The study period was anything but ordinary. Portugal endured an unusually dry stretch in which every month except March recorded below-average precipitation, and the Portuguese Institute for the Ocean and Atmosphere logged six distinct heat waves between September 2021 and September 2022. Following World Meteorological Organisation guidelines, the researchers classified heat waves as periods lasting at least two consecutive days with maximum temperatures five degrees Celsius above the mean. The events they documented ranged from six to sixteen days in duration, and remarkably, one occurred in December 2021, demonstrating that extreme heat no longer respects seasonal boundaries. This relentless sequence of thermal shocks, layered on top of chronic drought, created a natural experiment that the team could exploit by sampling monthly across a full year.

The Minho River estuary, which forms roughly forty kilometres of the border between Portugal and Spain, is a mesotidal, partially mixed system that tends toward a salt wedge during high flows. The researchers selected two dominant submerged plants anchored at opposite ends of the salinity gradient. Fucus ceranoides, a brown alga typical of sheltered rocky estuaries from northern Portugal to Norway, grows on rocks near the river mouth in the polyhaline zone. Egeria densa, a submerged macrophyte native to subtropical South America that first appeared in the international section of the Minho in the early 1990s, forms dense monospecific mats along the upper estuary’s tidal freshwater zone. Because Egeria densa’s downstream distribution stops at the oligohaline-limnetic boundary, roughly sixteen kilometres from the mouth, saline intrusion during dry months poses a constant threat to its foothold.

Monthly sampling from September 2021 to September 2022 was conducted at low tide, with five replicate quadrats of 0.1 square metres collected by hand for each plant. In the laboratory, the team removed, sorted and counted every associated invertebrate to the lowest taxonomic level possible, then dried plant and animal material at sixty degrees Celsius for forty-eight hours to determine dry weight. Ash-free dry mass was obtained by burning samples in a muffle furnace at 550 degrees Celsius. In parallel, a multi-parameter probe recorded water temperature, salinity, dissolved oxygen, pH, conductivity and oxidation-reduction potential at each site, while suspended organic matter was quantified by filtering one-litre water samples through pre-weighed glass fibre filters. Statistical treatment included UPGMA clustering with Bray-Curtis similarity to group months by environmental and faunal composition, Kendall’s rank correlations for non-normal data, and Kruskal-Wallis tests with Dunn post hoc comparisons for biomass differences.

The environmental data told a clear story of stress. In the lower estuary, water temperature ranged from 10.9 degrees Celsius in January to 20.6 degrees in June, and salinity swung dramatically from 13.55 in March to 37.18 in September 2021. Upstream, conditions were harsher still: temperatures spanned 7.7 degrees in November to 25.3 degrees in August, and an August salinity peak of 0.16, modest by marine standards but anomalous for a freshwater zone, contrasted with values of 0.04 to 0.08 in other months. Oxidation-reduction potential, a proxy for the chemical redox state of the water, dipped in every heat wave month in the lower estuary and collapsed to 87.1 millivolts in July upstream, far below the 228 to 348 millivolts seen otherwise. Cluster analysis confirmed that July and August stood apart environmentally from all other months.

Against this backdrop, the two plants responded in opposite ways. Fucus ceranoides biomass varied little, averaging 589.77 grams of dry weight per square metre, with a November maximum of 770.71 and a July minimum of 429.4. The only significant differences separated April and July from most other months, and no correlations emerged between algal biomass and any measured environmental variable. The authors attribute this resilience to the euryhaline tolerance of a species adapted to daily tidal salinity fluctuations, noting that similar biomasses were recorded in a survey of the same site a decade earlier. Egeria densa, by contrast, declined relentlessly, falling from 289.9 grams of dry weight per square metre in October 2021 to just 84.8 by September 2022, with significant decreases from March onward. Its biomass correlated positively with pH and suspended organic matter but negatively with water temperature, suggesting that despite the species’ reputation for tolerating warm water, abrupt heat waves push it beyond its limits in estuarine conditions.

The invertebrate communities living within these plants proved even more sensitive than the plants themselves. In the Fucus zone, thirteen species were recorded, including the exotic barnacle Austrominius modestus, and diversity dipped during the December, July and August heat waves. Yet one species bucked the trend spectacularly: the amphipod Echinogammarus marinus, whose abundance and biomass peaked during the hottest months, driving annual abundance to 461.68 individuals per square metre, nearly double the figure from the 2011-2012 survey. This adaptable euryhaline crustacean appears poised to thrive under climate change, potentially released from predation as the shore crab Carcinus maenas declined. Upstream, the Egeria beds hosted twenty-three species, nine of them exotic, including two, the snail Ferrissia californica and the leech Barbronia weberi, recorded in the estuary for the first time. Total invertebrate counts upstream plummeted from 2,490 individuals in June to just 106 in September, and the number of taxa collapsed in the final month.

Perhaps the most provocative finding concerns which animals flourished amid the chaos. The exotic gastropods Physella acuta and Menetus dilatatus surged in abundance during the December and May heat waves, confirming earlier evidence that non-native freshwater snails tolerate elevated temperatures better than European natives. However, both species crashed when salinity rose in the upper estuary during summer, with Menetus dilatatus abundance negatively correlated with salinity and Physella acuta biomass negatively correlated with temperature. Other freshwater residents fared worse: the bryozoan Paludicella articulata and the mite Lebertia insignis were both strongly and negatively correlated with rising temperatures, while several species, including the planarian Girardia sinensis and the damselfly Ischnura elegans, depended directly on Egeria densa biomass for their persistence. As the macrophyte withered, so did the animals that rely on it as habitat.

The broader implications reach beyond a single river. Salt intrusion in estuaries is projected to increase worldwide, and prolonged heat waves reduce river flow, amplifying saline penetration into upstream habitats. The study suggests that while compensatory mechanisms such as functional redundancy and species replacement can buffer communities against moderate disturbances, increasingly severe heat waves disrupt these safety nets, leaving freshwater assemblages vulnerable to lasting structural change. In the Minho, the synergy of heat and salinity appears to favour a small cast of hardy generalists, including invasive snails and one opportunistic amphipod, at the expense of the diverse communities that healthy submerged vegetation supports. The authors caution that because sampling covered only one exceptional year, continuous monitoring is needed to establish baseline biomass levels for Egeria densa and to separate seasonal abundance cycles from genuine extreme-event impacts. Controlled experiments on Fucus ceranoides temperature tolerance would further sharpen predictions. For estuary managers across the Iberian Peninsula and beyond, the message is stark: the plants that anchor these ecosystems are not equally equipped for a hotter world, and the animals that depend on them will inherit whatever the heat leaves behind.

Subject of Research: Effects of marine and freshwater heat waves on estuarine macrophytes and their associated invertebrate fauna in the Minho River estuary

Article Title: Effects of heat wave events on the brown alga Fucus ceranoides and on the established exotic species Egeria densa and associated fauna in the Minho river estuary (NW Iberian Peninsula)

Article References: Gomes, N., Sousa-Pinto, I., & Antunes, C. (2025). Effects of heat wave events on the brown alga Fucus ceranoides and on the established exotic species Egeria densa and associated fauna in the Minho river estuary (NW Iberian Peninsula). Discover Ecology, 1(1), Article 4. https://doi.org/10.1007/s44396-025-00004-x

Image Credits: AI Generated

DOI: 10.1007/s44396-025-00004-x

Keywords: heat waves, estuarine ecology, Fucus ceranoides, Egeria densa, invasive species, invertebrates, salinity intrusion, climate change, Minho River, macrophytes, biodiversity, NW Iberian Peninsula

The research team, led by Amanda Martins of the Federal University of Bahia together with Marcos Krull and Francisco Barros, combined two rarely united sources of evidence: a long-term benthic monitoring dataset stretching back to 2004, and controlled laboratory experiments that measured exactly how much organic matter individual worms eat and how much sediment they displace. The monitoring data came from ten sampling stations along the estuary’s salinity gradient, surveyed on six separate occasions between 2004 and 2022. At each station, researchers collected cores of sediment, sieved the samples through a 0.5 millimeter mesh, and identified the invertebrates to the family level. Polychaetes made up more than half of all benthic macroinvertebrates in the system, making them the natural focal group for the study.

To translate counts of worms into estimates of ecosystem-wide function, the team turned to Bayesian generalized linear mixed models, a statistical framework that allows researchers to predict abundance across unsampled areas while quantifying uncertainty. The models incorporated environmental predictors including salinity and sediment grain size fractions—ranging from coarse sand upstream to fine sand and mud downstream—drawn from high-resolution environmental layers covering the modeled estuarine portion of the system. The researchers tested both Poisson and negative binomial distributions, with and without zero-inflation parameters, and compared generalized additive and generalized linear mixed models. Model selection relied on leave-one-out cross-validation using Pareto-smoothed importance sampling, and the winning models were fitted with Markov Chain Monte Carlo methods using a Hamiltonian sampler, with four independent chains and weakly informative priors to regularize the estimates.

The result of this statistical machinery was a population estimate of roughly 2,062.5 million individual polychaetes across the estuarine system, with a 95 percent credible interval spanning about 1.87 to 2.63 billion individuals. Seven families dominated the analysis: Orbiniidae, Spionidae, Eunicidae, Pilargidae, Lumbrineridae, Onuphidae, and Goniadidae. Orbiniidae alone accounted for an estimated 1,045.9 million individuals, followed by Onuphidae with 671.6 million. Densities were highest in the marine-influenced regions of the estuary, particularly for these two dominant families, setting the stage for a strongly spatially patterned delivery of ecological functions.

The functional measurements themselves came from a previous experimental study in which worms collected from the Jaguaripe estuary were kept under controlled laboratory conditions. Organic matter decomposition was quantified as the consumption rate of standardized pieces of shrimp tissue over 96 hours, corrected for natural decomposition using controls. Bioturbation—the reworking of sediment—was measured as the volume of sediment displaced by each species, reconstructed from computed tomography scans of the burrow systems the worms produced. This CT-based approach allowed the researchers to visualize and measure the three-dimensional architecture of worm burrows without disturbing them, providing an unusually precise metric of a notoriously difficult-to-quantify process.

When the per-individual rates were multiplied by the model-predicted abundances across the estuary, the numbers were striking. Polychaetes consumed an estimated 232.56 tons of organic material over 96 hours, with Orbiniidae responsible for 123.16 tons—about 53 percent of the total—and Onuphidae contributing another 89.81 tons, or nearly 39 percent. Bioturbation totaled an estimated 6,098.3 cubic meters of sediment over the same period, again dominated by Orbiniidae at 4,495.1 cubic meters and Onuphidae at 1,070.7 cubic meters, together accounting for more than 91 percent of the total. Smaller families such as Eunicidae, Spionidae, and Lumbrineridae contributed comparatively little to bioturbation, while Eunicidae and Pilargidae were minor players in organic matter consumption as well.

The spatial patterns were equally revealing. Both functions peaked in regions of elevated salinity, in the euhaline and polyhaline zones near the estuary’s mouth. Orbiniidae and Onuphidae delivered their largest contributions at salinities between 25 and 40, whereas Spionidae showed a strikingly different profile, contributing up to 80 percent of decomposition and bioturbation in the lower-salinity upstream reaches, between roughly 5 and 20 on the salinity scale. Sediment grain size also mattered: the dominant families responded to different granulometric fractions, with Orbiniidae and Onuphidae key in coarse sand and mud regions respectively, and Spionidae playing its characteristic upstream role in very fine sand. No single sediment variable was consistently selected across all taxa, underscoring that each family responds to its own combination of environmental conditions.

Correlation analyses of the functional contributions revealed another important pattern: functional redundancy. In the marine lower estuary, the contributions of Orbiniidae, Onuphidae, and Lumbrineridae rose and fell together, with correlation coefficients for bioturbation reaching 0.93 between Orbiniidae and Onuphidae and 0.92 between Lumbrineridae and Onuphidae. This overlap means that if one family were lost, others performing similar roles could partially compensate, buffering the ecosystem against disturbance. In the oligohaline upstream zones, by contrast, fewer families contributed to the functions, and redundancy was correspondingly lower—making those reaches potentially more vulnerable to species loss. Spionidae, with low correlations to all other taxa, stands out as functionally irreplaceable in the upstream areas where it dominates.

The study’s authors are candid about its limitations. Only seven families and two functions were analyzed, other taxa in the estuary may contribute substantially, and variables such as food resources beyond the measured organic matter could not be included. The decomposition measurements represent early-stage, macrofauna-mediated breakdown driven by direct consumption at the sediment surface, not the full complexity of benthic metabolism. Body size, which likely influences per-individual function, could not be incorporated because weight data were unavailable for several taxa. Still, the researchers argue, the demonstration that laboratory experiments can be combined with long-term monitoring and Bayesian modeling to produce ecosystem-level estimates is itself a methodological advance, offering a template for other systems where functional data remain scarce—particularly in the tropics, where most species-specific functional measurements to date have been conducted in temperate terrestrial environments.

The broader implications extend to conservation and climate adaptation. Estuaries are hotspots of organic matter input and nutrient cycling, disproportionately productive relative to their area, yet they face mounting pressures from eutrophication, metal pollution, shrimp farming, and sea-level rise. Because polychaete bioturbation aerates deeper sediment layers, reduces compaction, and accelerates organic matter processing, the loss of these worms could tip sediments toward anoxia, with cascading effects on entire benthic communities. The functional maps generated by the study highlight regions of high activity that could guide conservation priorities, and the identification of low-redundancy upstream zones flags areas where monitoring should be intensified. As climate change reshapes salinity gradients and species distributions, the authors suggest that modeling the distribution and functioning of organisms under future scenarios will be essential for predicting trends—and for protecting the billions of small engineers whose invisible labor keeps coastal ecosystems running.

Subject of Research: Ecosystem-level estimation of polychaete-mediated organic matter decomposition and bioturbation in a tropical estuary

Article Title: Scaling up polychaete contributions to estuarine ecosystem functions

Article References: Martins, A., Krull, M., & Barros, F. (2026). Scaling up polychaete contributions to estuarine ecosystem functions. Discover Ecology, 2(1), Article 6. https://doi.org/10.1007/s44396-026-00023-2

Image Credits: AI Generated

DOI: 10.1007/s44396-026-00023-2

Keywords: polychaetes, estuarine ecology, bioturbation, organic matter decomposition, ecosystem functioning, Bayesian modeling, functional redundancy, salinity gradient, macrofauna, tropical estuary, sediment reworking, conservation

The Muriganga estuary, part of the Indian Sundarbans delta, is a macro-tidal system fed by freshwater from the Ganges, known locally as the Hooghly River. Under stable conditions it remains mesohaline, meaning its salinity sits in a moderate range, and slightly alkaline with a pH above 8. The estuary had already endured a punishing sequence of storms: Cyclone Fani in 2019, then Bulbul and Amphan in 2020 and 2021. Paul and colleagues had anticipated this vulnerability. In 2019 they established a dedicated ‘Cyclone Ecology’ research program with three permanent sampling stations arranged along a north-south axis of the estuary, specifically so that when the next storm arrived, they would have baseline data in hand rather than scrambling to reconstruct it afterward.

That foresight proved decisive. Yass made landfall near Dhamra Port in Odisha with maximum sustained winds of 130 to 140 kilometers per hour, gusting to 155, before sweeping across the West Bengal coast and passing directly over the program’s sampling stations. The timing was brutal in more ways than one. The storm coincided with an astronomical high tide of 5.71 meters at nearby Sagar Island, and Yass drove a storm surge of 3.5 to 5 meters, with tidal waves rising 1 to 2 meters above the astronomical tide. Nearly 1,195 millimeters of rain fell in a short period, triggering massive inundation on both banks of the estuary. In the Bay of Bengal itself, sea surface temperature dropped by about 3 degrees Celsius, while salinity and water density rose by roughly 1 practical salinity unit and 2 kilograms per cubic meter respectively.

The team’s sampling design was deliberately asymmetric, reflecting the realities of fieldwork in a disaster zone. Pre-cyclone samples were collected on three occasions between November 2020 and April 2021, providing a solid baseline of the estuary’s copepod community. After Yass, the flooding destroyed roads to Namkhana, the town adjacent to the estuary, and the COVID-19 pandemic complicated the mobilization of scientific resources. Despite these obstacles, the researchers managed four post-cyclone sampling trips at six-day intervals between 4 and 22 June 2021. On each visit they towed a 200-micrometer plankton net fitted with a mechanical flowmeter from a dinghy boat after sunset on the high tide, sampling sub-surface water at depths of 0.3 to 0.5 meters. Water temperature, salinity and pH were recorded simultaneously with a multi-parameter probe, and copepods were preserved in 4 percent formalin for species-level identification under the microscope.

The physical fingerprint of the storm was unmistakable. Before Yass, salinity in the estuary ranged from 7.50 to 17.90; afterward it climbed to a sustained range of 15.90 to 20.80, transforming the system from mesohaline to polyhaline for weeks. This saltwater intrusion from the adjacent Bay of Bengal echoed the aftermath of Cyclone Aila in 2009, but stood in contrast to Fani, Bulbul and Amphan, none of which had shifted the estuary’s salinity regime in this way. Water temperature, by comparison, showed no drastic change, remaining between 29.10 and 31.20 degrees Celsius in the post-cyclone period, consistent with the typical seasonal behavior of a tropical estuary. The pH stayed slightly alkaline, likely a consequence of the persistent high salinity, a pattern previously observed after Cyclone Phailin pushed salt water toward the estuary mouth.

What happened to the copepods was more surprising. Previous cyclones hitting the Muriganga had consistently knocked down both species richness and total abundance, and similar declines have been reported after storms in estuaries across India, South Africa and Taiwan. After Yass, the opposite occurred. Species richness rose from a pre-cyclone range of 17 to 20 species to a post-cyclone range of 20 to 24. Shannon diversity climbed from 2.37 to 2.61 up to 2.79 to 3.00, while Simpson’s index and Pielou’s evenness both increased as well. Most strikingly, total copepod abundance more than doubled, from 307,044 individuals per cubic meter before the storm to 650,635 individuals per cubic meter afterward. The authors caution that the asymmetry in temporal replicates before and after the storm means these figures should be interpreted carefully, but the direction of change contradicts nearly every previous cyclone study in the region.

The identity of the dominant species changed too. Before Yass, the tiny calanoid copepod Bestiolina similis was the most abundant member of the community, contributing up to 20.52 percent of total abundance, with Paracalanus parvus and Acartiella tortaniformis as co-dominants. After the storm, Acartiella tortaniformis took over as the most abundant species, followed by the cyclopoid Oithona brevicornis and Paracalanus parvus. Relative abundances of Canthocalanus pauper, Temora turbinata, Corycaeus crassiusculus and various Oithona species all rose. At the family level, the Paracalanidae, which had made up 49 to 59 percent of the community before the storm, fell to 28 to 35 percent, while the Oithonidae surged from 3 to 8 percent up to 13 to 19 percent. Two species, Labidocera euchaeta and Oithona nana, appeared in post-cyclone samples despite being entirely absent before. Labidocera euchaeta is not a resident of the Muriganga at all; it is a marine intruder, and the polyhaline conditions created by the storm surge likely allowed it to thrive temporarily, much as marine migrant species had flooded into South Africa’s St. Lucia estuary after Cyclone Imboa.

The rise of the Oithonidae carries a plausible ecological explanation. Oithona species are omnivores that feed heavily on organic debris, dinoflagellates, diatoms and proto-zooplankton, and cyclones typically flush large quantities of organic matter into estuarine waters. The same pattern emerged after Cyclone Aila, when the relative abundance of Oithona brevicornis jumped from zero to 10.5 percent, and after Cyclone Hudhud struck Chilika Lagoon, where cyclopoid copepods also increased. In other words, Yass may not have simply scattered the community; it may have restructured the food web’s resource base in a way that favored debris-feeding cyclopoids over the calanoids that had dominated under stable conditions.

Perhaps the most conceptually important finding concerns spatial homogenization. Before the cyclone, multivariate analyses using non-metric multidimensional scaling, Bray-Curtis dissimilarity and PERMANOVA revealed significant station-to-station differences in copepod assemblages, with a Pseudo-F of 17.66 and a p-value of 0.005, indicating that each sampling site hosted a distinct community. After Yass, that spatial structure vanished entirely: the PERMANOVA returned a Pseudo-F of just 0.41 with a p-value of 0.95, and SIMPER analysis showed average within-period similarity jumping from 37.04 percent to 77.09 percent. The storm, through its massive flooding and mechanical forcing, appears to have blended the copepod community into a single homogeneous assemblage across the estuary, a short-term homogenization of the spatial niche also observed after Cyclone Amphan.

Beyond its findings about one estuary, the study delivers a pointed methodological message. The rapid recolonization and restructuring observed within days to weeks suggests that bi-weekly, monthly or seasonal sampling frequencies are too coarse to capture the short-term ecological dynamics that follow a cyclone. The authors recommend that sampling begin within the first week after landfall and continue at daily or weekly intervals for weeks or months, with the timeline calibrated to the estuary’s water residence time, the distance from the landfall site and the complexity of the copepod community. They even suggest deploying Lagrangian floats to record real-time changes as a cyclone passes. Because India has no institutional mechanism for regular estuarine monitoring, the researchers argue that the United Nations Ocean Decade, running from 2021 to 2030, should serve as the baseline for establishing such programs. With cyclone intensity increasing in the Bay of Bengal, the copepods of the Muriganga may be among the most sensitive early-warning indicators of how tropical estuaries will weather a stormier future.

Subject of Research: Short-term effects of tropical cyclone Yass on copepod community structure in the Muriganga estuary, India

Article Title: Short-term impact of cyclone Yass on the copepod community of Muriganga estuary of India

Article References: Paul, S., Karan, S., & Bhattacharya, B. D. (2026). Short-term impact of cyclone Yass on the copepod community of Muriganga estuary of India. Discover Ecology, 2(1), Article 7. https://doi.org/10.1007/s44396-026-00024-1

Image Credits: AI Generated

DOI: 10.1007/s44396-026-00024-1

Keywords: Cyclone Yass, copepods, Muriganga estuary, Indian Sundarbans, zooplankton, estuarine ecology, Bay of Bengal, tropical cyclones, Oithona brevicornis, salinity, biodiversity, cyclone ecology