The research team, led by scientists at the Department of Geology of the University of Dhaka, collected 34 sediment samples along a land-sea transect spanning three environmentally contrasting areas: Cox’s Bazar on the eastern coast, Kuakata in the central deltaic zone, and the Sundarbans mangrove forest in the west. Sampling took place during winter, within the intertidal zone at elevations of roughly 0.5 to 4 meters above mean sea level, with cores extracted to a depth of 30 centimeters using a hand auger at low tide. Each sample was dried at 60 degrees Celsius for 24 hours, and 100-gram aliquots were sieved through meshes ranging from 63 to 1000 micrometers at one-phi intervals, following the Wentworth grain-size scale. Mean grain size and sorting were calculated using the Folk and Ward method.

The three study areas could hardly be more different. Cox’s Bazar, home to one of the longest natural beaches on Earth at 145 kilometers, is dominated by fine and medium sand, with fine sand reaching up to 75.1 percent of a sample and averaging 48.2 percent, while mud fractions are scarce. Kuakata displays a mixed picture: sediments at the Tin Nodir Mohona river mouth average 62.4 percent coarse sand, whereas the nearby Jhau Bon and Lebu Bon sites are rich in fine and very fine sand. The Sundarbans, the world’s largest mangrove forest covering roughly 6017 square kilometers, is a low-energy setting where fine and very fine sand dominate, averaging 59.5 and 27.2 percent respectively. These textural contrasts reflect fundamentally different hydrodynamic regimes, from wave-dominated open coast to tide-swept estuary to sheltered, freshwater-influenced mangrove plain.

When the researchers examined the foraminifera preserved in these sediments, the diversity patterns mapped almost perfectly onto the sediment map. Cox’s Bazar yielded 36 species across 26 genera, the highest diversity of the three areas, with Fisher’s alpha index ranging from 9.95 to 17.06 and Shannon values between 2.71 and 3.23. Kuakata produced 32 species from 20 genera with intermediate indices, while the Sundarbans assemblage contained only 19 species from 14 genera, the lowest diversity, with Fisher’s alpha falling as low as 4.89. The single most diverse site was Inani beach in Cox’s Bazar, and the least diverse was Kotka in the Sundarbans, a contrast that the authors link directly to substrate texture and the ecological conditions it implies.

The composition of the assemblages was equally revealing. The genera Ammonia and Elphidium dominated throughout the entire coastline, demonstrating their remarkable ecological tolerance, but each region carried a distinctive signature of companion taxa. Cox’s Bazar was characterized by Rotalinoides, Ammonia, Elphidium, and Asterorotalia, with mean relative abundances of 17.9, 17.4, 12.8, and 9.3 percent respectively. Kuakata was overwhelmingly dominated by Elphidium and Cribroelphidium at 33.8 and 20.7 percent, alongside several Ammonia species. In the Sundarbans, Ammonia reached individual-sample abundances of up to 55.6 percent, accompanied by Rosalina, Haplophragmoides, Entzia, Haynesina, and Trochammina, a suite of stress-tolerant and salt-marsh agglutinated forms typical of fine-grained, organically enriched, freshwater-influenced environments.

To test whether these patterns were statistically robust rather than coincidental, the team deployed a battery of complementary analyses. Spearman’s rank correlation across the combined 34-sample dataset showed that 18 of the 30 analyzed genera had at least one statistically significant association with a sediment fraction. Medium sand emerged as the fraction with the strongest biological signal: twelve genera, including Amphistegina, Spiroloculina, Cibicides, Rotalinoides, Neorotalia, and Operculina, showed significant positive correlations with it, while Ammonia, Elphidium, Rosalina, and Haplophragmoides were significantly and negatively correlated, reflecting their preference for finer substrates. The genus Gyroidella stood out as the only taxon significantly and positively correlated with coarse sand while being negatively correlated with both fine and very fine sand, marking it as a specialist of high-energy, coarse-grained settings.

Canonical correspondence analysis reinforced these findings in striking visual terms. The first two ordination axes together accounted for 90.9 percent of the constrained species-environment variance, with Cox’s Bazar samples aligning strongly with medium sands and Sundarbans samples clustering consistently with finer grains. Species vectors in the biplot closely tracked the sediment gradients, and the analysis independently reproduced the genus-substrate affinities identified by the correlation analysis. Finally, a two-way PERMANOVA based on Bray-Curtis dissimilarity and 9999 permutations confirmed that both location and sediment category significantly structured the assemblages, with p-values of 0.0001 and 0.0031 respectively. Crucially, the interaction term was not significant, meaning that sediment grain size exerts the same directional effect on community composition regardless of which of the three coastal environments is considered.

Why should grain size matter so much to organisms barely visible to the naked eye? The authors point to an indirect but well-supported mechanism: fine-grained sediments have greater surface area and adsorption capacity and are typically associated with higher organic matter concentrations, which in turn can deplete oxygen in the uppermost sediment layer. Although organic matter and dissolved oxygen were not directly measured in this study, a limitation the authors openly acknowledge, the dominance of opportunistic and stress-tolerant taxa such as Ammonia, Elphidium, and agglutinated forms in the Sundarbans is consistent with assemblages known from organically enriched coastal settings worldwide. The reduced diversity there likely reflects these stressed conditions, where a few tolerant species outcompete the rest.

The assemblages also encode hydrodynamic history. At Inani beach in Cox’s Bazar, the researchers found transported planktonic foraminifera, including Globigerina bulloides and Hantkenina alabamensis, species that live in the open water column and could only arrive on the beach through active wave and tidal transport. Their presence signals a well-connected, open-marine setting characteristic of the inner to middle neritic zone, rather than evidence of local productivity. In Kuakata, the combination of Elphidium, Cribroelphidium, Ammonia, and various miliolids, together with the complete absence of planktonic species, points to estuarine to very inner neritic conditions. In the Sundarbans, salt-marsh agglutinated foraminifera such as Trochammina inflata, Miliammina, Haplophragmoides, and Entzia were confined to areas under strong freshwater influence, echoing ecological patterns reported from karstic coasts of the Adriatic.

Beyond its scientific interest, the study carries practical weight for a coastline facing mounting natural and anthropogenic pressures, from sea-level rise to pollution and sediment starvation. By establishing sediment texture as a tractable, measurable, and directly comparable control on foraminiferal diversity, the work provides a framework for using these microfossils as bioindicators in environmental assessment. The authors recommend that future investigations incorporate direct measurements of organic matter, dissolved oxygen, salinity, and sediment mineralogy to refine the picture. For now, the message from the northern Bay of Bengal is clear: the grains on the seafloor, and the energy that arranges them, set the rules by which life at the smallest scale thrives or struggles, and reading those grains through the lens of foraminifera offers a powerful window into the health of coastal ecosystems under change.

Subject of Research: Relationship between sediment grain size and benthic foraminiferal diversity across contrasting coastal environments of Bangladesh

Article Title: Spatial patterns of benthic foraminiferal diversity in relation to sediment type across contrasting coastal environments of Bangladesh

Article References: Al Kabir, M. S., Saha, S. K., Tushar, M. A. N., & Maria, R. S. (2026). Spatial patterns of benthic foraminiferal diversity in relation to sediment type across contrasting coastal environments of Bangladesh. Discover Geoscience, 4(1), Article 344. https://doi.org/10.1007/s44288-026-00712-1

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00712-1

Keywords: benthic foraminifera, sediment grain size, Bay of Bengal, Bangladesh coast, Sundarbans, Cox's Bazar, Kuakata, PERMANOVA, canonical correspondence analysis, coastal biomonitoring, microfossils, Ganges-Brahmaputra-Meghna delta

The study was conducted by Thomamo B. Benjamin, Kelemogile Mmolawa, Gaolathe Tsheboeng, Marks Ditlhogo, and Francis O. Arimoro, who sampled three fixed stations along the river between February and May 2026. The stations were deliberately chosen to represent a gradient of urban influence: an upstream reach near the A1 Highway with relatively low-density development, a midstream reach cutting through the most densely built-up part of the city, and a downstream reach near the Tlokweng Road corridor that integrates the cumulative drainage of the entire urban catchment. Because the Segoditshane flows through residential, commercial, institutional, and industrial zones, it functions as the city’s principal stormwater conduit, collecting runoff, wastewater influence, and diffuse pollutants along its roughly 14-kilometer corridor through the metropolitan area.

The physicochemical results reveal a river under measurable strain. Dissolved oxygen fell from a mean of 7.59 milligrams per liter at the upstream station to roughly 4.8 to 5.0 milligrams per liter at the midstream and downstream sites, a decline of about 36 percent that the authors link to rising organic loading. Electrical conductivity climbed from around 734 to 742 microsiemens per centimeter upstream and midstream to 952 microsiemens per centimeter downstream, an increase of nearly 30 percent that reflects the accumulation of dissolved salts, nutrients, metals, and other urban-derived ions. Biochemical oxygen demand rose from 3.28 milligrams per liter upstream to 8.75 midstream, chemical oxygen demand more than doubled from 18.5 to 45 milligrams per liter, and total dissolved solids increased from 455 to 590 milligrams per liter downstream. Nitrate concentrations also rose steadily along the continuum, while phosphate peaked at the midstream station.

These chemical shifts tracked a semi-quantitative urban pressure assessment developed by the team. The upstream site scored 3 on a ten-point pressure scale, with roughly 25 percent impervious surface cover, a surrounding population density of about 1,500 people per square kilometer, and only one or two major stormwater outfalls. The midstream site, embedded in the densest urban fabric, scored 8, with approximately 70 percent impervious cover, 4,500 people per square kilometer, and five to seven outfalls. The downstream site scored highest at 9, combining about 60 percent impervious cover, seven to ten stormwater outfalls, high wastewater influence, and full integration of upstream drainage. The close agreement between these pressure scores and the measured water chemistry suggests that catchment development and drainage connectivity are central to the environmental degradation documented in the river.

Against this backdrop, the biological story is striking. Over four monthly sampling campaigns, the team collected 1,238 macroinvertebrate individuals representing 16 families from five insect orders, identified to family level using South African Scoring System Version 5 taxonomic keys. Abundance collapsed along the urban gradient, from 718 individuals upstream to 436 midstream and just 84 downstream. The upstream station was dominated by pollution-tolerant Diptera, particularly non-biting midges of the family Chironomidae with 250 individuals and mosquitoes of the family Culicidae with 156, alongside moderate numbers of predatory water bugs and damselflies. The midstream station supported a more balanced assemblage including mayflies of the family Baetidae, dragonflies and damselflies, and several families found nowhere else in the study, such as Gomphidae, Micronectidae, Scirtidae, and Gyrinidae. The downstream reach held only six families, with Baetidae, Chironomidae, Libellulidae, and a handful of others accounting for nearly all individuals.

Diversity metrics added an important nuance to this pattern. Although the upstream station recorded the highest raw abundance, its Shannon diversity of 1.844 was the lowest of the three sites, a consequence of dominance by a few opportunistic taxa. The midstream station, by contrast, posted the highest values for nearly every metric: 11 families, a Margalef richness index of 1.645, Shannon diversity of 2.265, Simpson diversity of 0.886, and Pielou’s evenness of 0.945. This midstream diversity maximum is one of the study’s most notable findings, suggesting a non-linear response along the river continuum. Rather than a simple slide from good to bad conditions, the pattern points to intermediate disturbance dynamics, in which moderate environmental stress and greater habitat heterogeneity at the midstream reach widen the range of niches available, allowing sensitive and tolerant taxa to coexist while competitive exclusion is dampened.

A modified application of the South African Scoring System reinforced this interpretation. The midstream station achieved a SASS score of 58, twelve scoring taxa, and an average score per taxon of 4.83, corresponding to a condition of Good under the SASS5 framework. The upstream station scored 35 with an ASPT of 3.89, rated Fair, while the downstream station fell to a score of 23 and an ASPT of 3.83, rated Fair to Poor. The authors caution that because their pooled multi-habitat sampling differed from the standardized SASS5 kick-sampling protocol, these indices should be read as comparative indicators of relative condition rather than formal ecological status classifications. Even so, the convergence of abundance, richness, diversity, and biotic index data on the same spatial pattern lends weight to the overall conclusion.

To connect community structure with environmental conditions, the researchers employed Canonical Correspondence Analysis, a multivariate ordination technique that relates species distributions to measured gradients. Based on twelve station-by-month observations, the analysis showed that the first two canonical axes together explained 97.82 percent of the constrained family-environment variation, with Axis 1 accounting for 65.72 percent and Axis 2 for 32.10 percent, both statistically significant under Monte Carlo permutation tests. Axis 1 loaded most strongly on ammonia, pH, dissolved oxygen, depth, and phosphate, while Axis 2 was associated with electrical conductivity, flow velocity, depth, nitrate, and phosphate. Because several environmental variables were strongly correlated with one another, the authors interpret the ordination as an exploratory description of interacting gradients rather than a test of independent drivers. Upstream assemblages aligned with higher pH and oxygen, midstream communities with flow, depth, and nutrients, and downstream assemblages with elevated conductivity and nitrate.

The findings fit squarely within the theory of environmental filtering, which holds that local abiotic conditions act as a sieve, favoring species whose traits allow survival under prevailing conditions while excluding those that cannot cope. In the Segoditshane River, the filters include oxygen depletion, nutrient enrichment, rising salinity indicated by conductivity, altered flow and depth, and habitat simplification driven by impervious surfaces and channel modification. Tolerant families such as Chironomidae and Culicidae thrived where organic enrichment and standing water created favorable breeding conditions, while moderately sensitive predators like Gomphidae and Aeshnidae were largely confined to the midstream reach, and the relatively sensitive Baetidae and Simuliidae persisted mainly where localized hydraulic refugia offered patches of suitable habitat. Importantly, the authors warn that the higher proportional contribution of sensitive taxa downstream should not be mistaken for recovery; it more likely reflects the loss of other groups from an already depauperate community.

The study carries practical weight for a rapidly urbanizing semi-arid region where conventional chemical monitoring alone can underestimate cumulative ecological degradation. Because macroinvertebrates integrate environmental stress over weeks and months, they provide a biological memory that instantaneous water samples lack. The authors argue that their results establish a baseline for biomonitoring in the Segoditshane River and support integrated catchment management spanning water-quality protection, stormwater management, habitat restoration, and long-term ecological monitoring. They also outline future priorities, including year-round monitoring across the full hydrological cycle, greater spatial replication, habitat-specific sampling, trait-based and species-level approaches, and the incorporation of environmental DNA metabarcoding to sharpen taxonomic resolution. As Gaborone continues to grow, the Segoditshane’s insect communities may prove to be among the most honest witnesses to the city’s environmental trajectory.

Subject of Research: Urban pollution signatures and environmental filtering of benthic macroinvertebrates along abiotic gradients in a semi-arid river in Botswana

Article Title: Urban pollution signatures and environmental filtering of benthic macroinvertebrates along abiotic gradients in a semi-arid river in botswana

Article References: Benjamin, T. B., Mmolawa, K., Tsheboeng, G., Ditlhogo, M., & Arimoro, F. O. (2026). Urban pollution signatures and environmental filtering of benthic macroinvertebrates along abiotic gradients in a semi-arid river in botswana. Environmental Challenges, 25, Article 101661. https://doi.org/10.1016/j.envc.2026.101661

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101661

Keywords: benthic macroinvertebrates, environmental filtering, urban pollution, semi-arid river, water quality, biomonitoring, SASS5, dissolved oxygen, electrical conductivity, Botswana, Gaborone, canonical correspondence analysis