Phytoplankton Succession Research Articles

Diatom and coccolithophore species fluxes in the Subtropical Frontal Zone, east of New Zealand

The Chatham Rise supports some of New Zealand's most economically valuable commercial fish stocks, fuelled by the highly productive waters of the Subtropical Frontal Zone (STFZ). Climate change-related shifts in phytoplankton community structure and function are predicted and may affect Chatham Rise productivity and deep-sea carbon sequestration by the biological carbon pump. However, little is known about how two major phytoplankton groups, diatoms and coccolithophores, will respond, since the seasonality of their productivity and export fluxes to the seafloor are not well known in the SW Pacific sector. This study is the first to report on the seasonal cycle of phytoplankton species fluxes in the STFZ, using a 12-month sediment trap record (1996–1997) on the northern (subtropical-influenced) and southern (subantarctic-influenced) flanks of the Chatham Rise. Diatom and coccolith flux assemblages were characterised and compared with biogeochemical fluxes from the same sites over the same time-frame. Northern phytoplankton flux composition was typical of the subtropics, with most particulate organic carbon (POC) and biogenic silica export associated with diatom fluxes. Fluxes to the northern trap were largely composed of coastal and benthic diatom taxa, reinforcing previous interpretations suggesting lateral advection and sediment resuspension along and across the Chatham Rise slope, influenced also by the Wairarapa Eddy. At the southern site, phytoplankton fluxes were on average an order of magnitude higher than at the northern site and comprised a combination of subantarctic and frontal zone diatom taxa. The majority of the annual diatom flux in the southern trap occurred during a spring Pseudo-nitzschia “pulse” associated with high biogenic silica and POC fluxes, which are inferred to be from subsurface chlorophyll maxima. High coccolith flux at the southern STFZ site was consistent with ecosystem processes observed from other subantarctic export regimes. Phytoplankton successions inferred from the sediment trap records suggest that coccolithophores precede diatoms in spring, which is contrary to expectations from classical studies where the opposite temporal trend is suggested. While maximum diatom species flux co-occurred with peak POC flux at both sites in spring, the estimated contribution of organic carbon associated with diatoms to POC flux was negligible (<0.05%). Estimated coccolithophore POC flux was also low, suggesting that either other phytoplankton groups and/or zooplankton-mediated processes (e.g. fecal pellet fluxes) must contribute more substantially to POC export in the STFZ. Our findings provide new insights into phytoplankton export flux pathways to the seafloor in the Chatham Rise region, with implications for regional biogeochemistry, paleoceanographic and export modelling studies.

Assessing the phytoplankton community and diversity in relation to physico-chemical parameters in a tropical reservoir of the River Ganga basin, India

Phytoplankton species composition and diversity and its relation to physico-chemical parameters were studied in Jargo Reservoir located on River Jargo, a tributary of River Ganga basin, India. A total of 137 phytoplankton species belonging to 8 divisions were recorded indicating rich phytoplankton diversity in the North Indian reservoir. Bacillariophyta contributed the highest number of species followed by Chlorophyta. Filamentous algae such as Geitlerinema sp., Pseudoanabaena sp., Oscillatoria sp. and Phormidium sp. were highly abundant. The contribution of the top ten species ranged between 63% and 98% of total phytoplankton density. PCA analysis of physico-chemical revealed six principal components (HCO3, TDS, BOD, DOM, air temperature and pH) which significantly affected the succession of phytoplankton in Jargo Reservoir. The CCA analysis of phytoplankton species and physico-chemical parameters revealed that Ochrophyta and Miozoa were highly influenced by high CO2 and PO4. Higher phytoplankton diversity and species richness were observed during the late winter season and rainy season, respectively. Our investigation recommends that fish production in the reservoir could be enhanced by stocking with fingerlings of silver carp, which has the potential to utilise the unconsumed population of abundant Myxophyta.

Genotype and host microbiome alter competitive interactions between Microcystis aeruginosa and Chlorella sorokiniana

Cyanobacterial harmful algal blooms (cyanoHABs) continue to increase in frequency and magnitude, threatening global freshwater ecosystems and services. In north-temperate lakes cyanobacteria appear in early summer, succeeding green algae as the dominant phytoplankton group, a pattern thought to be mediated by changes in temperature and bioavailable nutrients. To understand additional drivers of this successional pattern our study used reciprocal invasion experiments to examine the competitive interaction between Microcystis aeruginosa, a dominant contributor to cyanoHABs, and the green alga Chlorella sorokiniana. We considered two factors that may impact these interactions: (1) strain variation, with a specific emphasis on the presence or absence of the gene for the hepatotoxin microcystin, and (2) host-associated bacteria. We used toxic M. aeruginosa PCC 7806 (microcystin producing strain), a non-toxic mutant of PCC 7806, non-toxic M. aeruginosa PCC 9701 (non-microcystin producing strain), and C. sorokiniana. Each organism was available free of all bacteria (i.e., axenic) and with a re-introduced defined bacterial community to generate their xenic counterparts. Competitive interactions were assessed with reciprocal invasion experiments between paired xenic and paired axenic populations of C. sorokiniana and one of the two Microcystis strains, each assessed separately. Flow cytometry and random forest models were used to rapidly discriminate and quantify phytoplankton population densities with 99% accuracy. We found that M. aeruginosa PCC 7806, but not strain PCC 9701, could proliferate from low abundance in a steady-state population of C. sorokiniana. Further, the presence of bacteria allowed M. aeruginosa PCC 7806 to grow to a higher population density into an established C. sorokiniana population than when grown axenic. Conversely, when M. aeruginosa was dominant, C. sorokiniana was only able to proliferate from low density into the PCC 9701 strain, and only when axenic. The mutant of PCC 7806 lacking the ability to produce microcystin behaved similarly to the toxic wild-type, implying microcystin is not responsible for the difference in competitive abilities observed between the two wild-type strains. Quantification of microcystins (MCs) when PCC 7806 M. aeruginosa was introduced into the C. sorokiniana culture showed two-fold more MCs per cell when host-associated bacteria were absent compared to present in both species cultures. Our results show that the ability of M. aeruginosa to compete with C. sorokiniana is determined by genomic differences beyond genes involved in microcystin toxin generation and indicate an important role of host-associated bacteria in mediating phytoplankton interspecies interactions. These results expand our understanding of the key drivers of phytoplankton succession and the establishment and persistence of freshwater harmful cyanobacterial blooms.

Seasonal and Inter-Annual Variability of the Phytoplankton Dynamics in the Black Sea Inner Basin

We explore the patterns of Black Sea phytoplankton growth as driven by the thermohaline structure and circulation system and the freshwater nutrient loads. Seasonal and inter-annual variability of the phytoplankton blooms is examined using hydrodynamic simulations that resolve mesoscale eddies and online coupled bio-geochemical model. This study suggests that the bloom seasonality is homogeneous across geographic locations of the Black Sea inner basin, with the strongest bloom occurring in winter (February–March), followed by weaker bloom in spring (April–May), summer deep biomass maximum (DBM) (June–September) and a final bloom in autumn (October–November). The winter phytoplankton bloom relies on vertical mixing of nitrate from the intermediate layers, where nitrate is abundant. The winter bloom is highly dependent on the strength of the cold intermediate layers (CIL), while spring/summer blooms take advantage of the CIL weakness. The maximum phytoplankton transport across the North Western Shelf (NWS) break occurs in September, prior to the basin interior autumn bloom. Bloom initiation in early autumn is associated with the spreading of NWS waters, which in turn is caused by an increase in mesoscale eddy activity in late summer months. In summary, the intrusion of low salinity and nitrate-rich water into the basin interior triggers erosion of the thermocline, resulting in vertical nitrate uplifting. The seasonal phytoplankton succession is strongly influenced by the recent CIL disintegration and amplification of the Black Sea circulation, which may alter the natural Black Sea nitrate dynamics, with subsequent effects on phytoplankton and in turn on all marine life.

Seasonal and spatial distributions of morpho-functional phytoplankton groups and the role of environmental factors in a subtropical river-type reservoir.

Phytoplankton is capable of responding to aquatic conditions and can therefore be used to monitor freshwater reservoir water quality. Numerous classification techniques, including morpho-functional approaches, have been developed. This study examined changes in phytoplankton assemblages and water quality, which were sampled quarterly from July 2018 to April 2019. The purpose was to contrast the applicability of three classification approaches (functional, morpho-functional and morphological-based functional groupings) for understanding the spatial and seasonal distribution of the biomass variance in phytoplankton functional groups and their driving environmental factors in the ecological zones of the Shanxi Reservoir through multivariate analysis. The results showed that the phytoplankton biomass was highest in the watercourse zone and lowest in the transition zone. Furthermore, the Shanxi Reservoir was characterized by several cyanobacteria (Microcystis spp.) and numerous bacillariophytes (Asterionella sp., Navicula spp. and Aulacoseira granulata). After evaluating the advantages and disadvantages of morpho-functional classifications, we determined that water temperature appeared to be an essential factor, and the morphology-based functional group approach provided the best results for demonstrating phytoplankton succession, despite having lower sensitivity than the others. Nevertheless, these approaches are all appropriate for identifying and monitoring phytoplankton community structure in aquatic systems of reservoirs with complex terrains.

Top-down versus bottom-up control in planktonic systems: some case studies from the English Lake District

In this review, I use examples drawn from long-term studies in the English Lakes to highlight the limitations of top-down control in planktonic systems. These limitations are most obvious at the base of the food chain where there are large differences in the potential growth rates of the phytoplankton and zooplankton. Three of the examples are taken from a 40-year study of the impact of perch on Daphnia in the North Basin of Windermere, one on the rates of change measured in an experimental enclosure and one from an 18-year study of Daphnia dynamics in Esthwaite Water. The Windermere examples show that the commercial perch fishery established in 1941 had little effect on the abundance of Daphnia but the enrichment of the lake by war-time ploughing led to a short-lived increase in their reproductive rate. The other examples demonstrate that time plays an important part in all trophic interactions and shows that established patterns of phytoplankton succession are frequently disrupted by changes in the intensity of wind-induced mixing. The results are discussed in relation to recent reviews of the efficacy of biomanipulation and the challenges of analysing complex interactions in dynamic systems.

Seasonal succession of phytoplankton in two temperate artificial lakes with different water sources.

To explore the seasonal succession and drivers of phytoplankton communities in Guanzhong, Shaanxi Province, China, the water quality and phytoplankton communities in two typical waterbodies (Juehe Wetland Park in Xian and Xiaoxihu Lake in Xianyang) were monitored from 31 August 2017 to 31 July 2018. We identified 43 genera of phytoplankton from 7 phyla in Juehe Wetland Park and 45 genera of phytoplankton from 7 phyla in Xiaoxihu Lake. Phytoplankton communities in the two waterbodies showed significant patterns of seasonal succession. Cyanophyta (Microcystis spp.) dominated in August-September 2017 in Juehe Wetland Park, and Bacillariophyta (Navicula spp.) dominated in October 2017-July 2018. Cyanophyta (Microcystis spp.) dominated in Xiaoxihu Lake in August-November 2017, and Bacillariophyta (Synedra spp.) dominated in December 2017-July 2018. Redundancy analysis (RDA) showed that the temperature and total phosphorus were closely related to the seasonal succession of Cyanophyta and Bacillariophyta. In addition, the temperature, dissolved total phosphorus, and total phosphorus were closely related to the seasonal succession of Microcystis spp., Navicula spp., and Synedra spp.

Succession of phytoplankton in a shallow lake under the alternating influence of runoff and reverse water transfer

Abstract Both runoff and water diversion can interfere with the physical and chemical environment of a lake and affect aquatic organisms. In this study, previously obtained data were used to analyze the phytoplankton community, water quality, water level, and temperature in Dongping Lake (DPH) before, during, and after the water diversion caused by the South-to-North Water Transfer Project. The results showed that the total density and diversity index of phytoplankton decreased in the water transfer period, and was related to low temperature. Temperature also affected the recovery of phytoplankton community structure when the water transfer period ended. In a water transfer cycle, changes in dominant genera were more drastic than that of a whole phytoplankton community, and dominant genera were sensitive to total phosphorus (TP) and total nitrogen (TN) changes. Water transfer alleviated the deterioration of water quality in DPH, but water transfer process increased the risk of water pollution. Runoff from Dawen River carried TN, TP, and chemical oxygen demand (COD) into DPH in the rainy season, which indirectly affected phytoplankton, while it also carried phytoplankton directly into DPH. Overall, these findings provide a clear understanding of the impact of water transfer projects on ecology in shallow lakes.

Underestimated methane production triggered by phytoplankton succession in river-reservoir systems: Evidence from a microcosm study

The impoundment of dammed rivers accelerates phytoplankton succession from river-dominated to lake-dominated species. Little is known about the role of phytoplankton succession in methane (CH4) production. In this study, we performed a 61-day microcosm investigation to simulate the collapse processes of Cyclotella meneghiniana (river-dominated algae) and Chlorella pyrenoidosa and Microcystis aeruginosa (lake-dominated algae). The results suggested that different methanogenic conditions were induced by the collapse of river-and lake-dominated algae. The rapid settlement of C. meneghiniana induced aerobic conditions in the water that inhibited anaerobic CH4 production and intensified CH4 oxidation as a result of an increase in pmoA. However, the decomposition of C. pyrenoidosa and M. aeruginosa depleted dissolved oxygen and provided abundant labile organic matter, which jointly elevated mcrA and the mcrA/pmoA ratio. Under this condition, anaerobic CH4 production was the dominant pathway for the mineralization of algae-derived carbon. Finally, the CH4 produced per unit of particulate total carbon (identified as the carbon content of the algal biomass) by C. pyrenoidosa and M. aeruginosa was 16.29-fold and 8.56-fold higher, respectively, than that produced by C. meneghiniana. These observations provided evidence that lake-dominated algae played a more vital role in CH4 production than river-dominated algae when algal succession occurred. This discovery might be a new and vital, yet largely underestimated CH4 emission pathway in river-reservoir systems, that should be considered when evaluating the effect of hydraulic projects on greenhouse gas emissions.

Seasonal phytoplankton succession in Netravathi – Gurupura estuary, Karnataka, India: Study on a three tier hydrographic platform

The Netravathi-Gurupura estuary is a tropical, micro to meso-tdal estuary situated at Mangalore, southwest coast of India. The estuary outlets into the Arabian Sea and it is regarded as one of the major upwelling systems of India. Our study focusses on the seasonal phytoplankton succession in Netravathi-Gurupura estuary through studying the hydrochemical and hydrobiological physiognomies of riverine, estuarine and marine components. Hypoxic conditions prevailed at the estuarine zone during monsoon seasons (D.O.< 3 mg L−1). Salinity was very low in the estuarine and marine zone during the 2018 monsoon season and hefty silicate loadings (>100 μmol L−1) contributed to the proliferation and blooming of diatoms like Skeletonema costatum in the estuary. A strong negative correlation between phytoplankton composition and silicate was observed during the 2018 monsoon season (r = −0.977). The substantial entrainment of TDS (>10000 ppm), relatively higher salinity (33–35 psμ) and high values of TN: TP (42.0–67.0 μmol L−1) and proliferation of dinoflagellates were the characteristics of non-monsoon seasons. A significant positive association of phytoplankton was observed with NO3 (r = 0.989) and TN: TP (r = −0.997). In terms of phytoplankton species diversity, our study classifies estuarine zone as least diverse zone, particularly during the 2018 monsoon season (D > 0.55, 1/D < 1.82). Phytoplankton succession from diatoms (monsoon) to dinoflagellate (non-monsoon) is attributed to the alteration in physic-chemical and nutrient parameters as a function of seasonal variation.

Variation in ω-3 and ω-6 Polyunsaturated Fatty Acids Produced by Different Phytoplankton Taxa at Early and Late Growth Phase.

Phytoplankton synthesizes essential ω-3 and ω-6 polyunsaturated fatty acids (PUFA) for consumers in the aquatic food webs. Only certain phytoplankton taxa can synthesize eicosapentaenoic (EPA; 20:5ω3) and docosahexaenoic acid (DHA; 22:6ω3), whereas all phytoplankton taxa can synthesize shorter-chain ω-3 and ω-6 PUFA. Here, we experimentally studied how the proportion, concentration (per DW and cell-specific), and production (µg FA L−1 day−1) of ω-3 and ω-6 PUFA varied among six different phytoplankton main groups (16 freshwater strains) and between exponential and stationary growth phase. EPA and DHA concentrations, as dry weight, were similar among cryptophytes and diatoms. However, Cryptomonas erosa had two–27 times higher EPA and DHA content per cell than the other tested cryptophytes, diatoms, or golden algae. The growth was fastest with diatoms, green algae, and cyanobacteria, resulting in high production of medium chain ω-3 and ω-6 PUFA. Even though the dinoflagellate Peridinium cinctum grew slowly, the content of EPA and DHA per cell was high, resulting in a three- and 40-times higher production rate of EPA and DHA than in cryptophytes or diatoms. However, the production of EPA and DHA was 40 and three times higher in cryptophytes and diatoms than in golden algae (chrysophytes and synyrophytes), respectively. Our results show that phytoplankton taxon explains 56–84% and growth phase explains ~1% of variation in the cell-specific concentration and production of ω-3 and ω-6 PUFA, supporting understanding that certain phytoplankton taxa play major roles in the synthesis of essential fatty acids. Based on the average proportion of PUFA of dry weight during growth, we extrapolated the seasonal availability of PUFA during phytoplankton succession in a clear water lake. This extrapolation demonstrated notable seasonal and interannual variation, the availability of EPA and DHA being prominent in early and late summer, when dinoflagellates or diatoms increased.

Phytoplankton community, structure and succession delineated by partial least square regression in Daya Bay, South China Sea.

Daya Bay is facing the influence of human activities and nature changes, which result in phytoplankton adjusting to the changing environment. The data about environmental changes and phytoplankton were obtained from four seasonal cruises in 2013 in the bay. It is helpful to explore seasonal succession of phytoplankton driven by the determining environmental factors in this bay. Temperature is a significant indicator of season change. The limiting factor of phytoplankton growth totally changed from P (PO4-P) limiting during the southwest monsoon to Si (SiO3-Si) limiting during northeast monsoon. The order of diatoms and dinoflagellates was the dominant phytoplankton groups in Daya Bay. The dominant species included chain-forming diatoms (Skeletonema, Pseudo-nitzschia, Thalassionema, Chaetoceros and Rhizosolenia) were found all the year round and filamentous cyanobacteria (Trichodesmium) in spring and autumn. Partial least square regression (PLS) found that salinity, temperature and nutrients were important driving force for phytoplankton seasonal succession.

Spatial and temporal variations in the plankton community because of water and sediment regulation in the lower reaches of Yellow River

The water and sediment regulation (WSR) regime at the Xiaolangdi Hydropower Station in the Yellow River impacts significantly on the aquatic ecosystems in the middle and lower reaches of the river. In this study, we evaluated the influence of the WSR regime on aquatic ecosystems, using the plankton community as an indicator because of their sensitivity to changes in the environment. The plankton distribution and the environmental conditions in the middle and lower reaches of the Yellow River (a total of 810 km) were investigated in three sampling surveys; namely, before, during, and after WSR at the power station. The results showed that the WSR regime influenced both the phytoplankton and zooplankton. For example, the species succession of phytoplankton, from Bacillariophyta to Cyanophyta, then to Chlorophyta, was accelerated, and zooplankton were transported from the reservoir, leading to a temporary increase in the zooplankton biomass downstream. The WSR influenced the phytoplankton and zooplankton over about 540 and 15 km of the river, respectively. A structural equation model was built for the three stages of WSR from data of the plankton biomass and environmental factors. The results showed that environmental factors such as dissolved oxygen (with a coefficient of 0.86 pre-WSR, −0.47 at inter-WSR, and 0.82 at post-WSR), temperature (with coefficient of −0.76 at inter-WSR), chlorophyll-a (with coefficient of 0.80 at inter-WSR) and conductivity (with coefficient of −0.82 at post-WSR) were important for the plankton biomass. Under the WSR, the concentrations of nutrients such as dissolved silicate, nitrogen, and phosphorus changed, with impacts on the plankton biomass. Even when the environmental and nutrient conditions were optimal, the WSR regime also weakened the predation of zooplankton on phytoplankton and changed the relationships within the plankton community.

The "Neglected Viruses" of Taihu: Abundant Transcripts for Viruses Infecting Eukaryotes and Their Potential Role in Phytoplankton Succession.

Drivers of algal bloom dynamics remain poorly understood, but viruses have been implicated as important players. Research addressing bloom dynamics has generally been restricted to the virus-infection of the numerically dominant (i.e. bloom forming) taxa. Yet this approach neglects a broad diversity of viral groups, limiting our knowledge of viral interactions and constraints within these systems. We examined hallmark virus marker genes in metatranscriptomic libraries from a seasonal and spatial survey of a Microcystis aeruginosa bloom in Lake Tai (Taihu) China to identify active infections by nucleocytoplasmic large DNA viruses (NCLDVs), RNA viruses, ssDNA viruses, bacteriophage, and virophage. Phylogenetic analyses revealed a diverse virus population with seasonal and spatial variability. We observed disproportionately high expression of markers associated with NCLDVs and ssRNA viruses (consistent with viruses that infect photosynthetic protists) relative to bacteriophage infecting heterotrophic bacteria or cyanobacteria during the height of the Microcystis bloom event. Under a modified kill-the-winner scheme, we hypothesize viruses infecting protists help suppress the photosynthetic eukaryotic community and allow for the proliferation of cyanobacteria such as Microcystis. Our observations provide a foundation for a little considered factor promoting algal blooms.

Metabolic analyses by metatranscriptomics highlight plasticity in phosphorus acquisition during monospecific and multispecies algal blooms

Algal blooms have emerged as a global phenomenon affecting coastal areas, while the regulatory mechanisms are poorly understood. To explore the effects of environmental factors, especially phosphate concentrations, on the outbreak and maintenance of algal blooms, this study used a metatranscriptomic approach to analyze the molecular responses of phytoplankton during two blooms in 2013 near Qinhuangdao, China. Pico/nanophytoplanktons (< 10 μm) were dominant numerically in the two algal blooms. Significant shifts in KEGG pathway expression were observed with the succession of phytoplankton, suggesting high temporal plasticity in the expressed metabolic capacity. The KEGG pathway expression pattern in the multispecies bloom on August 22 showed higher gene expression of primary metabolic pathways and lower gene expression of secondary metabolic pathways than that in the monospecific bloom on July 20. Pico/nanophytoplankton showed species-specific transcriptional responses to the shifts in N/P ratios and phosphate concentrations. Our results demonstrate how the species specificity and temporal plasticity of resource utilization capacities enable pico/nanophytoplankton to induce monospecific and multispecies blooms under different phosphate conditions. This study provides a basis for further work on the gene responses of multispecies assemblages of algae to different environmental parameters during algal bloom succession.

Phytoplankton functional groups succession and their driving factors in a shallow subtropical lake

The succession of phytoplankton functional groups and environmental variables were investigated from January to December 2018 in a shallow subtropical lake, East Taihu Lake, located in the Yangtze Delta of China. Altogether, 125 genera, 21 functional groups, and 7 predominant functional groups, D (Fragilaria acus, Synedra acus, and Nitzschia spp.), P (Fragilaria capucina and Aulacoseira granulate), MP (Oscillatoria spp., Diploneis ovalis, Gomphonema angustatum, Nitzschia palea, and Achnanthes spp.), Y (Cryptomonas ovata, Cryptomonas erosa, Chroomonas acuta, and Ceratium spp.), H2 (Anabaena spp.), M (Microcystis spp.), and W1 (Euglenoids spp.), were identified, and these predominant functional groups exhibited strong seasonal variations. Groups Y and W1 dominated from winter to spring when temperature and light were low. The external pollution load and high levels of nutrients entered the lake in June, allowing group M to become dominant, even ultimately triggering Microcystis blooming. Meanwhile, the abundance of aquatic plants in summer, combined with high levels of transparency and light availability, resulted in Group M being replaced by Group P and then Groups M and H2 rebounding in autumn. The redundancy analysis revealed that water temperature, nutrients, and light availability (Zeu/Zmix) were the key factors promoting phytoplankton succession. The ecological assessment results demonstrated a good ecological status based on the assemblage index (Q average = 3.0). This work emphasizes the fact that temperature, nutrients, and light were the driving factors for phytoplankton dynamics, and aquatic plants and external pollution were important external factors for the succession of phytoplankton.

Composition and Seasonal Succession of Phytoplankton of Taganrog Bay in the Sea of Azov and the Downstream Reaches of the Don River under a Changing Climate

The results of a study of phytoplankton in Taganrog Bay in the Sea of Azov and the downstream reaches of the Don River carried out in January–November 2017 are presented. A total of 210 species of eukaryotic algae and cyanoprokaryotes are identified. An analysis of the taxonomic, ecological, and geographical composition of the plankton algal flora and seasonal dynamics of biomass of phytoplankton taxonomic and size structures has revealed no changes in the ratio of large taxonomic and ecological–geographic groups in the algal flora when compared with previous years (1980s to early 2000s). However, the proportion of myxotrophic phytophagellates, cyanoprokaryotes from the planktotrichet complex, and small species increased in algal communities. It is shown that the water salinity is the main factor influencing the formation of species richness and total biomass and structure of phytoplankton. The maximum diversity and biomass of phytoplankton are recorded in the horohalinicum zone.

Ocean acidification increases domoic acid contents during a spring to summer succession of coastal phytoplankton

Enrichment of the oceans with CO2 may be beneficial for some marine phytoplankton, including harmful algae. Numerous laboratory experiments provided valuable insights into the effects of elevated pCO2 on the growth and physiology of harmful algal species, including the production of phycotoxins. Experiments close to natural conditions are the next step to improve predictions, as they consider the complex interplay between biotic and abiotic factors that can confound the direct effects of ocean acidification. We therefore investigated the effect of ocean acidification on the occurrence and abundance of phycotoxins in bulk plankton samples during a long-term mesocosm experiment in the Gullmar Fjord, Sweden, an area frequently experiencing harmful algal blooms. During the experimental period, a total of seven phycotoxin-producing harmful algal genera were identified in the fjord, and in accordance, six toxin classes were detected. However, within the mesocosms, only domoic acid and the corresponding producer Pseudo-nitzschia spp. was observed. Despite high variation within treatments, significantly higher particulate domoic acid contents were measured in the mesocosms with elevated pCO2. Higher particulate domoic acid contents were additionally associated with macronutrient limitation. The risks associated with potentially higher phycotoxin levels in the future ocean warrants attention and should be considered in prospective monitoring strategies for coastal marine waters.

Succession of phytoplankton community during intensive shrimp (Litopenaeus vannamei) cultivation and its effects on cultivation systems

As the basic biological element in shrimp cultivation systems, phytoplankton performs irreplaceable ecological functions in aquaculture ecosystem, but knowledge of the its community succession is currently not sufficient. To fill this gap, we performed longitudinal dense sampling in six intensive shrimp (Litopenaeus vannamei) aquaculture ponds to track the time-series shift of phytoplankton community at the community and species levels and to discuss the effects of its succession on rearing environment and shrimp. We detected a distinct successional pattern in the phytoplankton community by principal coordinates analysis and time-lag regression analysis, and this pattern could be divided into 3 clusters. Canonical correspondence analysis indicated that water temperature, dissolved oxygen, salinity, pH and bacterioplankton (Actinobacteria, Gammaproteobacteria, Verrucomicrobiae, and Acidimicrobiia) were the extrinsic factors that were correlated with the phytoplankton community variation. The α-diversity and taxonomic composition of communities in different clusters were discrepant. By random forest regression, we identified biomarkers of phytoplankton succession which was corelated with shrimp cultivation time. Spearman's rank correlation showed that at least one environmental variable was correlated with the biomarkers, which emphasized the interaction between phytoplankton and rearing environment. Moreover, network analysis revealed that correlations among individual phytoplankton and biotic and abiotic factors at different clusters were also discrepant. The decline in biomarkers indicated a bloom of pathogenic Gammaproteobacteria at the end of cultivation, which contribute to disease outbreaks. Overall, our findings imply that the succession of the phytoplankton community during shrimp cultivation follows a process in which phytoplankton succeed from initial establishment driven by abiotic and biotic factors, and this process might affect the function of the aquaculture ecosystem and the health of reared shrimp to some extent.

Abiotic environmental factors override phytoplankton succession in shaping both free-living and attached bacterial communities in a highland lake

Bacterial communities are an important part of biological diversity and biogeochemical cycling in aquatic ecosystems. In this study, the relationship amongst the phytoplankton species composition and abiotic environmental factors on seasonal changes in the community composition of free-living and attached bacteria in Lake Erhai were studied. Using Illumina high-throughput sequencing, we found that the impact of environmental factors on both the free-living and attached bacterial community composition was greater than that of the phytoplankton community, amongst which total phosphorus, Secchi disk, water temperature, dissolved oxygen and conductivity strongly influenced bacterial community composition. Microcystis blooms associated with subdominant Psephonema occurred during the summer and autumn, and Fragilaria, Melosira and Mougeotia were found at high densities in the other seasons. Only small numbers of algal species-specific bacteria, including Xanthomonadaceae (Proteobacteria) and Alcaligenaceae (Betaproteobacteria), were tightly coupled to Microcystis and Psephonema during Microcystis blooms. Redundancy analysis showed that although the composition of the bacterial communities was controlled by species composition mediated by changes in phytoplankton communities and abiotic environmental factors, the impact of the abiotic environment on both free-living and attached bacterial community compositions were greater than the impact of the phytoplankton community. These results suggest that the species composition of both free-living and attached bacterial communities are affected by abiotic environmental factors, even when under strong control by biotic factors, particularly dominant genera of Microcystis and Psephonema during algal blooms.