Seasonal metabolic dynamics of microeukaryotic plankton: a year-long metatranscriptomic study in a temperate sea.

Seasonal fluctuations profoundly affect marine microeukaryotic plankton composition and metabolism, but accurately tracking these changes has been a long-standing challenge. In this study, we present a year-long metatranscriptomic data set from the Southern Bight of the North Sea, shedding light on the seasonal dynamics in temperate plankton ecosystems. We observe distinct shifts in active plankton species and their metabolic processes in response to seasonal changes. We characterized the metabolic signatures of different seasonal phases in detail, thereby revealing the metabolic versatility of dinoflagellates, the heterotrophic dietary strategy of Phaeocystis during its late-stage blooms, and stark variations in summer and fall diatom abundance and metabolic activity across nearby sampling stations. Our data illuminate the varied contributions of microeukaryotic taxa to biomass production and nutrient cycling at different times of the year and allow delineation of their ecological niches. IMPORTANCE: Ecosystem composition and metabolic functions of temperate marine microeukaryote plankton are strongly influenced by seasonal dynamics. Although monitoring of species composition of microeukaryotes has expanded recently, few methods also contain seasonally resolved information on ecosystem functioning. We generated a year-long spatially resolved metatranscriptomic data set to assess seasonal dynamics of microeukaryote species and their associated metabolic functions in the Southern Bight of the North Sea. Our study underscores the potential of metatranscriptomics as a powerful tool for advancing our understanding of marine ecosystem functionality and resilience in response to environmental changes, emphasizing its potential in continuous marine ecosystem monitoring to enhance our ecological understanding of the ocean's eukaryotic microbiome.

A model of time-dependent macromolecular and elemental composition of phytoplankton.

Phytoplankton Chl:C:N:P ratios are important from both an ecological and a biogeochemical perspective. We show that these elemental ratios can be represented by a phytoplankton physiological model of low complexity that includes major cellular macromolecular pools. In particular, our model resolves time-dependent intracellular pools of chlorophyll, proteins, nucleic acids, carbohydrates/lipids, and N and P storage. Batch culture data for two diatom and two prasinophyte species are used to constrain parameters that represent specific allocation traits and strategies. A key novelty is the simultaneous estimation of physiological parameters for two phytoplankton groups of such different sizes. The number of free parameters is reduced by assuming (i) allometric scaling for maximum uptake rates, (ii) shared half-saturation constants for synthesis of functional macromolecules, (iii) shared exudation rates of functional macromolecules across the species. The rationale behind this assumption is that across the different species, the same or similar processes, enzymes, and metabolites play a role in key physiological processes. For the turnover numbers of macromolecular synthesis and storage exudation rates, differences between diatoms and prasinophytes need to be taken into account to obtain a good fit. Our model fits suggest that the parameters related to storage dynamics dominate the differences in the C:N:P ratios between the different phytoplankton groups. Since descriptions of storage dynamics are still incomplete and imprecise, predictions of C:N:P ratios by phytoplankton models likely have a large uncertainty.

Evaluating a river's ecological health: A multidimensional approach.

Evaluating the health of river surface water is essential, as rivers support significant biological resources and serve as vital drinking water sources. While the Water Quality Index (WQI) is commonly employed to evaluate surface water quality, it fails to consider biodiversity and does not fully capture the ecological health of rivers. Here we show a comprehensive assessment of the ecological health of surface water in the lower Yangtze River (LYR), integrating chemical and biological metrics. According to traditional WQI metrics, the LYR's surface water generally meets China's Class II standards. However, it also contains 43 high-risk emerging contaminants; nitrobenzenes are found at the highest concentrations, representing 25-90% of total detections, while polycyclic aromatic hydrocarbons present the most substantial environmental risks, accounting for 81-93% of the total risk quotient. Notably, the plankton-based index of biological integrity (P-IBI) rates the ecological health of the majority of LYR water samples (59.7%) as 'fair', with significantly better health observed in autumn compared to other seasons (p < 0.01). Our findings suggest that including emerging contaminants and P-IBI as additional metrics can enhance the traditional WQI analysis in evaluating surface water's ecological health. These results highlight the need for a multidimensional assessment approach and call for improvements to LYR's ecological health, focusing on emerging contaminants and biodiversity rather than solely on reducing conventional indicators.

Seasonality regulates the distinct assembly patterns of microeukaryotic plankton communities in the Three Gorges Reservoir, China.

The hydrographic and environmental factors along the Three Gorges Reservoir (TGR) have been significantly altered since the Three Gorges Dam (TGD) began working in 2006. Here, we collected 54 water samples, and then measured the environmental factors, followed by sequencing of the 18S rRNA gene and subsequent analysis of community assembly mechanisms. The findings indicated that the majority of environmental variables (such as AN, TP, Chl-a, CODMn, and Cu) exhibited both temporal and spatial variations due to the influences of the TGD. The distribution of different environmental factors and microeukaryotic plankton communities is influenced by the changing seasons. The community structure in TGR showed variations across three seasons, possibly due to variations in their environmental preferences, inherent dissimilarities, and seasonal succession. Furthermore, different communities exhibited a comparable distance-decay trend, suggesting that distinct taxa are likely to exhibit a similar spatial distribution. In addition, the community formation in TGR was influenced by both deterministic and stochastic factors, with the balance between them being mainly controlled by the season. Specifically, deterministic processes could explain 33.9-51.1% of community variations, while stochastic processes could contribute 23.5-32.2%. The findings of this research demonstrated that the varying ecological processes' significance relied on environmental gradients, geographical scale, and ecological conditions. This could offer a fresh outlook on comprehending the composition, assembly mechanisms, and distribution patterns of microeukaryotic plankton in reservoir ecosystems.

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.

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.

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.

Responses of plankton community to threshold metal concentrations of cadmium and lead in a mesocosm experiment at Bay of Bengal.

Metals are essential at trace levels to aquatic organisms for the function of many physiological and biological processes. But their elevated levels are toxic to the ecosystem and even brings about shifts in the plankton population. Threshold limits such as Predicted No Effect Concentration (PNEC - 0.6 µg/l of Cd; 2.7 µg/l of Pb), Criterion Continuous Concentration (CCC - 3.0 µg/l of Cd; 4.5 µg/l of Pb) and Criterion Maximum Concentration (CMC - 23 µg/l of Cd; 130 µg/l of Pb) prescribed for Indian coastal waters were used for the study. Short-term mesocosm experiments (96 h) were conducted in coastal waters of Visakhapatnam to evaluate responses of the planktonic community on exposure to threshold concentrations of cadmium and lead for the first time. Four individual experimental bags of 2500 L capacity (Control, PNEC, CCC & CMC) were used for the deployment and ambient water samples were analysed simultaneously to evaluate the impacts of the threshold levels in the natural waters. Chaetoceros sp. were dominant group in the control system whereas, Prorocentrum sp. Ceratium sp. Tintinopsis sp. Chaetoceros sp. and Skeletonema sp. were major groups in the test bags. Throughout the experiment the phytoplankton community did not show any significant differences with increased nutrients and plankton biomass (Chl-a <8.64 mg/m3). Positive response of plankton community was observed in the experimental bags. High abundance of diatoms were observed in PNEC, CCC & CMC bags at 48 h and the abundance decreased with shift in the species at 72-96 h. The catalase activity in phytoplankton (5.99 nmol/min/ml) and the zooplankton (4.77 nmol/min/ml) showed induction after exposure to PNEC. The present mesocosm study is confirmed that short-term exposure to threshold metal concentration did not affects the phytoplankton community structure in PNEC, but CCC and CMC affects the community structure beyond 24 h. The insights from this study will serve as a baseline information and help develop environmental management tools. We believe that long-term mesocosm experiments would unravel metal detoxification mechanisms at the cellular level and metal transfer rate at higher trophic levels in real-world environment.

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.

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 characterized 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 utilization is present but not universal in marine yeasts, consistent with existing knowledge of nonmarine 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.

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.

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).

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.

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).

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.

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.