Picophytoplankton Abundance Research Articles

The effects of decomposing invasive jellyfish on biogeochemical fluxes and microbial dynamics in an ultra-oligotrophic sea

Abstract. Over the past several decades, jellyfish blooms have intensified spatially and temporally, affecting functions and services of ecosystems worldwide. At the demise of a bloom, an enormous amount of jellyfish biomass sinks to the seabed and decomposes. This process entails reciprocal microbial and biogeochemical changes, typically enriching the water column and seabed with large amounts of organic and inorganic nutrients. Jellyfish decomposition was hypothesized to be particularly important in nutrient-impoverished ecosystems, such as the Eastern Mediterranean Sea – one of the most oligotrophic marine regions in the world. Since the 1970s, this region has been experiencing the proliferation of a notorious invasive scyphozoan jellyfish, Rhopilema nomadica. In this study, we estimated the short-term decomposition effects of R. nomadica on nutrient dynamics at the sediment–water interface. Our results show that the degradation of R. nomadica has led to increased oxygen demand and acidification of overlying water as well as high rates of dissolved organic nitrogen and phosphate production. These conditions favored heterotrophic microbial activity and bacterial biomass accumulation, and triggered a shift towards heterotrophic biodegrading bacterial communities, whereas autotrophic picophytoplankton abundance was moderately affected or reduced. This shift may further decrease primary production in the water column of the Eastern Mediterranean Sea. Deoxygenation, acidification, nutrient enrichment, and microbial community shifts at the sediment–water interface may have a detrimental impact on macrobenthic communities. Based on these findings, we suggest that jelly-falls and their decay may facilitate an additional decline in ecosystem functions and services.

Estimation of cell abundances of picophytoplankton based on the absorption coefficient of phytoplankton in the South China sea

Picophytoplankton are essential components of phytoplankton in the oligotrophic South China Sea (SCS). Understanding the variation in the picophytoplankton community structure will provide important information about primary production and the biogeochemical cycling of carbon in the SCS. Based on a field dataset from the SCS, we developed empirical algorithms using the absorption coefficient of phytoplankton at 443 nm [aph(443)] as the input to estimate the cell abundances of picophytoplankton, including Prochlorococcus (Pro), Synechococcus (Syn), and autotrophic picoeukaryotes (PE), in the SCS. Evaluation of algorithm performances demonstrated good agreement with field measurements. The root mean square errors and the mean absolute errors (MAEs) between the algorithm derivations and measurements were 0.44, 0.35, and 0.29, and 2.67, 2.02, and 1.88 for the cell abundances of Pro, Syn, and PE, respectively. The match-up comparisons showed that the satellite-derived cell abundances of picophytoplankton (e.g., MAEs ranged from 1.73 to 2.76) also agreed with the field data. We also analyzed the influences of both temperature and nutrient concentration on algorithm performance. The influence of temperature on the algorithms was not significant because the data were mainly collected in the summer, and the analysis should be repeated in the future with data from other seasons. The algorithm for estimating cell abundance of Syn was sensitive to variations in nutrient levels and herbivory pressure in coastal waters where nano- or microphytoplankton dominated. The input of our algorithms, aph(443), was easily obtained from field measurements and remote sensing. These algorithms provide a relatively easy way to estimate the cell abundances of picophytoplankton and provide data for studying the picophytoplankton community structure in the SCS.

A machine-learning approach to modeling picophytoplankton abundances in the South China Sea

Picophytoplankton, the smallest phytoplankton (<3 µm), contribute significantly to primary production in the oligotrophic South China Sea. To improve our ability to predict picophytoplankton abundances in the South China Sea and infer the underlying mechanisms, we compared four machine learning algorithms to estimate the horizontal and vertical distributions of picophytoplankton abundances. The inputs of the algorithms include spatiotemporal (longitude, latitude, sampling depth and date) and environmental variables (sea surface temperature, chlorophyll, and light). The algorithms were fit to a dataset of 2442 samples collected from 2006 to 2012. We find that the Boosted Regression Trees (BRT) gives the best prediction performance with R2 ranging from 77% to 85% for Chl a concentration and abundances of three picophytoplankton groups. The model outputs confirm that temperature and light play important roles in affecting picophytoplankton distribution. Prochlorococcus, Synechococcus, and picoeukaryotes show decreasing preference to oligotrophy. These insights are reflected in the vertical patterns of Chl a and picoeukaryotes that form subsurface maximal layers in summer and spring, contrasting with those of Prochlorococcus and Synechococcus that are most abundant at surface. Our forecasts suggest that, under the “business-as-usual” scenario, total Chl a will decrease but Prochlorococcus abundances will increase significantly to the end of this century. Synechococcus abundances will also increase, but the trend is only significant in coastal waters. Our study has advanced the ability of predicting picophytoplankton abundances in the South China Sea and suggests that BRT is a useful machine learning technique for modelling plankton distribution.

Ocean acidification reduces growth and grazing impact of Antarctic heterotrophic nanoflagellates

Abstract. High-latitude oceans have been identified as particularly vulnerable to ocean acidification if anthropogenic CO2 emissions continue. Marine microbes are an essential part of the marine food web and are a critical link in biogeochemical processes in the ocean, such as the cycling of nutrients and carbon. Despite this, the response of Antarctic marine microbial communities to ocean acidification is poorly understood. We investigated the effect of increasing fCO2 on the growth of heterotrophic nanoflagellates (HNFs), nano- and picophytoplankton, and prokaryotes (heterotrophic Bacteria and Archaea) in a natural coastal Antarctic marine microbial community from Prydz Bay, East Antarctica. At CO2 levels ≥634 µatm, HNF abundance was reduced, coinciding with increased abundance of picophytoplankton and prokaryotes. This increase in picophytoplankton and prokaryote abundance was likely due to a reduction in top-down control of grazing HNFs. Nanophytoplankton abundance was elevated in the 634 µatm treatment, suggesting that moderate increases in CO2 may stimulate growth. The taxonomic and morphological differences in CO2 tolerance we observed are likely to favour dominance of microbial communities by prokaryotes, nanophytoplankton, and picophytoplankton. Such changes in predator–prey interactions with ocean acidification could have a significant effect on the food web and biogeochemistry in the Southern Ocean, intensifying organic-matter recycling in surface waters; reducing vertical carbon flux; and reducing the quality, quantity, and availability of food for higher trophic levels.

Picophytoplankton in Blagopoluchia Bay (Novaya Zemlya Archipelago) and Adjacent Part of the Kara Sea

Picophytoplankton abundance, biomass, and chlorophyll a concentration as well as contribution of picoalgae to total chlorophyll a, and species composition of phototrophic picoeukaryotes (cells size less than 3 µm) were studied in Blagopoluchia Bay (Novaya Zemlya archipelago) and the northwestern part of the Kara Sea in September 2017. In the Bay, the highest picophytoplankton abundance and biomass were found in the surface layer and averaged 0.75 × 109 cell/m3 and 1.49 mg С/m3 respectively. In the northwestern part of the Kara Sea, the highest abundance and biomass of picophytoplankton were observed in the 8 m layer and were 2.29 × 109 cells/m3 and 2.76 mg С/m3 respectively. The contribution of picophytoplankton to total chlorophyll a concentration varied from 12 to 24% in the Bay and reached 46% in the Kara Sea. The Illumina sequencing of the V4 region of 18S rRNA gene revealed eight classes of phototrophic picoeukaryotes. Mamiellophyceae dominated in the number of reads. Arctic endemic (Micromonas polaris) and widespread (Bathycoccus prasinos) species were revealed in both studied areas.

Temporal and Spatial Variation Patterns of Picophytoplankton and Their Correlations with Environmental Factors During the Wet Season in East Lake Dongting

Picophytoplankton (<3 μm), comprising picocyanobacteria (PCY) and photosynthetic picoeukaryotes (PPEs), are considerably important in the material circulation and energy flow of aquatic ecosystems. To explore the temporal and spatial variation patterns of picophytoplankton and their correlations with environmental factors in lotic Yangtze-connected lakes, field in-situ investigations were performed on a monthly basis during the wet season (May to August) in 2019 in East Lake Dongting, a Yangtze-connected lake. The results indicated that both the Chla biomass and abundances of picophytoplankton exhibited significant spatial and temporal variability (P<0.05). The picophytoplankton Chla biomass showed an average concentration of 8.52 μg·L-1 and accounted for 41.6% to total phytoplankton on an average. From May to August, Chla biomass of picophytoplankton kept increasing with increasing temperature, especially in the north and south of the lake, and it was the lowest in the east of the lake. PCY dominated picophytoplankton abundance in East Lake Dongting and was 3.4 times the abundance of PPEs on an average. Similar spatial and temporal variation patterns were observed between PCY and PPEs. The abundances of PCY and PPEs both increased first and then decreased during the wet season. Spatially, picophytoplankton showed a trend to migrate from the northern lake to the southern lake from May to July, and the abundance significantly declined in August and peaked mainly in the north of the lake. The analysis results showed that picophytoplankton in East Lake Dongting exhibited significant spatial and temporal variability during the wet season; the water level and N：P ratio were determined to be the most important factors explaining the variation of the abundance proportion of PCY and PPEs.

Biogeographic variations of picophytoplankton in three contrasting seas: the Bay of Bengal, South China Sea and Western Pacific Ocean

Marine picophytoplankton are abundant in many oligotrophic oceans, but the known geographical patterns of picophytoplankton are primarily based on small-scale cruises or time-series observations. Here, we conducted a wider survey (5 cruises) in the Bay of Bengal (BOB), South China Sea (SCS) and Western Pacific Ocean (WPO) to better understand the biogeographic variations of picophytoplankton. Prochlorococcus (Pro) were the most abundant picophytoplankton (averaging [1.9-3.6] × 104 cells ml-1) across the 3 seas, while average abundances of Synechococcus (Syn) and picoeukaryotes (PEuks) were generally 1-2 orders of magnitude lower than Pro. Average abundances of total picophytoplankton were similar between the BOB and SCS (4.7 × 104 cells ml-1), but were close to 2-fold less abundant in the WPO (2.5 × 104 cells ml-1). Pro and Syn accounted for a substantial fraction of total picophytoplankton biomass (70-83%) in the 3 contrasting seas, indicating the ecological importance of Pro and Syn as primary producers. Pro were generally abundant in oligotrophic open waters; however, the exceptional presence of Pro near the SCS coast was potentially associated with the Kuroshio intrusion. Syn and PEuk abundances were higher near freshwater-dominated areas, which was likely due to dilution waters. Water temperature and cold eddies were also major drivers responsible for the biogeographic distributions of picophytoplankton. Although Pro, Syn and PEuks showed negative correlations with nutrient concentrations, their maximal abundances in vertical distribution showed positive correlations with the nutricline depth, indicating that nutrient availability plays a 2-faceted role in regulating the biogeographic variation in picophytoplankton.

Picophytoplankton distribution at the Ob section and in western part of the Kara sea

The spatial distribution of picophytoplankton abundance, biomass, chlorophyll a and contribution of picoalgae to total chlorophyll a was studied in the outer Ob estuary with an adjacent shelf and in the western part of the Kara Sea. In August-September picophytoplankton abundance and biomass varied from 0.1 to 17.3106 cell/l and from 0.06 to 9.20 mg С/m3, respectively. Cyanobacteria dominated in plankton picofraction, its contribution to total picophytoplankton biomass did not exceed 11%. The highest contribution of picophytoplankton to the total phytoplankton abundance was observed at a lower (11 mg/m2) chlorophyll a concentration in the euphotic layer. The spatial heterogeneity of picoforms contribution was determined by the silicon concentration.

Small Phytoplankton Shapes Colored Dissolved Organic Matter Dynamics in the North Atlantic Subtropical Gyre.

The North Atlantic subtropical gyre (NASTG) is a model of the future ocean under climate change. Ocean warming signals are hidden within the blue color of these clear waters and can be tracked by understanding the dynamics among phytoplankton chlorophyll ([Chl]) and colored dissolved organic matter (CDOM). In NASTG, [Chl] and CDOM are strongly correlated. Yet, this unusual correlation for open oceans remains unexplained. Here, we test main hypotheses by analyzing high spatiotemporal resolution data collected by Biogeochemical‐Argo floats between 2012 and 2018. The direct production of CDOM via phytoplankton metabolism is the main occurring mechanism. More importantly, CDOM dynamics strongly depend on the abundance of picophytoplankton. Our findings thus highlight the critical role of these small organisms under the ocean warming scenario. Picophytoplankton will enhance the production of colored dissolved compounds and, ultimately, impact on the ocean carbon cycle.

Picophytoplankton Distribution at the Ob River Section and in the Western Part of the Kara Sea

The spatial distribution of picophytoplankton abundance, biomass, chlorophyll a and the contribution of picoalgae to the total chlorophyll a was studied in the outer Ob River estuary with the adjacent shelf and in the western part of the Kara Sea. In August–September, the picophytoplankton abundance and biomass varied from 0.1 to 17.3 × 106 cell/L and from 0.06 to 9.20 mg С/m3, respectively. Photosynthetic picoeukaryotes dominated in the plankton picofraction; the contribution of cyanobacteria to the total picophytoplankton biomass did not exceed 11%. The highest contribution of picophytoplankton to the total phytoplankton abundance was observed at a lower (<11 mg/m2) euphotic zone integrated chlorophyll a. The spatial heterogeneity of picoforms contribution was determined by the silicon concentration.

The Spatial Distribution of Plankton Picocyanobacteria on the Shelf of the Kara, Laptev, and East Siberian Seas

The spatial distribution of picocyanobacteria at the arctic longitude section passing through the shelf of the Kara, Laptev, and East Siberian seas from 58° to 168° E was studied. The average abundance of picocyanobacteria was 0.48 ± 1.2 × 109 cells/m3 in the Kara Sea, 0.16 ± 0.24 × 109 cells/m3 in the Laptev Sea, and 0.25 ± 0.43 × 109 cells/m3 in the East Siberian Sea. The fluctuations of picocyanobacterial abundance were determined by their presence in allochthonous sources: river flow and transformed North Atlantic waters. The highest abundance was observed in the areas of the runoff influence of the Ob, Khatanga, Indigirka, and Kolyma Siberian rivers: 0.5 × 109, 0.2 × 109, 0.4 × 109, and 1.6 × 109 cells/m3, respectively. The average contribution of picocyanobacteria to the total abundance and biomass of picophytoplankton in the western part of the Kara Sea was 37 and 36%, respectively. In other regions, the average contribution of picocyanobacteria to the total abundance and biomass of phototrophic picoplankton did not exceed 7 and 6%, respectively. A highly reliable (p$$ \ll $$ 0.01) positive correlation between the abundance and biomass of picocyanobacteria and the water temperature (p = 0.003) was revealed in the entire dataset obtained.

Phytoplankton Community Structure Is Driven by Stratification in the Oligotrophic Mediterranean Sea.

The phytoplankton community composition, structure, and biomass were investigated under stratified and oligotrophic conditions during summer for three consecutive years in the Mediterranean Sea. Our results reveal that the phytoplankton community structure was strongly influenced by vertical stratification. The thermocline separated two different phytoplankton communities in the two layers of the euphotic zone, characterized by different nutrient and light availability. Picoplankton dominated in terms of abundance and biomass at all the stations sampled and throughout the photic zone. However, the structure of the picoplanktonic community changed with depth, with Synechococcus and heterotrophic prokaryotes dominating in surface waters down to the base of the thermocline, and Prochlorococcus and picoeukaryotes contributing relatively more to the community in the deep chlorophyll maximum (DCM). Light and nutrient availability also influenced the communities at the DCM layer. Prochlorococcus prevailed in deeper DCM waters characterized by lower light intensities and higher picophytoplankton abundance was related to lower nutrient concentrations at the DCM. Picoeukaryotes were the major phytoplankton contributors to carbon biomass at surface (up to 80%) and at DCM (more than 40%). Besides, contrarily to the other phytoplankton groups, picoeukaryotes cell size progressively decreased with depth. Our research shows that stratification is a major factor determining the phytoplankton community structure; and underlines the role that picoeukaryotes might play in the carbon flux through the marine food web, with implications for the community metabolism and carbon fate in the ecosystem.

Picophytoplankton of the Laptev sea in the autumn

The abundance and biomass of picophytoplankton, the total concentration of chlorophyll “a”, and the contribution of the picofraction to chlorophyll “a” were studied in the Laptev Sea in 2017. These were the first data for the western sea area. An increase in picophytoplankton abundance in the eastern part of the sea in comparison with that in 1991 was not revealed. The contribution of the picofraction to chlorophyll “a” was the highest (to 56%) in areas with a low total concentration of chlorophyll “a”.

Dynamic responses of picophytoplankton to physicochemical variation in the eastern Indian Ocean.

Picophytoplankton were investigated during spring 2015 and 2016 extending from near‐shore coastal waters to oligotrophic open waters in the eastern Indian Ocean (EIO). They were typically composed of Prochlorococcus (Pro), Synechococcus (Syn), and picoeukaryotes (PEuks). Pro dominated most regions of the entire EIO and were approximately 1–2 orders of magnitude more abundant than Syn and PEuks. Under the influence of physicochemical conditions induced by annual variations of circulations and water masses, no coherent abundance and horizontal distributions of picophytoplankton were observed between spring 2015 and 2016. Although previous studies reported the limited effects of nutrients and heavy metals around coastal waters or upwelling zones could constrain Pro growth, Pro abundance showed strong positive correlation with nutrients, indicating the increase in nutrient availability particularly in the oligotrophic EIO could appreciably elevate their abundance. The exceptional appearance of picophytoplankton with high abundance along the equator appeared to be associated with the advection processes supported by the Wyrtki jets. For vertical patterns of picophytoplankton, a simple conceptual model was built based upon physicochemical parameters. However, Pro and PEuks simultaneously formed a subsurface maximum, while Syn generally restricted to the upper waters, significantly correlating with the combined effects of temperature, light, and nutrient availability. The average chlorophyll a concentrations (Chl a) of picophytoplankton accounted for above 49.6% and 44.9% of the total Chl a during both years, respectively, suggesting that picophytoplankton contributed a significant proportion of the phytoplankton community in the whole EIO.

Picophytoplankton of the Laptev Sea in Autumn

The abundance and biomass of picophytoplankton, the total concentration of chlorophyll a, and the contribution of the picofraction to chlorophyll a were studied in the Laptev Sea in 2017. These were the first data for the western sea area. An increase in picophytoplankton abundance in the eastern part of the sea in comparison with that in 1991 was not revealed. The contribution of the picofraction to chlorophyll a was the highest (to 56%) in areas with a low total concentration of chlorophyll a.

Occurrence and distribution of viruses and picoplankton in tropical freshwater bodies determined by flow cytometry

This study aimed to examine the drivers in shaping the occurrence and distribution of total viruses and picoplankton in tropical freshwater ecosystems. Flow cytometry was used to quantify the concentrations of total viruses, picoheterotrophs, picophytoplankton, and picocyanobacteria. Three land use patterns (urban-, agriculture- and parkland-dominated) were evaluated using ArcGIS. Significant correlations were observed between water-borne microbial targets and water quality parameters (0.175 ≤ |r| ≤ 0.441), nutrients (0.250 ≤ r ≤ 0.570) and land use factors (0.200 ≤ |r| ≤ 0.460). In particular, the concentrations of total viruses and picoheterotrophic cells were higher in catchments whereas the abundances of picophytoplankton and picocyanobacteria were higher in reservoirs. Total viruses and picoplankton had higher concentrations in urban- and agriculture-dominated areas, probably due to anthropogenic inputs and agricultural inputs, respectively. Although surface water is a complex matrix influenced by niche-based (i.e., physicochemical properties, nutrients, land use impact etc.) and neutral-based factors (i.e., ecological drift, dispersal and species), land use patterns could help to elucidate the occurrence and distribution of the total microbial community at the macroscopic level. Meanwhile, inter-correlations among viruses, picoplankton and picoheterotrophs (0.715 ≤ r ≤ 0.990) also substantiates their mutual interactions in influencing the microbial community. Furthermore, the relationships between total microbial cells and bacterial and viral indicators were also investigated. Concentrations of total viruses, picoplankton and picoheterotrophs were positively correlated with bacterial indicators (0.427 ≤ r ≤ 0.590) and viral indicators (0.201 ≤ r ≤ 0.563). These results indicated that faecal and viral contamination could contribute to the numbers of total viruses and bacteria.

Mychonastes dominates the photosynthetic picoeukaryotes in Lake Poyang, a river-connected lake.

Lake Poyang, which is connected to the Yangtze River, is the largest freshwater lake in China and experiences large and frequent changes in water levels. The seasonal diversity and composition of photosynthetic picoeukaryotes (PPEs) in Lake Poyang were investigated from flow-cytometry-sorted samples using MiSeq high-throughput sequencing. Flow cytometric counting indicated that PPEs accounted for 97% of the total picophytoplankton abundance in spring, reaching a maximum value of 6.30 × 104 cellsmL-1. PPEs in Lake Poyang showed lower diversity than those in other investigated lakes in the lower reaches of the Yangtze River and were dominated by one OTU (66.29%) affiliated with Mychonastes (Chlorophyceae). Other minor classes of PPEs were found to be sporadically abundant in specific seasons, i.e. Chrysophyceae prevailed in spring and summer, while Eustigmatophyceae was mainly present in winter. This study reports coccal green algae of Mychonastes in Lake Poyang; additionally, these algae are reportedly representative of the prominent plankton in this river-connected lake ecosystem but are often overlooked due to their lack of morphological features. Finally, the sequencing results from the sorted samples of Lake Poyang revealed that the proportion of PPEs was quite low, with an average of 36% of total reads. Many OTUs belonging to heterotrophic picoeukaryotes were also identified in the sorted samples, most of which were affiliated with terrestrial fungi, including Basidiomycota and Ascomycota. The spores of these fungi can disperse in the aquatic environment during the flood seasons, yet their effect on PPEs is still unclear.

Spatial distribution patterns of phytoplankton biomass and primary productivity in six coral atolls in the central South China Sea

Nutrients’ concentration, phytoplankton biomass, and primary productivity were investigated in six coral atolls in the central South China Sea during the southwest monsoon in the early summer. Nutrients’ concentrations were generally very low in these atolls, and showed no obvious variation among outer reef slope, reef flat, and lagoon. The spatial variation of phytoplankton biomass and primary productivity were significant among the different regions of the atolls, and generally followed the order of outer reef slope < reef flat < lagoon. Phytoplankton biomass might be a more sensitive indicator than nutrients for the habitat differences in coral atoll ecosystem. The lagoon of Huangyan Atoll showed the highest phytoplankton biomass and primary productivity. The phytoplankton biomass and primary productivity were mainly contributed by pico-phytoplankton. Moreover, our results suggested that there might have a threshold value of Chl a for pico-phytoplankton abundance. When Chl a was higher than this threshold value (about 0.5 µg L−1), the pico-phytoplankton abundance should not linearly increase with the Chl a concentration. Due to the significantly high-phytoplankton biomass and primary productivity, the anthropogenic eutrophication should be paid attention in the lagoon of Huangyan Atoll.

Responses of the picophytoplankton community to temperature fronts in the northeastern Arabian Sea during the northeast monsoon

We investigated the responses of the picophytoplankton (< 3 µm) community to a temperature filament and front through high resolution spatial (~ 1 NM) sampling (November-23 to December-11, 2012) in the northeastern Arabian Sea (69°E, 18.85°N to 20.25°N). Samples were collected at discrete depths within the 100 m water column. Synechococcus dominated the picophytoplankton community numerically and in terms of biomass along the entire transect. To investigate the patterns of variability in picophytoplankton distribution, depending on the water mass characteristics, the entire transect was divided into four zones (1) south of filament (SFIL) with warm oligotrophic waters, (2) filament (FIL) with cooler and low saline waters, (3) north of filament (NFIL) with relatively cooler waters than the SFIL and (4) front (FRO) with relatively cooler and less saline waters than the FIL. Depth-integrated abundance and biomass of Synechococcus were relatively higher within the FIL and FRO whereas Prochlorococcus and picoeukaryotes were abundant in SFIL and NFIL. Redundancy analysis of environmental variables and picophytoplankton abundance showed that lower saline water mass within the mesoscale features harbored relatively higher Synechococcus abundance and biomass. Two Synechococcus ecotypes were distinguished based on the fluorescence intensity of the accessory pigment, phycoerythrin; the one with higher intensity (open ocean ecotype) dominating in the FIL and the other with lower intensity in the FRO (coastal ecotype). The relatively lower saline surface water mass at the FRO, probably a result of coastal advection, could have introduced the latter ecotype. Vertically, a positive correlation of Prochlorococcus with nutrients and Synechococcus with temperature corroborates their higher and lower abundance and biomass, respectively in the deeper waters. The positive correlation of Synechococcus with the total chlorophyll biomass indicates a similar response to environmental variables within the mesoscale features. This study shows that picophytoplankton contribution (16–24%) to the total phytoplankton carbon biomass in tropical mesoscale features is likely to have important consequences on the planktonic food web function.

Community structure of photosynthetic picoeukaryotes differs in lakes with different trophic statuses along the middle-lower reaches of the Yangtze River.

Photosynthetic picoeukaryotes (PPEs) play an important role in aquatic ecosystem functioning. There is still a relative lack of information on freshwater PPEs, especially in eutrophic lakes. We used a combination of flow cytometric sorting and pyrosequencing to investigate the PPEs community structure in more than 20 mesotrophic and eutrophic lakes along the middle-lower reaches of the Yangtze River in China. The abundance of PPEs ranged between 2.04×103 and 5.92×103 cells mL-1. The contribution of PPEs to total picophytoplankton abundance was generally higher in eutrophic lakes than in mesotrophic lakes. The sequencing results indicated that the Shannon diversity of PPEs was significantly higher in mesotrophic lakes than in eutrophic lakes. At the class level, PPEs were mainly dominated by three taxonomic groups, including Cryptophyceae, Coscinodiscophyceae and Chlorophyceae, and 15 additional known phytoplankton classes, including Synurophyceae, Dinophyceae, Chrysophyceae, Trebouxiophyceae and Prymnesiophyceae, were identified. Coscinodiscophyceae dominated in the most eutrophic lakes, while Chrysophyceae, Dinophyceae and other classes of PPEs were more abundant in the mesotrophic lakes. We also observed several PPEs operational taxonomic units, and those affiliated with Cyclotella atomus, Chlamydomonas sp. and Poterioochromonas malhamensis tended to be more prevalent in the eutrophic lakes. The canonical correspondence analysis and Mantel analysis highlighted the importance of environmental parameters as key drivers of PPEs community composition.