Response of coastal phytoplankton to pollution from various sources in the coastal Bay of Bengal.

The coastal ocean receives nutrient pollutants from various sources, such as aerosols, municipal sewage, industrial effluents and groundwater discharge, with variable concentrations and stoichiometric ratios. The objective of this study is to examine the response of phytoplankton to these pollutants in the coastal water under silicate-rich and silicate-poor coastal waters. In order to achieve this, a microcosm experiment was conducted by adding the pollutants from various sources to the coastal waters during November and January, when the water column physicochemical characteristics are different. Low salinity and high silicate concentration were observed during November due to the influence of river discharge contrasting to that observed during January. Among the various sources of pollutants used, aerosols and industrial effluents did not contribute silicate whereas groundwater and municipal sewage contained high concentrations of silicate along with nitrate and phosphate during both the study periods. During November, an increase in phytoplankton biomass was noticed in all pollutant-added samples, except municipal sewage, due to the limitation of growth by nitrate. On the other hand, an increase in biomass and abundance of phytoplankton was observed in all pollutant-added samples, except for aerosol, during January. Increase in phytoplankton abundance associated with decrease in biomass was observed in aerosol-added sample due to co-limitation of silicate and phosphate during January. A significant response of Thalassiothrix sp. was observed for industrial effluent-added sample during November, whereas Chaetoceros sp. and Skeletonema sp. increased significantly during January. Higher increase in phytoplankton biomass was observed during November associated with higher availability of silicate in the coastal waters in January. Interestingly, an increase in the contribution of dinoflagellates was observed during January associated with low silicate in the coastal waters, suggesting that the concentration of silicate in the coastal waters determines the response of the phytoplankton group to pollutant inputs. This study suggested that silicate concentration in the coastal waters must be considered, in addition to the coastal currents, while computing dilution factors for the release of pollutants to the coastal ocean to avoid occurrence of unwanted phytoplankton blooms.

Microeukaryotic plankton evolutionary constraints in a subtropical river explained by environment and bacteria along differing taxonomic resolutions.

Microeukaryotic plankton communities are keystone components for keeping aquatic primary productivity. Currently, variations in microeukaryotic plankton diversity have often been explained by local ecological factors but not by evolutionary constraints. We used amplicon sequencing of 100 water samples across five years to investigate the ecological preferences of the microeukaryotic plankton community in a subtropical riverine ecosystem. We found that microeukaryotic plankton diversity was less associated with bacterial abundance (16S rRNA gene copy number) than bacterial diversity. Further, environmental effects exhibited a larger influence on microeukaryotic plankton community composition than bacterial community composition, especially at fine taxonomic levels. The evolutionary constraints of microeukaryotic plankton community increased with decreasing taxonomic resolution (from 97% to 91% similarity levels), but not significant change from 85% to 70% similarity levels. However, compared with the bacterial community, the evolutionary constraints were shown to be more affected by environmental variables. This study illustrated possible controlling environmental and bacterial drivers of microeukaryotic diversity and community assembly in a subtropical river, thereby indirectly reflecting on the quality status of the water environment by providing new clues on the microeukaryotic community assembly.

Haplosporidium nelsoni and Perkinsus marinus occurrence in waters of Great Bay Estuary, New Hampshire.

In Great Bay Estuary, New Hampshire, USA, Haplosporidium nelsoni and Perkinsus marinus are 2 active pathogens of the eastern oyster Crassostrea virginica (Gmelin), that cause MSX (multinucleated sphere with unknown affinity 'X') and dermo mortalities, respectively. Whereas studies have quantified infection intensities in oyster populations and determined whether these parasites exist in certain planktonic organisms, no studies thus far have examined both infectious agents simultaneously in water associated with areas that do and do not have oyster populations. As in other estuaries, both organisms are present in estuarine waters throughout the Bay, especially during June through November, when oysters are most active. Waters associated with oyster habitats had higher, more variable DNA concentrations from these pathogenic organisms than waters at a non-oyster site. This finding allows for enhanced understanding of disease-causing organisms in New England estuaries, where oyster restoration is a priority.

Data-driven determination of zooplankton bioregions and robustness analysis.

Identifying biogeographic regions through cluster analysis of species distribution data is a common method for partitioning ecosystems. Selecting the appropriate cluster analysis method requires a comparison of multiple algorithms. In this study, we demonstrate a data-driven process to select a method for bioregionalization based on community data and test its robustness to data variability following these steps: •We aggregated and curated zooplankton community observations from expeditions in the Northeast Pacific.•We determined the best bioregionalization approach by comparing nine cluster analysis methods using ten goodness of clustering indices.•We evaluated the robustness of the bioregionalization to different sources of sampling and taxonomic variability by comparing the bioregionalization of the overall dataset with bioregionalizations of subsets of the data. The K-means clustering of the log-chord transformed abundance was selected as the optimal method for bioregionalization of the zooplankton dataset. This clustering resulted in the emergence of four bioregions along the cross-shelf gradient: the Offshore, Deep Shelf, Nearshore, and Deep Fjord bioregions. The robustness analyses demonstrated that the bioregionalization was consistent despite variability in the spatial and temporal frequency of sampling, sampling methodology, and taxonomic coverage.

Physiological and morphological plasticity in response to nitrogen availability of a yeast widely distributed in the open ocean.

Yeasts are prevalent in the open ocean, yet we have limited understanding of their ecophysiological adaptations, including their response to nitrogen availability, which can have a major role in determining the ecological potential of other planktonic microbes. In this study, we characterised the nitrogen uptake capabilities and growth responses of marine-occurring yeasts. Yeast isolates from the North Atlantic Ocean were screened for growth on diverse nitrogen substrates, and across a concentration gradient of three environmentally relevant nitrogen substrates: nitrate, ammonium, and urea. Three strains grew with enriched nitrate while two did not, demonstrating that nitrate utilisation is present but not universal in marine yeasts, consistent with existing knowledge of non-marine yeast strains. Naganishia diffluens MBA_F0213 modified the key functional trait of cell size in response to nitrogen concentration, suggesting yeast cell morphology changes along chemical gradients in the marine environment. Meta-analysis of the reference DNA barcode in public databases revealed that the genus Naganishia has a global ocean distribution, strengthening the environmental applicability of the culture-based observations. This study provides novel quantitative understanding of the ecophysiological and morphological responses of marine-derived yeasts to variable nitrogen availability in vitro, providing insight into the functional ecology of yeasts within pelagic open ocean environments.

Monitoring Bioindication of Plankton through the Analysis of the Fourier Spectra of the Underwater Digital Holographic Sensor Data.

The study presents a bioindication complex and a technology of the experiment based on a submersible digital holographic camera with advanced monitoring capabilities for the study of plankton and its behavioral characteristics in situ. Additional mechanical and software options expand the capabilities of the digital holographic camera, thus making it possible to adapt the depth of the holographing scene to the parameters of the plankton habitat, perform automatic registration of the "zero" frame and automatic calibration, and carry out natural experiments with plankton photostimulation. The paper considers the results of a long-term digital holographic experiment on the biotesting of the water area in Arctic latitudes. It shows additional possibilities arising during the spectral processing of long time series of plankton parameters obtained during monitoring measurements by a submersible digital holographic camera. In particular, information on the rhythmic components of the ecosystem and behavioral characteristics of plankton, which can be used as a marker of the ecosystem well-being disturbance, is thus obtained.

Do weathered microplastics impact the planktonic community? A mesocosm approach in the Baltic Sea.

Microplastics (MPs) are ubiquitous pollutants of increasing concern in aquatic systems. However, little is still known about the impacts of weathered MPs on plankton at the community level after long-term exposure. In this study, we investigated the effects of weathered MPs on the structure and dynamics of a Baltic Sea planktonic community during ca. 5 weeks of exposure using a mesocosm approach (2 m3) mimicking natural conditions. MPs were obtained from micronized commercial materials of polyvinyl chloride, polypropylene, polystyrene, and polyamide (nylon) previously weathered by thermal ageing and sunlight exposure. The planktonic community was exposed to 2 µg L-1 and 2 mg L-1 of MPs corresponding to measured particle concentrations (10-120 µm) of 680 MPs L-1 and 680 MPs mL-1, respectively. The abundance and composition of all size classes and groups of plankton and chlorophyll concentrations were periodically analyzed throughout the experiment. The population dynamics of the studied groups showed some variations between treatments, with negative and positive effects of MPs exhibited depending on the group and exposure time. The abundance of heterotrophic bacteria, pico- and nanophytoplankton, cryptophytes, and ciliates was lower in the treatment with the higher MP concentration than in the control at the last weeks of the exposure. The chlorophyll concentration and the abundances of heterotrophic nanoflagellates, Astromoeba, dinoflagellate, diatom, and metazooplankton were not negatively affected by the exposure to MPs and, in some cases, some groups showed even higher abundances in the MP treatments. Despite these tendencies, statistical analyses indicate that in most cases there were no statistically significant differences between treatments over the exposure period, even at very high exposure concentrations. Our results show that weathered MPs of the studied conventional plastic materials have minimal or negligible impact on planktonic communities after long-term exposure to environmentally relevant concentrations.

Diversity and distribution analysis of eukaryotic communities in the Xiangshan Bay, East China sea by metabarcoding approach.

Eukaryotic communities play an important role in the coastal ecosystem of Xiangshan Bay, a narrow semi-closed bay famous for fisheries and marine farming. However, information on the diversity and composition of eukaryotic communities in Xiangshan Bay remains unclear. In this study, the metabarcoding approach was utilized to comprehensively investigate the eukaryotic plankton community structure and dominant taxa, particularly eukaryotic microalgae, in the Xiangshan Bay over a period of four months in 2018. The results showed that the three major phyla were Arthropoda, Chlorophyta, and Bacillariophyta. The richness indices revealed that species richness peaked in February and was at its lowest in May. Diversity indices showed that the samples collected in May had the lowest diversity. Centropages was detected in the samples of all months, however, its highest dominance was observed in the samples collected in February. In addition, compared to other months, a greater proportion of eukaryotic microalgae was witnessed in March. The three eukaryotic algae with highest abundances in March were Cyclotella, Prorocentrum, and Thalassiosira. Moreover, high diversity of pico-sized (0.2-2.0 µm) phytoplankton (which are often easily missed by microscopy) was discovered in this study by using metabarcoding approach. This study highlights the strength and significance of the metabarcoding approach to uncover a large number of eukaryotic species which remains undetectable during application of conventional approaches. The findings of this study reveals that the eukaryotic community structure varies noticeably in both time and space throughout sampling period, with temperature being the most important environmental factor influencing these changes. This study lays a solid foundation to understand eukaryotic plankton composition, temporal and spatial dynamics and the distribution mechanism of eukaryotic plankton community in Xiangshan Bay, providing theoretical reference for further studies related to marine ecology.

Trace and rare earth elements in phytoplankton from the Tyrrhenian Sea (Italy).

Plankton plays a very crucial role in bioaccumulation and transfer of metals in the marine food web and represents a suitable bioindicator of the occurrence of trace and rare earth elements in the ecosystem. Trace elements and REEs were analyzed by ICP-MS in phytoplankton samples from the northwestern Mediterranean Sea. Metal concentrations in phytoplankton were found strongly influenced by seasons and depth of collection (- 30 m, - 50 m). Principal component analysis (PCA) has shown that Al, As, Cr, Cu, Ga, and Sn concentrations were related to summer and autumn in samples collected at 30 m depth, while Fe, Mn, Ni, V, and Zn levels related strongly with summer and spring at 50 m depth. Fe, Al, and Zn were the most represented elements in all samples (mean values respectively in the ranges 4.2-8.2, 9.6-13, and 1.0-4.4 mg kg-1) according to their widespread presence in the environment and in the earth crust. Principal component analysis (PCA) performed on REEs showed that mostly all lanthanides' concentrations strongly correlate with summer and autumn seasons (- 30 m depth); the highest ∑REE concentration (75 µg kg-1) was found in winter. Phytoplankton REE normalized profile was comparable to those of other marine biota collected in the same area according to the suitability of lanthanides as geological tracers.

Changes in coastal ecosystems affected by overburden dumping from amber open-cut mining on the Sambia Peninsula (Baltic Sea).

The Sambia Peninsula (Kaliningrad region) is historically well known for its amber mining. The 2019 year was the last year of direct overburden disposal into the Baltic Sea as a part of technological amber mining process. The extremely high-suspended particulate matter concentrations during that disposal were recorded immediately after the discharge of significant volumes of pulp and reached 200 mg/L. The impact of pulp discharge had sequentially suppressed plankton communities development due to the high content of suspended solids and afterwards stimulated plankton development due to the glauconite infusion. Cladocera were the most sensitive group to the effects of suspended matter. According to the preliminary forecast, when the pulp discharge stops, the restoration of plankton communities may take from 1 to 2 seasons to 1 year for different groups. This is due to the timing of the removal of fine suspended particulate matter from sediments and the possibility of secondary entry during resuspension.

Community stability of free-living and particle-attached bacteria in a subtropical reservoir with salinity fluctuations over 3 years.

Changes in salinity have a profound influence on ecological services and functions of inland freshwater ecosystems, as well as on the shaping of microbial communities. Bacterioplankton, generally classified into free-living (FL) and particle-attached (PA) forms, are main components of freshwater ecosystems and play key functional roles for biogeochemical cycling and ecological stability. However, there is limited knowledge about the responses of community stability of both FL and PA bacteria to salinity fluctuations. Here, we systematically explored changes in community stability of both forms of bacteria based on high-frequency sampling in a shallow urban reservoir (Xinglinwan Reservoir) in subtropical China for 3 years. Our results indicated that (1) salinity was the strongest environmental factor determining FL and PA bacterial community compositions - rising salinity increased the compositional stability of both bacterial communities but decreased their α-diversity. (2) The community stability of PA bacteria was significantly higher than that of FL at high salinity level with low salinity variance scenarios, while the opposite was found for FL bacteria, i.e., their stability was higher than PA bacteria at low salinity level with high variance scenarios. (3) Both bacterial traits (e.g., bacterial genome size and interaction strength of rare taxa) and precipitation-induced factors (e.g., changes in salinity and particle) likely contributed collectively to differences in community stability of FL and PA bacteria under different salinity scenarios. Our study provides additional scientific basis for ecological management, protection and restoration of urban reservoirs under changing climatic and environmental conditions.

Metabolic trade-offs constrain the cell size ratio in a nitrogen-fixing symbiosis.

Biological dinitrogen (N2) fixation is a key metabolic process exclusively performed by prokaryotes, some of which are symbiotic with eukaryotes. Species of the marine haptophyte algae Braarudosphaera bigelowii harbor the N2-fixing endosymbiotic cyanobacteria UCYN-A, which might be evolving organelle-like characteristics. We found that the size ratio between UCYN-A and their hosts is strikingly conserved across sublineages/species, which is consistent with the size relationships of organelles in this symbiosis and other species. Metabolic modeling showed that this size relationship maximizes the coordinated growth rate based on trade-offs between resource acquisition and exchange. Our findings show that the size relationships of N2-fixing endosymbionts and organelles in unicellular eukaryotes are constrained by predictable metabolic underpinnings and that UCYN-A is, in many regards, functioning like a hypothetical N2-fixing organelle (or nitroplast).

Importance of integrating mixoplankton into marine ecosystem policy and management-Examples from the Marine Strategy Framework Directive.

Marine plankton capable of photosynthesis and predation ("mixoplankton") comprise up to 50% of protist plankton and include many harmful species. However, marine environmental management policies, including the European Union Marine Strategy Framework Directive (MSFD) and the USEPA, assume a strict dichotomy between autotrophic phytoplankton and heterotrophic zooplankton. Mixoplankton often differ significantly from these two categories in their response to environmental pressures and affect the marine environment in ways we are only beginning to understand. While the management policies may conceptually provide scope for incorporating mixoplankton, such action is rarely implemented. We suggest that the effectiveness of monitoring and management programs could benefit from explicit implementations regarding the ecological roles and impact of mixoplankton. Taking the MSFD as an example of marine management guidelines, we propose appropriate methods to explicitly include mixoplankton in monitoring and marine management. Integr Environ Assess Manag 2024;00:1-18. © 2024 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Habitat shift of adult Caligus undulatus (Copepoda: Siphonostomatoida: Caligidae) from host fish to plankton in response to host behavior.

Before 2019, adults of the sea louse Caligus undulatus were reported exclusively in plankton from ocean samples worldwide and were not known to parasitize fish hosts. In 2019, the first instance of this caligid parasitizing a fish host, Japanese sardinella Sardinella zunasi, was reported in the Seto Inland Sea, Japan. The presently reported study aimed to investigate the biology and ecology of adult C. undulatus in plankton communities in the Seto Inland Sea and surrounding waters from March 2020 to November 2021. The occurrence of sea lice in plankton communities was restricted to the period of August-January, mainly between October and December with maximum plankton abundance (10.5 ind. per 1000 m3) recorded on 30 November 2020. All post-naupliar stages of C. undulatus were found on the host fish, and they represented a typical life cycle pattern known for Caligus species. The sex ratios in both planktonic and parasitic adults were not significantly different. The frequency of occurrence of planktonic and parasitic adult females with egg strings was 68 and 46%, respectively. The number of eggs per string was significantly higher in parasitic adult females (mean ± SD: 16.9 ± 8.6) than in planktonic females (10.4 ± 10.8). These data suggest that adult females were detached from their hosts and continued to produce eggs without feeding. Seasonal migration of S. zunasi to brackish water for spawning may result in the detachment of mature caligids from the host and may be effective in protecting the offspring, which are less tolerant of less brackish water.

Environmental factors controlling seasonal and spatial variability of zooplankton in thermokarst lakes along a permafrost gradient of Western Siberia.

Humic thermokarst lakes of permafrost peatlands in Western Siberia Lowland (WSL) are major environmental controllers of carbon and nutrient storage in inland waters and greenhouse gases emissions to the atmosphere in the subarctic. In contrast to sizable former research devoted to hydrochemical and hydrobiological (phytoplankton) composition, zooplankton communities of these thermokarst lakes and thaw ponds remain poorly understood, especially along the latitudinal gradient, which is a perfect predictor of permafrost zones. To fill this gap, 69 thermokarst lakes of the WSL were sampled using unprecedented spatial coverage, from continuous to sporadic permafrost zone, in order to assess zooplankton (Cladocera, Copepoda, Rotifera) diversity and abundance across three main open water physiological seasons (spring, summer and autumn). We aimed at assessing the relationship of environmental factors (water column hydrochemistry, nutrients, and phytoplankton parameters) with the abundance and diversity of zooplankton. A total of 74 zooplankton species and taxa were detected, with an average eight taxa per lake/pond. Species richness increased towards the north and reached the maximum in the continuous permafrost zone with 13 species found in this zone only. In contrast, the number of species per waterbody decreased towards the north, which was mainly associated with a decrease in the number of cladocerans. Abundance and diversity of specific zooplankton groups strongly varied across the seasons and permafrost zones. Among the main environmental controllers, Redundancy Analysis revealed that water temperature, lake area, depth, pH, Dissolved Inorganic and Organic Carbon and CO2 concentrations were closely related to zooplankton abundance. Cladocerans were positively related to water temperature during all seasons. Copepods were positively related to depth and lake water pH in all seasons. Rotifers were related to different factors in each season, but were most strongly associated with DOC, depth, CH4, phytoplankton and cladoceran abundance. Under climate warming scenario, considering water temperature increase and permafrost boundary shift northward, one can expect an increase in the diversity and abundance of cladocerans towards the north which can lead to partial disappearance of copepods, especially rare calanoid species.

Chaotic turnover of rare and abundant species in a strongly interacting model community.

The composition of ecological communities varies not only between different locations but also in time. Understanding the fundamental processes that drive species toward rarity or abundance is crucial to assessing ecosystem resilience and adaptation to changing environmental conditions. In plankton communities in particular, large temporal fluctuations in species abundances have been associated with chaotic dynamics. On the other hand, microbial diversity is overwhelmingly sustained by a "rare biosphere" of species with very low abundances. We consider here the possibility that interactions within a species-rich community can relate both phenomena. We use a Lotka-Volterra model with weak immigration and strong, disordered, and mostly competitive interactions between hundreds of species to bridge single-species temporal fluctuations and abundance distribution patterns. We highlight a generic chaotic regime where a few species at a time achieve dominance but are continuously overturned by the invasion of formerly rare species. We derive a focal-species model that captures the intermittent boom-and-bust dynamics that every species undergoes. Although species cannot be treated as effectively uncorrelated in their abundances, the community's effect on a focal species can nonetheless be described by a time-correlated noise characterized by a few effective parameters that can be estimated from time series. The model predicts a nonunitary exponent of the power-law abundance decay, which varies weakly with ecological parameters, consistent with observation in marine protist communities. The chaotic turnover regime is thus poised to capture relevant ecological features of species-rich microbial communities.

Sea surface microplastics in the Galapagos: Grab samples reveal high concentrations of particles <200 µm in size.

Plastic pollution in the oceans is increasing, yet most global sea surface data is collected using plankton nets which limits our knowledge of the smaller and more bioaccessible size fraction of microplastics (<5 mm). We sampled the biodiverse coastal waters of the Galapagos Island of San Cristobal, comparing two different microplastic sampling methodologies; 1 l whole seawater grab samples filtered to 1.2 µm and sea surface plankton tows with a net mesh size of 200 µm. Our data reveal high concentrations of microplastics in Galapagos coastal waters surrounding the urban area, averaging 11.5 ± 1.48 particles l-1, with a four-order of magnitude increase in microplastic abundance observed using grab sampling compared with 200 µm plankton nets. This increase was greater when including anthropogenic cellulose particles, averaging 19.8 ± 1.86 particles l-1. Microplastic and anthropogenic cellulose particles smaller than 200 µm comprised 44 % of the particles from grab samples, suggesting previous estimates of microplastic pollution based on plankton nets likely miss and therefore underestimate these smaller particles. The particle characteristics and distribution of these smaller particles points strongly to a local input of cellulosic fibres in addition to the microplastic particles transported longer distances via the Humbolt current found across the surface seawater of the Galapagos. Improving our understanding of particle characteristics and distributions to highlight likely local sources will facilitate the development of local mitigation and management plans to reduce the input and impacts of microplastics to marine species, not just in the Galapagos but globally.

Enhancing water quality and ecosystems of reclaimed water-replenished river: A case study of Dongsha River, Beijing, China.

The use of reclaimed water for urban river replenishment has raised concerns regarding its impact on water quality and aquatic ecosystems. This study aims to reveal the improvements seen in an urban river undergoing a practical water eco-remediation after being replenished with reclaimed water. A one-year monitoring of water quality, phytoplankton, and zooplankton was carried out in Dongsha River undergoing eco-remediation in Beijing, China. The results showed that compared to the unrestored river, the concentrations of COD, NH4+-N, TP, and TN decreased by 28.22 ± 7.88 %, 40.24 ± 11.77 %, 44.17 ± 17.29 %, and 28.66 ± 10.39 % in the restoration project area, respectively. The concentration of Chlorophyll-a in the restoration area was maintained below 40 µg/L. During summer, when algal growth is vigorous, the density of Cyanophyta in the unrestored river decreased from 46.84 × 104cells/L to 16.32 × 104cells/L in the restored area, while that of Chlorophyta decreased from 41.61 × 104cells/L to 11.87 × 104cells/L, a reduction of 65.16 % and 71.47 %, respectively. The dominant phytoplankton species were replaced with Bacillariophyta, such as Synedra sp. and Nitzschia sp., indicating that the restoration of aquatic plants reduces the risk of Cyanophyta blooms. Zooplankton species also changed in the restoration area, especially during summer. The density of pollution-tolerant Rotifer and Protozoa decreased by 31.06 % and 27.22 %, while the density of clean water indicating Cladocera increased by 101.19 %. We further calculated the diversity and evenness index of phytoplankton and zooplankton within and outside the restoration area. The results showed that the Shannon-Weaver index for phytoplankton and zooplankton in the restoration area was 2.1 and 1.91, which was higher than those in the river (1.84 and 1.82). This further confirmed that aquatic plant restoration has positive effects. This study can provide a practical reference and theoretical basis for the implementation of water ecological restoration projects in other reclaimed water rivers in China.

Spatiotemporal assembly and functional composition of planktonic microeukaryotic communities along productivity gradients in a subtropical lake.

Microeukaryotes play crucial roles in the microbial loop of freshwater ecosystems, functioning both as primary producers and bacterivorous consumers. However, understanding the assembly of microeukaryotic communities and their functional composition in freshwater lake ecosystems across diverse environmental gradients remains limited. Here, we utilized amplicon sequencing of 18S rRNA gene and multivariate statistical analyses to examine the spatiotemporal and biogeographical patterns of microeukaryotes in water columns (at depths of 0.5, 5, and 10 m) within a subtropical lake in eastern China, covering a 40 km distance during spring and autumn of 2022. Our results revealed that complex and diverse microeukaryotic communities were dominated by Chlorophyta (mainly Chlorophyceae), Fungi, Alveolata, Stramenopiles, and Cryptophyta lineages. Species richness was higher in autumn than in spring, forming significant hump-shaped relationships with chlorophyll a concentration (Chl-a, an indicator of phytoplankton biomass). Microeukaryotic communities exhibited significant seasonality and distance-decay patterns. By contrast, the effect of vertical depth was negligible. Stochastic processes mainly influenced the assembly of microeukaryotic communities, explaining 63, 67, and 55% of community variation for spring, autumn, and both seasons combined, respectively. Trait-based functional analysis revealed the prevalence of heterotrophic and phototrophic microeukaryotic plankton with a trade-off along N:P ratio, Chl-a, and dissolved oxygen (DO) gradients. Similarly, the mixotrophic proportions were significantly and positively correlated with Chl-a and DO concentrations. Overall, our findings may provide useful insights into the assembly patterns of microeukaryotes in lake ecosystem and how their functions respond to environmental changes.