers collected surface water samples from each wetland, fixed them immediately with Lugol’s solution, and examined them under an inverted light microscope. Taxonomic identification relied on established keys, while abundance counts followed the Utermöhl sedimentation method, with at least 400 individuals counted per sample to achieve greater than 90 percent accuracy. Biomass was estimated by measuring cell dimensions of dominant species and applying geometric formulae to calculate biovolume.

In total, the team identified 195 phytoplankton taxa across the 24 wetlands. Chlorophyceae, the green algae, proved the richest class with 50 taxa, followed by Bacillariophyceae, the diatoms, with 36, Dinophyceae, the dinoflagellates, with 20, and Zygnematophyceae with 17. Species richness ranged from 4 to 47 taxa per site, with brackish and saline wetlands supporting markedly fewer species, averaging 8 taxa, compared with freshwater sites, which averaged 25. Total abundance varied enormously, from roughly 500 to 300,000 individuals per millilitre, with a general tendency for saline waters to exhibit lower counts. Hierarchical clustering based on Bray-Curtis dissimilarity revealed two distinct groups: freshwater systems clustered independently of their location, size, or habitat type, while non-freshwater systems formed a separate cluster dominated by a few tolerant taxa, including the diatom genera Navicula and Nitzschia and the prymnesiophyte Chrysochromulina.

To identify the drivers of community assembly, the researchers fitted generalized linear mixed-effects models to pairwise Bray-Curtis dissimilarities across 276 site comparisons, incorporating ten environmental and anthropogenic predictors. These included salinity, habitat type, wetland area, pairwise geographic distance, elevation, distance from the sea, overlap with Nominally Protected Areas under the Natura 2000 framework, surrounding land cover classified from Copernicus Corine data, a composite Human Footprint index, and phytoplankton biomass as a proxy for trophic state. The best-fit model achieved a marginal R-squared of 0.57 and a conditional R-squared of 0.97, indicating that the measured fixed effects explained a substantial portion of compositional variation while random effects absorbed additional site- and island-specific influences.

Salinity emerged as the strongest environmental filter. Comparisons between freshwater and non-freshwater sites yielded significantly higher dissimilarity, with a standardized beta coefficient of 0.82, than comparisons within either salinity category, which did not differ from each other. Protection status also proved significant: wetland pairs differing in Natura 2000 designation, with one site inside and one outside a Nominally Protected Area, were more dissimilar than pairs sharing the same status. Pairwise geographic distance contributed an approximately equal effect size, with a beta of 0.15. When the analysis was restricted to freshwater-only sites, protection status became the strongest predictor, followed by geographic distance, suggesting that within uniform salinity regimes, human-related filtering and dispersal limitation jointly structure these microscopic communities.

Complementary matrix-based analyses reinforced these findings. Distance-based redundancy analysis identified salinity, broad-scale spatial structuring captured by principal coordinates of neighbour matrices, and protection status as significant predictors across all sites. Variance partitioning showed that environmental variables explained the largest unique fraction of variation across all sites, at 14.8 percent, followed by a smaller spatial component of 4.7 percent. For freshwater-only sites, however, the spatial signal strengthened dramatically, with pure spatial predictors accounting for 19.2 percent of variation compared with just 5.9 percent for environmental predictors. This shift indicates that once the dominant salinity filter is removed, dispersal processes become the primary force shaping community turnover among island wetlands.

The most striking results concerned distance-decay relationships. Community similarity declined steeply with geographic distance up to approximately 25 kilometres, a threshold coinciding with the maximum distance among within-island wetland comparisons. Beyond this threshold, which corresponds to between-island comparisons, similarity remained uniformly low irrespective of distance. Mantel correlograms confirmed the same structure, showing significant positive spatial autocorrelation only at the shortest distance classes, followed by rapid decline to non-significant or negative correlations at broader scales. This plateau demonstrates that seawater imposes a strong dispersal barrier despite the theoretical capacity of phytoplankton for long-distance passive transport, directly challenging the long-standing paradigm that everything is everywhere but the environment selects.

The researchers further discovered that the strength of distance-decay among freshwater sites depended on protection status. The steepest slope occurred when neither site fell within protected areas, closely followed by comparisons involving protected sites. Within-island community similarity was significantly higher than between-island similarity, and this tendency persisted across all combinations of Natura 2000 status. The authors interpret this pattern cautiously, noting that many Nominally Protected Areas may in practice operate as paper parks with limited enforcement, so wetlands inside protected areas likely experience weaker distance-decay simply because they are more remote and subject to lower human influence rather than active management.

Synthesising these results, the study proposes a hierarchical triple insularity framework in which phytoplankton assembly is shaped by nested spatial filters: strong among-wetland environmental filtering dominated by salinity gradients, island-level dispersal limitation moderated by human accessibility, and cross-island isolation enforced by marine corridors. The findings provide empirical support for microbial regionalism, or provincialism, the idea that microscopic communities exhibit spatially structured patterns similar to those of macro-organisms, shaped by both environmental and historical processes. Only four taxa, the diatoms Navicula and Nitzschia, the dinoflagellate Peridinium, and the chlorophyte Chlorella, proliferated across more than half of the sampled wetlands, sharing traits that favour survival in eutrophic and human-impacted environments, including resting stages and robust cell structures. Their dominance signals a shift toward generalist, disturbance-tolerant communities in island landscapes increasingly shaped by human activity. The researchers argue that regionalism, typically reserved for macro-organisms, also governs phytoplankton biogeography in fragmented systems, and that island wetlands should be recognised as discrete ecological units of high conservation value, isolated not only by geography but also by socio-economic gradients such as land-use intensity and accessibility.

Subject of Research: Phytoplankton regionalism in natural mesocosm-like wetlands of an island archipelago in the Aegean Sea

Article Title: Phytoplankton regionalism in natural mesocosm-like wetlands of an island archipelago in the Aegean Sea

Article References: Proios, K., Parmakelis, A., Papadaki, C., Maranti, A., Christopoulos, A., Stefanidou, N., Mazaris, A. D., & Genitsaris, S. (2026). Phytoplankton regionalism in natural mesocosm-like wetlands of an island archipelago in the Aegean Sea. Frontiers of Biogeography, 19, Article e164401. https://doi.org/10.21425/fob.19.164401

Image Credits: AI Generated

DOI: 10.21425/fob.19.164401

Keywords: Phytoplankton, regionalism, natural, mesocosm-like, wetlands, island, archipelago, Aegean, scientific research

The research team, led by Mackenzie Poirier, Marina Cvetkovska, and Douglas Campbell, set out to test a deceptively simple hypothesis. When light is extremely scarce, the sequential arrivals of photons at any given Photosystem II complex are separated by long intervals of time. During those gaps, the charge separations that photosynthesis depends on can quietly fall apart through recombination reactions, undoing the chemical work that a photon just accomplished. The researchers suspected that polar algae, having evolved in perpetual cold and near-darkness, might have found ways to suppress these wasteful photochemical misses, allowing them to squeeze productive electron transport out of photons that arrive seconds apart.

To understand why this matters, it helps to picture how Photosystem II actually works. Each complex contains a reaction center, P680, that absorbs excitation energy funneled in from light-harvesting pigments. When excited, P680 hands an electron down a chain of acceptors, ultimately passing it to a mobile quinone carrier, while a tyrosine residue extracts a replacement electron from a manganese-calcium-oxygen cluster known as the Oxygen Evolving Complex. Four consecutive charge separations drive this cluster through four increasingly oxidized states, labeled S0 through S3, before it can strip four electrons from two water molecules and release a single molecule of oxygen. The entire water-splitting cycle therefore demands four photons delivered in sequence to the same Photosystem II, a feat that becomes extraordinarily difficult when photons arrive seconds apart and the intermediates must persist through the intervening darkness.

Those persisted intermediates are precisely where the trouble begins. Charge recombination reactions, in which the separated electron and hole fall back together, are energetically wasteful steps backward in the S-state cycle. Worse, some recombinations generate chlorophyll triplets that react with molecular oxygen to produce singlet oxygen, a highly reactive and damaging molecule that can destroy the photosynthetic machinery itself. The probability of these recombinations depends on temperature, because each pathway has a characteristic activation energy. As temperatures fall below those thresholds, recombinations become less likely, which means that cold, counterintuitively, can actually stabilize the charge separations that photosynthesis needs to function.

Testing this idea required a clever experimental design. The researchers grew nine taxa of diatoms and green algae, some isolated from polar habitats and others from temperate waters, under controlled conditions of temperature, light, and salinity. They then dark-adapted each culture, allowing the Oxygen Evolving Complexes to relax predominantly to the S1 state, which synchronizes the entire population of Photosystem II complexes. From that synchronized starting point, they fired a series of twenty to thirty-two short, saturating flashes of blue light centered at 445 nanometers, each flash delivering a photon to every Photosystem II in the sample. Because each S-state has a slightly different intrinsic fluorescence yield, the sequential flashes produced a characteristic four-step oscillation in the maximum quantum yield of photochemistry, which the team tracked using a fast repetition rate fluorometer.

As the flashes continued, however, the oscillations inevitably decayed. Each photochemical miss, whether from a recombination or a double hit, knocked one Photosystem II out of step with its neighbors, scrambling the population-level signal toward an averaged value. The researchers quantified this decay using wavelet analysis, a mathematical technique that decomposes a time series into its constituent frequencies without assuming that statistical properties remain constant over time. By comparing the observed wavelet power at a periodicity of four against a white noise model, they could determine how many flashes occurred before the synchronized S-state cycling dropped below statistical significance. They called this measure the damping index, and it served as a direct readout of how many successive photochemical charge separations the population could sustain before misses overwhelmed the signal.

The results were striking. Across all taxa, the damping index declined as the spacing between flashes widened, which the team converted into equivalent effective light levels ranging from roughly 0.02 to 1 micromole photons per square meter per second, several orders of magnitude below full sunlight and well beneath the traditional threshold for the bottom of the photic zone. It also declined as measurement temperature rose. But the polar taxa, including the sea-ice diatom Fragilariopsis cylindrus and the Antarctic green algae Chlamydomonas priscui, Chlamydomonas malina, and Chlamydomonas ICE-MDV, maintained synchronized S-state cycling over far wider ranges of conditions than their temperate counterparts. The temperate diatom Thalassiosira pseudonana and the freshwater green alga Chlamydomonas reinhardtii lost their four-step oscillations entirely at measurement temperatures near or above their growth optima, while the polar species kept cycling even at the lowest light levels tested.

Perhaps the most intriguing finding emerged when the researchers compared peak performance. Both polar and temperate taxa achieved maximum damping indices of roughly ten to twelve flashes, suggesting that the intrinsic capacity of Photosystem II to maintain synchronized S-state cycling is fundamentally similar across lineages. The difference lies not in the machinery itself but in the temperatures at which each taxon can grow. Polar algae, adapted to thrive at temperatures near zero, naturally operate in the regime where recombination probabilities are suppressed and charge separations persist. Temperate species, growing at warmer temperatures, never access that regime. When the researchers measured both Chlamydomonas priscui and Chlamydomonas reinhardtii at a common temperature of 12 degrees Celsius, their wavelet power patterns became remarkably similar, confirming that the sustained synchronization in the polar species reflects its ability to grow cold rather than any intrinsic difference in Photosystem II architecture.

The study also probed the role of salinity, which varies dramatically across polar habitats and fluctuates with freezing and thawing cycles. Previous work had shown that high salinity induces restructuring of Photosystem I supercomplexes in Antarctic Chlamydomonas species, increasing cyclic electron flow and nonphotochemical quenching as photoprotective mechanisms. In the new experiments, salinity showed a statistically significant influence on damping indices when analyzed across all Chlamydomonas taxa, but within individual species only Chlamydomonas priscui, which tolerates the widest range of growth salinities, showed a direct effect. This suggests that the apparent salinity signal across taxa largely reflects differences in how each species responds to its native salinity rather than a universal direct influence of salt on Photosystem II synchronization.

The broader implications reach into one of the most consequential questions in polar ecology: how spring blooms will respond to a warming Arctic. Field observations have shown that net photosynthetic accumulation can begin very early in the season under ice, at photon fluxes that push the boundaries of photobiological knowledge. These winter survivors seed the massive spring blooms that dominate annual primary production in polar regions. The new findings suggest that low growth temperature itself is a key enabler, allowing polar phytoplankton to exploit extreme low light by suppressing the wasteful recombinations that would otherwise drain their photosynthetic energy. As climate change thins sea ice and extends the window of light available under it, the species best equipped to photosynthesize at low temperatures and low light may gain a decisive competitive advantage, capturing nutrients first and shaping the entire food web that depends on them. In the polar night, it turns out, being cold is not a liability but a strategy.

Subject of Research: Photosystem II efficiency and photochemical miss suppression in polar versus temperate phytoplankton under extreme low light and low temperature

Article Title: Photosystem II function under extreme low light and low temperatures; polar vs. temperate phytoplankton

Article References: Poirier, M., Ryan, N. M., Osmers, P., Plassart, A., Guérin, S., Babin, M., Berthold, M., Savoie, M., Cvetkovska, M., & Campbell, D. A. (2026). Photosystem II function under extreme low light and low temperatures; polar vs. temperate phytoplankton. Frontiers in Photobiology, 4, Article 1787189. https://doi.org/10.3389/fphbi.2026.1787189

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1787189

Keywords: Photosystem II, phytoplankton, polar algae, psychrophiles, chlorophyll fluorescence, S-state cycling, charge recombination, low light photosynthesis, sea ice diatoms, Chlamydomonas, polar night, spring bloom

VOCs are low-molecular-weight carbon compounds that easily evaporate into the gas phase. In the ocean, they are estimated to represent 30 to 40 percent of the dissolved organic carbon pool and serve as both metabolic substrates and chemical signals for marine bacteria. Once produced, they can be consumed by heterotrophic microbes, react with other dissolved chemicals, or escape across the sea surface into the atmosphere. There, they participate in the formation of secondary organic aerosols, which influence cloud formation and climate, and modulate the availability of the hydroxyl radical, a key oxidant that controls the atmospheric lifetime of methane and helps maintain stratospheric ozone. Despite their importance, the biological processes governing which VOCs are produced, and in what quantities, remain poorly resolved.

To isolate the effect of growth rate on VOC production, the team turned to a controlled laboratory approach. They grew two model marine phytoplankton species, the green alga Dunaliella tertiolecta and the diatom Thalassiosira weissflogii, in semi-continuous cultures under constant low light (10 micromol photons per square meter per second) or high light (180 micromol photons per square meter per second). By passaging cultures repeatedly through fresh medium, the researchers ensured that cells had undergone at least ten divisions under identical conditions, achieving what biologists call steady-state growth: a physiological state where light absorption is perfectly balanced with downstream carbon metabolism and cell division proceeds at a constant, predictable rate. This design eliminated the confounding effects of short-term light fluctuations, nutrient limitation, or stress responses that often complicate VOC measurements in the field.

The results were unexpected. Under low light, D. tertiolecta grew at 0.6 per day and produced a diverse array of VOCs, including acetone, isoprene, and methanethiol, with oxygen-containing compounds making up 72 percent of the total VOC pool. But when the same species was grown under high light at a doubled growth rate of 1.2 per day, the researchers detected no VOCs at all. This was the opposite of what the team had hypothesized. Meanwhile, T. weissflogii, the diatom, produced VOCs under both light conditions, with its total VOC pool reaching 5.5 times that of low-light D. tertiolecta under high light. Even more strikingly, the types of VOCs produced by the diatom remained nearly identical whether cells were growing slowly in dim light or quickly in bright light.

Only two VOCs were shared between the two species across all treatments, underscoring how species-specific VOC production can be. Seven VOCs were unique to D. tertiolecta and seven were unique to T. weissflogii. In the green alga, acetone dominated the low-light VOC profile, likely arising from fatty acid beta-oxidation as an auxiliary energy pathway when photosynthetic electron transport is limited. Isoprene, which the researchers interpret as linked to chlorophyll biosynthesis rather than high-light stress protection, appeared only in low-light cells where chlorophyll content was 7.1-fold higher than in high-light cells. Methanethiol, derived from the metabolism of dimethylsulfoniopropionate (DMSP), an abundant cellular osmolyte, was also detected only under low light, consistent with field observations showing elevated methanethiol during light-limited periods in the subarctic North Atlantic.

The diatom told a different story. T. weissflogii produced ethanol as its dominant VOC under both light conditions, with ethanol and one other oxygenated compound comprising 88 to 90 percent of the total VOC pool. Flux-based modeling of a closely related diatom, Thalassiosira pseudonana, has shown that ethanol release serves as a fermentation product to balance cellular redox state, specifically the ratio of NADH to ATP, across a wide range of growth rates. This generalized redox regulatory strategy appears to be a hallmark of diatom metabolism that D. tertiolecta lacks. Instead, the green alga channels excess photosynthetic electrons into glycerol synthesis, a pathway that keeps volatile metabolites from accumulating and diffusing out of the cell.

The researchers propose that these contrasting VOC production patterns reflect fundamental differences in how each species has adapted to its ecological niche. D. tertiolecta, a chlorophyte, is adapted to relatively stable, well-lit waters with moderate nutrient availability. Under these conditions, tight metabolic control minimizes metabolic overflow, and VOC production is essentially shut down when growth is fast and energy is abundant. Diatoms, by contrast, thrive in dynamic environments such as coastal waters and deep mixed layers where light availability shifts dramatically over hours and days. Their success in these variable conditions may depend on a more relaxed control of metabolite pools, allowing constitutive VOC release as a byproduct of the flexible redox balancing that enables rapid responses to changing light.

The findings carry significant implications for atmospheric and climate science. Current models of ocean-atmosphere VOC flux rely on limited knowledge of which phytoplankton taxa produce which compounds under what conditions. If VOC production is governed more by evolutionary life-history strategy than by instantaneous growth rate, then predicting how ocean VOC emissions will respond to climate-driven changes in light availability, mixed-layer depth, and species composition becomes far more complex. The results also suggest that light-limited growth, which occurs in deep chlorophyll maxima, during bloom senescence, and in seasonally dark polar waters, may be an important and previously unrecognized source of isoprene and other climate-active VOCs.

The study does leave open questions. The researchers did not attempt to separate VOCs produced directly by algal metabolism from those generated through photochemical degradation of dissolved organic carbon released by the cells, a sequential process that can sometimes account for the majority of acetaldehyde and acetone production in seawater. Future experiments exposing cell-free spent medium to light could disentangle these two production pathways. Additionally, the team notes that exposing D. tertiolecta to unpredictable light pulses, mimicking the variable conditions diatoms naturally encounter, might reverse the observed patterns, causing the green alga to produce a broader VOC array while the diatom remains unaffected. Testing this prediction across a wider range of phytoplankton taxa and nutrient conditions will be essential for building a comprehensive picture of how the ocean’s microscopic forests shape the air we breathe.

Subject of Research: Light-dependent volatile organic compound production in marine phytoplankton

Article Title: Light-driven growth rate-dependent volatile organic compound production reflects adaptative strategies in Dunaliella tertiolecta and Thalassiosira weissflogii

Article References: Almubarak, H. H., Buchholz, H. H., & Halsey, K. H. (2026). Light-driven growth rate-dependent volatile organic compound production reflects adaptative strategies in Dunaliella tertiolecta and Thalassiosira weissflogii. Frontiers in Photobiology, 4, Article 1810167. https://doi.org/10.3389/fphbi.2026.1810167

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1810167

Keywords: phytoplankton, volatile organic compounds, Dunaliella tertiolecta, Thalassiosira weissflogii, diatom, green alga, photosynthesis, light intensity, isoprene, acetone, ethanol, ocean-atmosphere exchange

The study, published in Frontiers in Remote Sensing, was led by Borja Sánchez-López and Marco Talone of the Institut de Ciències del Mar in Barcelona, together with colleagues from Spanish, Italian, and Slovenian institutions. Pigment concentrations matter because they reveal the composition of marine ecosystems and drive biogeochemical cycling: which phytoplankton groups dominate the water, how much carbon they fix, and how communities shift as oceans warm. Traditionally, the only way to measure them is to bottle seawater, filter it, and run it through High Performance Liquid Chromatography — a precise but slow and expensive technique that limits sampling to a few stations and cruises.

Satellites promise the opposite: daily, near-global coverage. But turning multispectral radiometry into pigment concentrations is a hard inverse problem, because many pigments share overlapping absorption features and vary by orders of magnitude in abundance. Machine learning has been applied before to sensors like MODIS and SeaWiFS, yet no study had systematically exploited OLCI’s additional bands — including wavelengths near 442, 560, 620, and 673 nanometers — for the simultaneous retrieval of multiple diagnostic pigments. The new work fills that gap with a controlled comparison of five architectures: Random Forest, eXtreme Gradient Boosting, a Dense Neural Network, a one-dimensional Convolutional Neural Network, and a Bidirectional Long Short-Term Memory network.

Training and evaluation relied on 185 co-located in situ measurements of remote-sensing reflectance and HPLC pigment concentrations collected between 2014 and 2021 across optically diverse European waters — the Mediterranean Sea, the Black Sea, the Atlantic, and the English Channel — within the BiOMaP programme. The radiometry was deliberately rigorous: hyperspectral TriOS RAMSES radiometers or multispectral BioSpherical Instruments C-OPS profilers, processed under strict protocols to keep uncertainty low, then linearly interpolated to OLCI’s 11 relevant center-wavelengths. The waters ranged from near-oligotrophic Balearic Sea conditions to the sediment- and CDOM-laden Adriatic and Black Sea, giving the models a genuinely diverse optical training domain.

The results were striking. The CNN consistently outperformed all other architectures across all evaluation metrics, achieving coefficients of determination between 0.70 and 0.93 and mean absolute percentage errors of 25 to 55 percent across the 13 target pigments. For the five best-retrieved compounds — chlorophyll a, chlorophyll c1+c2, fucoxanthin, beta-carotene, and diadinoxanthin — errors approach the roughly 20 percent inter-laboratory differences documented in HPLC round-robin experiments, suggesting the retrieval is close to the physical limit of what multispectral data can deliver. The CNN’s advantage over the BiLSTM also carries a scientific lesson: pigment retrieval appears governed by localized spectral features confined to a few consecutive bands, rather than long-range spectral dependencies.

A key methodological innovation was a novel two-phase deep learning training strategy. Pigment-specific modules were first trained independently, each optimized for a single compound with its own early stopping and hyper-parameters. The modules were then concatenated into a joint multi-output model with a shared dense head, first trained with frozen weights and later unfrozen for joint optimization at a reduced learning rate. This design prevents dominant, easily retrieved pigments — chiefly chlorophyll a, whose strong absorption features and wide concentration range give it statistical leverage — from degrading performance for rarer accessory pigments that are less represented in the dataset.

To extend the method to operational satellite products, the team applied transfer learning: the CNN pre-trained on the high-quality regional in situ data was fine-tuned on a global matchup dataset combining HPLC measurements from the MAREDAT, NOMAD, and TARA databases with Copernicus-GlobColour satellite reflectance. In a controlled comparison, the fine-tuned model systematically outperformed an identical architecture trained from scratch on satellite data alone across all 13 pigments. Mean absolute percentage errors were generally lower than 70 percent and roughly 20 to 25 percent smaller than the from-scratch version, and the coefficient of determination decreased by only about 0.2 relative to the in situ baseline, versus about 0.3 for the from-scratch model. The gains suggest the convolutional layers learned robust, transferable spectral-biogeochemical features that hold across oceanic regimes.

The team also built and validated a spectrally reduced five-wavelength version, using bands at 412, 442, 490, 555, and 670 nanometers, for compatibility with the Copernicus-GlobColour and ESA Ocean Colour Climate Change Initiative merged archives. That version retains comparable in situ performance for most pigments — with the largest degradation for 19′-butanoyloxyfucoxanthin — and preserves the transfer learning advantage after fine-tuning. A SHAP interpretability analysis confirmed that green reflectance near 560 nanometers provides the highest relative contribution across most pigments, with blue bands near 442 and 490 nanometers as key secondary contributors, consistent with established ocean color algorithm design.

Not every pigment surrendered easily. Zeaxanthin and 19′-butanoyloxyfucoxanthin remained challenging regardless of model complexity, most likely due to low concentrations, poor representation in the training data, and heavy spectral overlap with dominant pigments — a limitation the authors expect hyperspectral sensors such as NASA’s PACE OCI may partially resolve. The geographical concentration of the BiOMaP dataset in European waters may also limit global transferability, and the authors call for expanding coincident radiometric and HPLC measurements to underrepresented regions including the tropical Pacific, the Southern Ocean, and major upwelling systems.

Still, the implications reach far beyond one sensor. The five-wavelength model can be applied retrospectively to a continuous satellite record extending back to 1997, enabling 26-year climatological analysis of phytoplankton community composition. The authors also envisage applying their framework to current and future hyperspectral missions, noting that historical in situ HPLC measurements collected alongside multispectral radiometry — spectrally incompatible with hyperspectral sensors — need not be discarded: they can serve as pre-training foundations for hyperspectral CNN models, fine-tuned later on the smaller hyperspectral matchup datasets that PACE and ESA CHIME will progressively generate. The trained models and implementation code are publicly available on GitHub, opening the door for the ocean color community to map the ocean’s microscopic garden in unprecedented chemical detail.

Subject of Research: Simultaneous retrieval of 13 phytoplankton pigments from Sentinel-3 OLCI using convolutional neural networks and transfer learning

Article Title: Simultaneous retrieval of 13 phytoplankton pigments from Sentinel-3 OLCI using convolutional neural networks and transfer learning

Article References: Sánchez-López, B., Talone, M., Cerquides, J., Di Cicco, A., Martínez-Fornos, G., & Slavinec, P. (2026). Simultaneous retrieval of 13 phytoplankton pigments from Sentinel-3 OLCI using convolutional neural networks and transfer learning. Frontiers in Remote Sensing, 7, Article 1880618. https://doi.org/10.3389/frsen.2026.1880618

Image Credits: AI Generated

DOI: 10.3389/frsen.2026.1880618

Keywords: phytoplankton, Sentinel-3, OLCI, ocean colour, machine learning, convolutional neural network, transfer learning, remote sensing, Simultaneous, retrieval, pigments, convolutional

The research team, led by Ling Su of the Jiangxi Academy of Eco-Environmental Sciences and Planning, monitored twenty sampling sites across the reservoir’s Jinkou and Sima areas every month from May 2022 to April 2023. Their goal was to fill a conspicuous gap in freshwater ecology: while shallow, nutrient-polluted lakes in China have been studied extensively, deep reservoirs with complex thermal layering, engineered water-level swings, and steep underwater terrain remain poorly documented. Lushan West Reservoir, with a storage capacity of 7.92 billion cubic meters, an average depth of 18.59 meters, and a maximum depth of 45 meters, offered an ideal natural laboratory.

What the team found was a community of remarkable richness and volatility. In total, 116 phytoplankton species belonging to eight phyla were identified. Green algae of the phylum Chlorophyta led in sheer species count, accounting for 43.97 percent of the catalog, followed by diatoms of Bacillariophyta at 28.45 percent and cyanobacteria at 17.24 percent. Yet species richness told only part of the story. When it came to actual abundance, cyanobacteria dominated throughout the year, making up between 48.64 and 62.26 percent of cells in spring and surging to as much as 94.72 percent in winter.

The seasonal rhythm of total cell density was equally striking. Numbers peaked at 9.98 million cells per liter in September, crashed to roughly 1.72 million cells per liter by December, and bottomed out below one million cells per liter in the early spring months of February and April. A curious dip occurred in June, when density plummeted from 7.86 million to 1.08 million cells per liter. The researchers attribute this collapse to pre-flood dam operations, which flush water through the reservoir, shorten the time phytoplankton spend in productive zones, and dilute populations before they can recover.

To untangle which environmental variables were steering these shifts, the team employed redundancy analysis, a multivariate statistical technique suited to linear species-environment relationships. The model explained 62.4 percent of the total variation in community structure, with the first two ordination axes accounting for 57.5 percent. Four factors emerged as the principal drivers: water temperature, transparency, total nitrogen, and total phosphorus. Transparency carried the strongest conditional effect, followed closely by temperature, both statistically significant at the one percent level.

The thresholds the study identified read like an ecological rulebook. When water temperature climbed above 25 degrees Celsius, total phosphorus reached 0.22 milligrams per liter, and total nitrogen hit 1.29 milligrams per liter, cyanobacterial biomass peaked at 0.42 milligrams per liter and cell density hit its annual maximum. Conversely, when temperatures fell below 13 degrees Celsius and transparency exceeded 320 centimeters, diatom relative abundance climbed to 41.80 percent. These numbers align with the reservoir’s own seasonal extremes: summer water temperatures reached 28.42 degrees Celsius, while winter lows dipped to 12.68 degrees.

One of the study’s most intriguing findings concerns a seeming paradox. Total phosphorus showed no statistically significant seasonal variation when measured in integrated water-column samples, yet cyanobacterial dominance fluctuated dramatically with the seasons. The authors resolve this by pointing to thermal stratification. In summer, a stable thermocline forms in the deep reservoir, creating oxygen-poor conditions in the bottom layer that trigger the release of phosphorus from sediments. This internally loaded phosphorus accumulates below the thermocline and cannot fully mix into surface waters, which is why whole-column measurements miss it. But bioavailable dissolved phosphorus diffuses upward into the sunlit zone, where warm temperatures supercharge cyanobacteria’s phosphorus uptake efficiency, giving them a decisive competitive edge.

Niche analysis added a competitive dimension to the picture. Using Levins’ modified niche breadth index, the researchers found that diatoms maintained relatively broad ecological niches, with values ranging from 1.75 to 2.54, reflecting their ability to exploit a wide range of conditions. Cyanobacteria, however, achieved even wider niche widths, from 1.25 to 2.78, with the filamentous species Pseudanabaena sp. topping the scale at 2.78. This broad adaptation, combined with efficient resource utilization, explains how cyanobacteria sustain year-round dominance despite seasonal environmental swings. Green algae, by contrast, occupied narrower niches between 0.40 and 1.58, limiting their competitive reach.

The study also carries a sobering management message. Pollution source census data cited in the paper reveal that urban domestic wastewater is the dominant external nutrient source, contributing 936.15 tons of total nitrogen and 51.33 tons of total phosphorus annually to the watershed. Industrial discharges add another 44.43 tons of nitrogen and 1.11 tons of phosphorus per year. The authors recommend prioritizing sewage collection and treatment upgrades, agricultural buffer strips, riparian restoration, and controls on tourism-related discharges to curb the external nutrient inputs that fuel summer cyanobacterial blooms.

For a reservoir that serves as both a drinking water source and a scenic tourism destination, the stakes are considerable. Cyanobacterial blooms can release toxins, suppress zooplankton communities, and disrupt energy flow through the food web, while diatom dominance tends to stabilize primary productivity and aid nutrient recycling through the deposition of siliceous shells. By pinpointing the temperature and nutrient thresholds at which the balance tips, this research offers water managers a quantifiable early-warning framework, not just for Lushan West Reservoir but for the many deep, regulated reservoirs scattered across the mountainous landscapes of southern China.

Subject of Research: Seasonal dynamics of phytoplankton community structure and environmental drivers in a deep freshwater reservoir

Article Title: Spatiotemporal dynamics of phytoplankton community structure in freshwater reservoir and its relationship with environmental factors

Article References: Su, L., Li, L., Ji, Y., Xiong, P., Cheng, W., Wan, F., & Liu, H. (2026). Spatiotemporal dynamics of phytoplankton community structure in freshwater reservoir and its relationship with environmental factors. Frontiers in Water, 8, Article 1906226. https://doi.org/10.3389/frwa.2026.1906226

Image Credits: AI Generated

DOI: 10.3389/frwa.2026.1906226

Keywords: phytoplankton, cyanobacteria, diatoms, freshwater reservoir, eutrophication, water quality, thermal stratification, niche analysis, redundancy analysis, total phosphorus, total nitrogen, China

A team of researchers working within European research infrastructure projects has published a harmonized, individual-level dataset of phytoplankton traits collected from fourteen transitional water sites across the Mediterranean and Black Sea regions. Spanning Albania, Bulgaria, Greece, Italy, and Romania, the dataset contains 104,687 individual records covering 242 accepted taxa distributed across ten phyla, twenty-two classes, sixty-seven orders, one hundred families, and one hundred thirty-six genera. Crucially, the data were collected not just at the species level, as most trait databases are, but cell by cell, capturing the fine-grained variation within populations that ecologists increasingly recognize as essential for understanding how communities respond to environmental change.

Transitional waters, the dynamic ecosystems perched at the boundary between rivers and seas, are among the most variable and vulnerable habitats on Earth. Their salinity, nutrient loads, and hydrology fluctuate dramatically with freshwater inputs, tidal exchange, climatic variability, and human activity. The lagoons in this study, from Alimini and Torre Guaceto in southern Italy to Varna Bay in Bulgaria and Sinoe lagoon in Romania, are microtidal or non-tidal, nutrient-rich, depositional systems that experience intense anthropogenic pressure while simultaneously hosting remarkable biodiversity. Understanding how phytoplankton communities organize themselves across such steep environmental gradients is a central question in functional ecology, and it requires data of a kind that has been notoriously difficult to find.

The original sampling took place during two coordinated campaigns, in October 2004 and April 2005, conducted under the INTERREG IIIB CADSES TWReference Net Project coordinated by Prof. Alberto Basset and funded by the European Union’s Seventh Framework Programme. At each lagoon, between one and seven sampling stations were established according to a hierarchical design meant to capture both seasonal and spatial variability. Researchers collected water from a depth of half a meter using a Ruttner bottle, preserved samples in Lugol’s iodine solution, and stored them in darkness at four degrees Celsius until analysis. Physicochemical conditions, including temperature, salinity, pH, and dissolved oxygen, were measured in situ with a multiparametric probe, while nutrient samples were processed immediately upon arrival at the laboratory using standard methods.

The laboratory work is where the dataset acquires its unusual power. Following the classical Utermöhl method, subsamples were examined under an inverted Nikon T300E microscope at four hundred times magnification. For each sample, a minimum of four hundred cells were counted, measured, and identified to the lowest possible taxonomic level, ensuring statistically robust estimates of both community composition and cell dimensions. Each cell was assigned to one of twenty-four geometric shapes, from simple spheres and cubes to composite forms such as a cone paired with a half sphere, and its linear dimensions were recorded. These measurements fed into geometric equations that calculate biovolume, which was then converted into individual carbon content using the widely cited equations of Menden-Deuer and Lessard, and into biomass using a conversion factor of 1.03.

The resulting picture of phytoplankton life in these lagoons is strikingly region-dependent. By density, Chlorophyta, the green algae, dominated the dataset at 37.39 percent of total abundance, followed by Cyanobacteria at 23.58 percent and Cryptista at 15.12 percent. But biomass tells a very different story: Dinoflagellata contributed the largest share at 36.29 percent, followed by Heterokontophyta, the group containing diatoms, at 20.08 percent. This discrepancy reflects a fundamental ecological truth, that large-celled groups such as dinoflagellates and diatoms punch far above their numerical weight when it comes to carbon and energy flowing through the food web. Counting cells alone would badly misrepresent the functional structure of these communities.

Geographic contrasts were equally revealing. Mediterranean lagoons hosted communities with richness values typically between twenty-three and forty-nine taxa, whereas Black Sea lagoons were richer, ranging from forty-six to seventy-nine taxa, with the highest diversity observed in Varna. Mediterranean communities were more evenly distributed, with alternating dominance of Cryptista, Cyanobacteria, and Chlorophyta in density terms, while Black Sea systems, particularly Sinoe and Varna, showed stronger dominance by cyanobacteria and dinoflagellates, especially when biomass was considered. Shannon diversity values based on density ranged from 0.56 to 2.56 across lagoons, indicating substantial differences in community evenness from one system to the next.

Even cell shape itself carries an ecological signal. Across the whole dataset, cube-shaped cells accounted for 19.3 percent of total density and cylindrical cells 19.2 percent, together making up nearly forty percent of all individuals counted. Shape composition differed sharply between the two seas: the cone-plus-half-sphere morphology dominated Mediterranean sites at 43.8 percent of cell density, while cylindrical forms, associated with genera such as Cyclotella, Anabaena, and Planktolyngbya, and cube-shaped cells, represented mainly by Merismopedia and Crucigenia, predominated in the Black Sea. Grouped by geometric complexity, simple shapes accounted for roughly seventy-two percent of total density overall, but the balance shifted between regions, with simple shapes reaching 78.0 percent of mean relative density in Black Sea sites compared with a more even 50.8 percent split in the Mediterranean.

What elevates this dataset beyond a simple archive is the extraordinary effort devoted to making the data genuinely interoperable and reusable, compliant with the FAIR principles of findability, accessibility, interoperability, and reusability. The phytoplankton records are accompanied by a complementary abiotic dataset containing in situ measurements of salinity, conductivity, temperature, pH, dissolved oxygen, oxygen saturation, and total dissolved solids, along with nutrient concentrations including nitrite, nitrate, ammonium, dissolved inorganic nitrogen, phosphate, and silicate. Physiographic and hydrological characteristics of each lagoon, such as water depth, tidal range, inlet morphology, sinuosity index, surface area, and hydrological connectivity, are included alongside qualitative descriptors of anthropogenic pressure and protection status, including Ramsar designation, Natura 2000 site membership, and local protection plans.

The curation pipeline drew on an impressive array of controlled vocabularies and standards. Data were organized using the Data Template Builder tool of the Phytoplankton Virtual Research Environment, incorporating concepts from Darwin Core standards, the Traits Thesaurus, MarineRegions, EnvThes, and NERC vocabularies to generate semantically consistent data tables. Taxonomic validation was performed against the AlgaeBase repository, with both the original identification and the accepted name preserved in the dataset through dedicated Darwin Core properties, ensuring traceability. Morphological shape assignments were verified using the Atlas of Shapes web service, and trait computation was automated through a dedicated Python-based e-service. The dataset has already served as a reference case for harmonization and trait-based integration within the Phyto VRE and has underpinned previously published studies on morpho-functional analysis, guild classification, and hypothesis testing across multiple levels of biological organization.

Archived in comma-separated value format with metadata in the Ecological Metadata Language 2.2.0 standard, the two datasets are openly accessible through the LifeWatch Italy Data Portal and Metadata Catalogue under a Creative Commons Attribution 4.0 licence. For researchers studying functional diversity, trait-environment relationships, or the ecology of coastal ecosystems under mounting human pressure, the resource represents something rare: a standardized, cell-level record spanning five countries, two seas, and a full spectrum of environmental gradients. As transitional waters face intensifying eutrophication, hydrological alteration, and climate-driven change, having a decade-spanning, taxonomically validated, individually measured baseline from these vulnerable lagoons may prove invaluable, not only for charting what has been lost, but for predicting what these ecosystems, and the microscopic engines at their heart, will become.

Subject of Research: Individual-level phytoplankton trait data from Mediterranean and Black Sea transitional waters

Article Title: A harmonized individual-level phytoplankton trait dataset from Mediterranean and Black Sea transitional waters

Article References: Titocci, J., Laraib, M., Semeraro, T., & Basset, A. (2026). A harmonized individual-level phytoplankton trait dataset from Mediterranean and Black Sea transitional waters. Biodiversity Data Journal, 14, Article e196603. https://doi.org/10.3897/bdj.14.e196603

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e196603

Keywords: phytoplankton, trait-based ecology, transitional waters, coastal lagoons, Mediterranean Sea, Black Sea, FAIR data, biovolume, functional diversity, biodiversity data, cyanobacteria, dinoflagellates