Phytoplankton Diversity Research Articles

Phytoplankton Community in Relation to Environmental Variables in the Tidal Mangrove Creeks of the Pasur River Estuary, Bangladesh

The Pasur River estuary (PRE) provides vital fishery resources and supports millions of livelihoods in the southwestern coastal region of Bangladesh. Our research focused on phytoplankton community assemblages, alpha diversity indices, and the seasonal succession of major phytoplankton species in relation to physicochemical parameters in the tidal mangrove creeks of the Pasur River estuary. Spatial and temporal variations were assessed by water sampling at 17 stations in the study area from January to December 2019. The mean salinity level in the tidal mangrove creeks of the PRE was significantly (p < 0.05) higher during the dry season than during the wet season. Spatially, no significant variation (p > 0.05) was observed in the dissolved inorganic nitrogen and dissolved inorganic phosphorus between PRE and mangrove creeks, but temporally, the variables varied significantly (p < 0.05). Spatially, no significant variation (p > 0.05) was observed in the alpha diversity of the phytoplankton community but significantly (p < 0.05) varied temporally. Blue-green algae became dominant in the oligohaline conditions during the wet season, while diatoms were dominant during the dry season which severely depleted dissolved silica. In terms of phytoplankton species diversity, our study classifies the study areas as highly diversified zones. Phytoplankton succession from diatoms (dry season) to blue-green algae (wet season) is attributed to the changes in the physicochemical and nutrient parameters depending on seasonal environmental parameter fluctuations. This study illustrated that phytoplankton diversity and density varied with the degrees of habitat and seasonal changes, implying the potential impacts of anthropogenic activities and natural causes on their community structure in tropical estuaries and mangrove creeks.

Drivers of Phytoplankton Variability in and Near the Pearl River Estuary, South China Sea During Typhoon Hato (2017): A Numerical Study

AbstractThe Pearl River Estuary and adjacent seas in South China Sea are frequently affected by tropical cyclones (TCs). Previous in situ and remote sensing studies have found that typhoons can enhance phytoplankton biomass and induce blooms in this region. However, the mechanistic links between phytoplankton blooms and typhoons have not been well understood due to the interplay of multiple complex processes along the land‐ocean‐atmosphere interface. Here, we constructed an integrated modeling system with the marine ecosystem and sediment components for the China Great Bay Area. By using the integrated modeling system, we quantitatively investigated the phytoplankton response to hydrological conditions variations under Typhoon Hato (2017), a strong typhoon case. Passive tracer experiments showed that with high river discharge induced by heavy rainfall, the residence time within Lingding Bay is as short as 15 days, less than half of that under the climatological discharge. The increase in freshwater pulse washes out the phytoplankton biomass within Lingding Bay. While for the offshore region, the analysis of various source and sink terms for the offshore region's post‐typhoon response showed that the increase of phytoplankton biomass in the first week was because of the uplift of nutrient‐rich subsurface water, while in the second week, it was due to the seaward propagation of nearshore high phytoplankton biomass water. While riverine nutrients support phytoplankton growth in the third week, a large part of phytoplankton biomass was lost to zooplankton grazing, showing the system shifted from the bottom‐up control to the top‐down control.

Development of an index based on fish, macroinvertebrates, phytoplankton, and physicochemical properties to assess urban aquatic ecosystems in Jinan, China

AbstractBecause of the rapid growth in urbanization over recent decades, urban aquatic ecosystems have been subjected to unprecedented threats and challenges. In this study, we visited 48 sites, of which 14 were reservoir sites, in water bodies in Jinan, China, on six occasions in May, August, and October of 2014 and 2015. We sampled fish, macroinvertebrates, and phytoplankton assemblages and physicochemical factors at each site. We used the richness and diversity of fish, macroinvertebrates, and phytoplankton to evaluate the condition of the aquatic ecosystems of the river sites. We also evaluated the water quality of the reservoir sites using the physicochemical factors of the State Environmental Protection Administration of China. We classified the results into five levels—excellent, good, moderate, poor, and bad—with the comprehensive evaluation index (CEI). More than 40% of the sampling sites were in either poor or bad condition. Nearly half of the excellent or good sites were in the south of Jinan. The CEI was significantly different between the reference and impaired sites; it was significantly and positively correlated with the qualitative habitat evaluation index and was significantly and negatively correlated with conductivity, total alkalinity, and ammonia nitrogen.

Sensitivity of phytoplankton, zooplankton and macroinvertebrates to hydrogen peroxide treatments of cyanobacterial blooms

Addition of hydrogen peroxide (H2O2) is a promising method to acutely suppress cyanobacterial blooms in lakes. However, a reliable H2O2 risk assessment to identify potential effects on non-target species is currently hampered by a lack of appropriate ecotoxicity data. The aim of the present study was therefore to quantify the responses of a wide diversity of freshwater phytoplankton, zooplankton and macroinvertebrates to H2O2 treatments of cyanobacterial blooms. To this end, we applied a multifaceted approach. First, we investigated the 24-h toxicity of H2O2 to three cyanobacteria (Planktothrix agardhii, Microcystis aeruginosa, Anabaena sp.) and 23 non-target species (six green algae, eight zooplankton and nine macroinvertebrate taxa), using EC50 values based on photosynthetic yield for phytoplankton and LC50 values based on mortality for the other organisms. The most sensitive species included all three cyanobacterial taxa, but also the rotifer Brachionus calyciflores and the cladocerans Ceriodaphnia dubia and Daphnia pulex. Next, the EC50 and LC50 values obtained from the laboratory toxicity tests were used to construct a species sensitivity distribution (SSD) for H2O2. Finally, the species predicted to be at risk by the SSD were compared with the responses of phytoplankton, zooplankton and macroinvertebrates to two whole-lake treatments with H2O2. The predictions of the laboratory-based SSD matched well with the responses of the different taxa to H2O2 in the lake. The first lake treatment, with a relatively low H2O2 concentration and short residence time, successfully suppressed cyanobacteria without major effects on non-target species. The second lake treatment had a higher H2O2 concentration with a longer residence time, which resulted in partial suppression of cyanobacteria, but also in a major collapse of rotifers and decreased abundance of small cladocerans. Our results thus revealed a trade-off between the successful suppression of cyanobacteria at the expense of adverse effects on part of the zooplankton community. This delicate balance strongly depends on the applied H2O2 dosage and may affect the decision whether to treat a lake or not.

Chlorophyll-a concentrations in the Arabian Gulf waters of arid region: A case study from the northern coast of Qatar

Remote sensing of spectrally active Chlorophyll-a (Chl-a) in the gulf water of an arid region is important to understand the spatial and temporal variations of phytoplankton and biomass in the water. This study characterizes the spectral absorption of Chl-a and detects and maps the Chl-a of Al Arish–Al Ghariyah coastal region of northern Qatar using the data of Hyperion of EO-1, MultiSpectral Instrument (MSI) of Sentinel-2, and Operational Land Imager (OLI) of Landsat-8 satellites. The study of image spectra of Chl-a of coastal water of the Al Ghariyah region using the Hyperion image showed spectral band absorptions near 450 and 475 nm in the blue region and around 640, 730, 760, and 830 nm in the red region. The analysis of Hyperion data using image spectra by linear spectral unmixing (LSU) method showed the occurrence of Chl-a in very shallow and shallow water. The mapping of Chl-a of the Al Arish–Al​ Ghariyah coastal region was carried out using MSI and OLI data by NDCI, 2BDA, 3BDA, and FLH violet algorithms, which showed the concentrations and distributions of the Chl-a in the region. The performance of the algorithms was studied using WorldView-3 data, which provided the R2 values of 60% and the best suitability of the NDCI algorithm and MSI data to map the concentration of Chl-a. All the results were validated using field measurements, in-situ measurements, and laboratory analyses of field samples. The measurements of Chl-a showed the occurrence of poor occurrence of Chl-a and confirmed the imagery results. The remote sensing results with physical and chemical parameters of water samples allowed us to assess Chl-a concentrations in the region. This study evaluates the sensor’s capability and applications of satellite data and algorithms to map concentrations of Chl-a in the Gulf waters of the arid region.

Pulses outweigh cumulative effects of water diversion from river to lake on lacustrine phytoplankton communities.

Due to the allochthonous input of nutrients and species, the cumulative effects of water diversion on water-receiving lakes deserve attention. Taking the water diversion project from the Yangtze River to Lake Taihu (WDYT) as an example, we explored the temporal effects of WDYT on the phytoplankton community and physicochemical habitat of Lake Taihu in autumn and winter from 2013 to 2018. Although the short-term diversion significantly increased the risk of importing nutrients, the relatively high quality of the diversion water compared with other inflow rivers had improved the water quality of the water-receiving lake region. The seasonal water diversion significantly increased phytoplankton diversity and community network complexity and reshaped the lacustrine community to be diatom-dominated with their relative proportions of 24.1-64.9% during water diversion periods. The contributions of physicochemical habitat changes induced by water diversion to variations in phytoplankton communities were 24.0-28.0%. The differences in phytoplankton diversity, community composition and physicochemical habitat in the water-receiving lake region between the diversion and non-diversion years were more evident than those between the non-diversion years in the same season, when comparing the multivariate dispersion indices among them. However, the lacustrine phytoplankton community during non-diversion periods still has not been essentially altered after several years of diversion, so the pulse effects of short-term water diversion were more obvious than the long-term cumulative impacts. Better control of allochthonous nutrients, appropriate increase in inflow water, adhering to the long-term operation, should be effective to enhance ecological benefits of such water diversion projects.

Phytoplankton biomass and community composition in three Texas estuaries differing in freshwater inflow regime

Because many estuaries worldwide are experiencing large-scale alterations in freshwater inflows due to climatic and human influences on watersheds, it is critical to understand ecosystem-level responses to freshwater inflow conditions and variability. This study compared environmental conditions and phytoplankton biomass/community composition among three Texas estuaries with differing freshwater inflow regimes to understand the impacts of freshwater inflow magnitude on phytoplankton communities. It was hypothesized that: 1) nutrient concentrations and phytoplankton biomass would be highest in San Antonio Bay (SA), the high inflow estuary and lower in Nueces-Corpus Christi Bay (NC) and Baffin Bay (BB) due to lower average inflows, and 2) the phytoplankton community would be dominated by large and/or fast-growing taxa in SA, with a greater fraction of small and/or slow-growing taxa in NC and BB. Highest inorganic nutrient concentrations were generally observed in SA, while high organic nutrient concentrations were found in BB. Chlorophyll a was relatively high in both SA and BB (mean 16.9–18.5 μg L−1) while phytoplankton biovolume was highest in BB. Despite distinct freshwater inflow, salinity and nutrient regimes, differences in phytoplankton community composition were less pronounced. Nano- or microplankton were the dominant size class of phytoplankton in each system, and diatoms were the dominant functional group, accounting for 27–49% of total biovolume on average. There were indications that the phytoplankton community was more diverse in SA, especially following inflow events, providing evidence that inflow may act as a disturbance that leads to greater phytoplankton diversity. Results from this study also showed that while freshwater inflow is important for nutrient delivery, low inflow estuaries such as BB are still susceptible to effects of eutrophication due to long residence times and nutrient retention/recycling. Overall, the differing responses of each of these ecosystems to freshwater inflow highlight the importance of system-specific management plans and consistent monitoring programs in coastal estuaries.

Insights into coastal phytoplankton variations from 1979 to 2018 derived from Ba/Ca records in scallop shells (Chlamys islandica) from a fishing ground in the northern Gulf of St. Lawrence

Bivalve growth is affected by phytoplankton quality and availability, but long-term, coastal environmental time series related to these parameters are often lacking. Therefore, it is crucial to develop methods to accurately quantify trends in phytoplankton dynamics over time. This would be especially important for the fished scallop beds in the northern Gulf of St. Lawrence, where landings sharply declined since the early-2000s. Over the past decade, many studies have highlighted the potential of Ba/Ca ratios in bivalve shells as an environmental proxy for phytoplankton dynamics. This study presents records of Ba/Ca ratios in 31 young Chlamys islandica shells sampled in the Mingan Archipelago from 1979 to 2018. The Ba/Ca master chronology showed a decreasing trend since 2002, which could reflect changes in local phytoplankton bloom taxonomic composition, and coincides with the aforementioned decline in scallop landings. Investigations of environmental controls on barium incorporation into the shells highlight the importance of bottom, nutrient-rich waters to support diatom production or export in this fishing area. The use of such high-resolution seasonal records extracted from bio-archives may identify essential environmental mechanisms that will then assist with the development of an ecosystem-based fishery management strategy.

Unraveling environmental drivers of chlorophyll seasonal and interannual variability in the East China Sea

Phytoplankton, the dominant marine primary producers, are considered highly sensitive indicators of ecosystem conditions and changes. The East China Sea (ECS) includes a variety of oceanic and coastal domains that collectively challenge our understanding of phytoplankton dynamics and controls. This study evaluates the seasonal and interannual variability of phytoplankton in the ECS and the underlying environmental determinants based on 22-year satellite chlorophyll (Chl-a) data and concurrent environmental variables. A seasonal spring bloom was found in the ECS, classically driven by increased stratification, which is associated with increases in sea surface temperature (SST), photosynthetically available radiation (PAR), net heat flux (NHF), and reduced wind mixing. The most significant Chl-a interannual variability was present in a triangular area surrounded by three SST fronts in the southern ECS during springtime. Anomalously high Chl-a (~30% increase) occurred with increased SST and NHF and enhanced wind mixing during negative Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modes. This seems to be contrary to the stratification control model, which fits the seasonal spring bloom observed in this region. More front activities during the negative PDO and ENSO could be associated with Chl-a increase in this triangular area. Contrary to this mixing control scenario, a significant Chl-a increase (~36% increase) also developed during the positive PDO and ENSO modes after 2014 under conditions of higher SST, NHF, and weaker wind mixing following the stratification control scenario. This study used biologically relevant objective regionalization of a heterogeneous area to elucidate phytoplankton bloom dynamics and controls. Our analyses highlight the triangular area in the ECS for its region-specific linkages between Chl-a and multiple climate-sensitive environmental drivers, as well as the potential structural and functional variability in this region.

Dynamics of phytoplankton community in relation to seasonal variation and Laminaria japonica culturing in coastal area

Seaweed aquaculture is an important industry with great social and eco-environmental values. This study assessed the water quality and phytoplankton community during the four seasons around the Taohua Island where seaweed aquaculture has been practiced for near 70 years. Sampling was conducted in Laminaria japonica culturing area. Significant seasonal effects were observed among measured environmental factors and phytoplankton community. The species number and density of phytoplankton were positively correlated with temperature (T), Secchi depth (SD) and total nitrogen (TN) but were negatively correlated with dissolve oxygen (DO). In spring (the late culturing period), the seaweed culture increased pH, DO and salinity, while slightly reduced total phosphorous (TP). The phytoplankton abundance was lower in the culturing area than in control area, whereas the Shannon-Wiener diversity were found to be higher. Our study demonstrated that seasonal environment changes (especially T, SD, TN and DO) had strong influence on the functional characteristics of the phytoplankton community and L. japonica cultivation can absorb the nutrients and enhance phytoplankton diversity, thus alleviating eutrophication and reducing the risk of algal blooms. This is of significance to remediate the coastal eutrophicated and to restore the ecosystem.

Relationship of diatom phytoplankton (Bacillariophyceae) diversity with water quality and heavy metal in Angke and Grogol Rivers, Jakarta City

Jakarta City is passing by 2 rivers, they are Angke and Grogol Rivers. These rivers are a source of phytoplankton including diatom Bacillariophyceae. Whereas recently, those rivers have received heavy metal inputs that may affect the water quality and also the diatom diversity. Then this study aims to assess the relationship of diatom phytoplankton belonging to Bacillariophyceae diversity with water quality and heavy metal in Angke and Grogol Rivers, Jakarta City. The diversity was quantified using the Shannon-Wiener index (H’) and the relationships were analyzed using Principal Component Analysis (PCA). The results show the diversity of diatoms in both rivers were significantly different (P < 0.05) with the diatom diversity in Grogol (H’ = 1.175) being higher than in Angke (H’ = 0.8175). Mean heavy metal values in Grogol River were recorded higher (P < 0.05) than in Angke as observed in Cu (0.61 mg/L) and Pb. According to the PCA results, water turbidity along with Zn and Cu were the limiting factors for the diversity of diatom Bacillariophyceae.

Application of modified clay in intensive mariculture pond: Impacts on nutrients and phytoplankton

Nutrients and phytoplankton associated with mariculture development are important concerns globally, as they can significantly impact water quality and aquaculture yield. Currently, there is still insufficient information regarding the variations in nutrients and phytoplankton community of intensive mariculture systems, and effective treatment is lacking. Here, based on consecutive daily monitoring of two Litopenaeus vannamei ponds from July to October, the dynamic variations in nutrients and phytoplankton were elucidated. In addition, modified clay (MC) method was adopted to regulate the nutrients and phytoplankton community. The temporal variations in organic and inorganic nutrients presented fluctuating upward trends. Notably, organic nutrients were the dominant species, with average proportions of TON/P in TN/P were as high as 75.29% and 87.36%, respectively. Furthermore, a marked increase in the ratios of dinoflagellates to diatoms abundance were also observed in the control pond, concurrently with dominant organic nutrients, ascending N/P ratio and decreasing Si/N and Si/P ratios. In the MC-regulated pond, MC reduced the contents of both organic and inorganic nutrients. Furthermore, a distinct change pattern of dominant phytoplankton community occurred, with green algae becoming the most abundant phytoplankton in the MC-regulated pond. This study can provide new insights into an effective treatment for managing water quality and maintaining sustainable mariculture development.

Phytoplankton distribution along a salinity gradient in two Kenyan saltworks (Tana and Kurawa)

The current study assessed the diversity and abundance of phytoplankton genera in two Kenyan saltworks (Tana and Kurawa) in March and September 2021. Water samples were obtained from ponds with salinities ranging from 30 to 200 ppt by filtering 40 l of water using a 20-μm phytoplankton net. Seventy-six genera of phytoplankton were identified. Genera richness, evenness and diversity decreased with increasing salinity while phytoplankton abundance increased with increasing salinity. Higher phytoplankton densities were observed in the Tana than in the Kurawa saltworks. Ponds of &lt;100 ppt were dominated by Dinophyceae and Bacillariophyceae which accounted for &gt;90 % of the phytoplankton community. Ponds of salinities &gt;100 ppt were dominated by Cyanophyceae which accounted for &gt;90 % of the phytoplankton community. From the results it was concluded that Kenyan saltworks host diverse phytoplankton genera whose richness decreases with increasing salinity and varies with seasons. The present data describes variation of phytoplankton assemblages in salt ponds between two selected seasons, but several samplings throughout the year would be more appropriate to describe variations of phytoplankton with season in these salt ponds.

Water Quality in Relation to Phytoplankton Diversity of Gautam Kund, Dehradun (Uttarakhand), India

This study deals with the study of water quality of Gautam Kund in relation to its phytoplankton diversityfrom December 2018 to April 2019 at two sites. A Total of 76 genera were identified belonging to 8 classes, 20orders and 40 families during the present study period. Class Chlorophyceae contributes maximum tophytoplankton population. Total density of phytoplankton was ranged in between 207.0 to 1615.67 Unit l-1.Three different ecological indices were applied to phytoplankton data show that water of Gautam Kundranges from moderately polluted to clean water. Principal component analysis was applied to explorerelationship between physico-chemical parameters and phytoplankton diversity.

Phytoplankton Groups in the Estuarine Mangrove Creeks of Edakochi (Kerala), Southwest Coast of India: A Preliminary Study

The present work focused on the ecological studies in the mangroves of the Edakochi region, Kerala (India)with special reference to phytoplankton groups. This wetland is situated on the banks of Vembanad Lake,the largest Ramsar site of Kerala. Sampling was carried out from February to June 2017. 5 true mangrovespecies and 2 semi- mangrove species were noted from the Edakochi region. A total of 25 phytoplanktonspecies were recorded from different stations comprising 15 species of Bacillariophyceae, 7 species ofDinophyceae, 1 species of Cyanophyceae, 1 species of Chlorophyceae and 1 species of Ciliateae. In eachstation the species composition was found in an order of Bacillariophyceae &gt; Dinophyceae &gt;Cyanophyceae&gt; Chlorophyceae &gt;Ciliatae. Bacillariophyceae contributed 60% of the species, whereas Dinophyceaecontributed 28%. Cyanophyceae, Chlorophyceae and Ciliateae contributed 4% each. The highest numberof species (19) was recorded during March and the highest cell density was recorded during April (1143/l). The lowest cell density was recorded in June (844/l). From the study results, it was noted that the highestnumber of species in March was due to the favorably increased salinity. The shift in the salinity regime maybe the reason for the lowest number of species in June. Alternation in species diversity influences variousecosystem phenomenon, such as nutrient retention, primary productivity and vegetation dynamics. Thestudy findings depict good species diversity of phytoplankton in the Edakochi mangrove ecosystem.

Integrated modeling framework to evaluate the impacts of multi-source water replenishment on lacustrine phytoplankton communities

An extensive artificial water diversion project aimed at alleviating the shortage of ecological water in lakes can change the original hydrological and physicochemical states and further affect the structure and distribution of phytoplankton communities. Therefore, it is important to fully evaluate the ecological restoration effect of the water diversion project on the water-receiving area before implementing water replenishment. For this purpose, we developed an integrated model framework based on the Environmental Fluid Dynamics Code (EFDC) and Random Forest (RF). We adopted a probability distribution method to address the uncertainty during model coupling. This framework was implemented to simulate and predict the evolution of phytoplankton diversity in Baiyangdian Lake (BL) in China. To solve the problem that the phytoplankton biodiversity in BL decreased especially in summer due to human activity, the Yellow River into BL, the south-to-north water diversion project, and joint replenishment of upstream reservoirs have been implemented in recent years. Our framework was used to analyze the biodiversity restoration effects of multi-source water replenishment through different routings. The results show that spatiotemporal coverage should be considered to reduce uncertainty during model coupling. Water replenishment has a positive impact on the biodiversity of BL; however, there are effective areas for phytoplankton diversity restoration, which are related to water quality and quantity, water replenishment routing and internal hydrological connectivity. The Xiaobai River has the most significant water ecological restoration potential among the routings, compared with that of the Baigou and Fu Rivers. Appropriate water replenishment in spring will play a vital role in alleviating the decrease in phytoplankton biodiversity in summer owing to flood control.

Effects of large-scale Sargassum fusiforme culture on phytoplankton community structure and water quality

The large-scale cultivation of S. fusiforme was hypothesized to effectively improve water quality and environmental characteristics. Specifically, such cultivation could promote habitat restoration effects by increasing DO and pH levels in addition to decreasing nitrogen and phosphorus levels within cultivation area waters. Phytoplankton diversity could also reflect the quality and health of waters. This study explored the effects of large-scale cultivation of Sargassum fusiforme on the biodiversity and the ecosystem of a phytoplankton community in the coastal waters of Dongtou island, China. S. fusiforme cultivation increased DO and pH values within the area but decreased N contents of the cultivation area waters. A total of 75 phytoplankton species were identified based on morphological identification, while a parallel survey of phytoplankton communities using high-throughput cultivation-independent sequencing identified 601 phytoplankton OTUs. Large scale cultivation of S. fusiforme was associated with increased phytoplankton diversity as reflected by multiple alpha and beta diversity metrics. These data suggested that S. fusiforme cultivation could affect phytoplankton community structure when the S. fusiforme biomass reached maximum levels. Redundancy analysis (RDA) indicated that S. fusiforme played an important role in controlling the dynamics of phytoplankton communities in coastal ecosystems. Thus, the results suggested that S. fusiforme cultivation could enhance phytoplankton community biodiversity. Overall, this study demonstrated that large-scale cultivation of S. fusiforme in coastal regions was an effective approach for mitigating environmental problems in marine ecosystems.

Dumping of Dredge Spoil in the Pelagic Habitat: Focus on Trophic Status, Phytoplankton Diversity Responses and Generation of Blooms

This study presents the impacts of dredge spoil dumping in the pelagic habitat during a 27-month monitoring survey in eastern Mediterranean coastal waters (Saronikos Gulf, Aegean Sea), with a focus on changes in trophic status and eutrophication levels, phytoplankton diversity and bloom dynamics. A number of environmental parameters and phytoplankton metrics were significantly influenced by the dumping operations, specifically phytoplankton diversity indices (number of species, Diatoms:Dinoflagellates ratio) and total abundance, Chlorophyll-a, light transmission, dissolved oxygen and inorganic nutrients, N:P ratio, and the Eutrophication Ιndex (a metric for trophic status assessment). Phosphates started to increase after the first year of dumping operations, shifting the N:P ratio to values lower than 10. A similarity cluster analysis highlighted that the phytoplankton community structure during the pre-dumping and the early-dumping period was clearly discriminated from the period during and after the dumping operations. A clear shift with an increase in the Diatoms:Dinoflagellates ratio was observed immediately after the initiation of dumping operations, which maximized in the dumping site after two years of operations. Diatoms dominated the phytoplankton communities, reaching ~ 95% relative abundance in the dumping site. High biomass producers or potentially toxic diatom species proliferated forming blooms. Pseudo-nitzschia multiseries was the most frequent potentially toxic species. A multivariate analysis (RDA) highlighted that among a suite of phytoplankton metrics plotted against stressors relevant to dumping, the Eutrophication Index, Chlorophyll-a, the diversity index Diatoms:Dinoflagellates ratio and the abundance of the potentially toxic diatom P. multiseries emerged as the most suitable to reflect the responses of phytoplankton communities to dumping. Dredge spoil dumping at sea poses pressures to ecosystem components addressed by the European Marine Strategy Framework Directive (MSFD) monitoring programs. In such a context, this study further supports the role of phytoplankton diversity and blooms as sensitive monitoring elements for the environmental status assessment and dumping management in coastal waters.

Risk of eutrophication in the seawater of the coastal Red River aquaculture zone (Thai Binh province, Vietnam)

Coastal aquaculture represents an important economic activity in many countries. Nutrient (N, P and Si) concentrations and phytoplankton diversity and abundance in the coastal aquaculture zone, Tien Hai district, Thai Binh province (North Vietnam) were investigated during 2019–2020. Nutrient concentrations in five types of wastewater were also determined. The mean Eutrophication Index value of surface seawater at the 9 studied sites varied from 0.27 to 0.52, ranking as oligotrophic to mesotrophic. Five phytoplankton groups were observed (Bacillariophyceae (62% relative abundance), Dinophyceae (16%), Chlorophyceae (11%), Cyanobacteria (7%), and Euglenophyceae (4%)). The temporal and spatial distribution of phytoplankton diversity and abundance was governed by water temperature, pH, TSS, and TP and reflected the strong interactions between seawater and riverine water in the area. The presence of certain potentially toxic algal species combined with increasing inorganic nitrogen concentrations and N:P ratios further underlined the risk for harmful algal blooms. The investigation of different nutrient sources in this area revealed that while nitrate and total phosphorus concentrations are related to diffuse sources, point sources may be important in providing ammonium and dissolved phosphate concentrations to coastal seawater in this aquaculture zone. These results provide a scientific basis upon which better management strategies can be constructed in order to assure the sustainable development of aquaculture in the Red River coastal area, in particular, and in the coast of Vietnam, in general.

Responses of Functional and Taxonomic Phytoplankton Diversity to Environmental Gradients in Subtropical and Tropical Reservoirs

Understanding the influence of environmental conditions on biodiversity is a major task in ecology. We investigated how phytoplankton taxonomic (TD) and functional (FD) diversities vary with environmental factors in eight subtropical and tropical reservoirs. We hypothesized that i) environmental variables affect phytoplankton TD and FD; ii) FD provides better relationships to environmental changes than TD, and; iii) indices based on biomass are better related to the environment than those based on identities. The relationships between phytoplankton diversities and environmental drivers were assessed through generalized linear models. Our hypotheses were partly confirmed. TD and FD were, in fact, dependent on the environment, with higher values occurring in warmer, clearer, and more enriched systems, under lower zooplankton grazing pressure; but FD based on identities was not predicted better from environmental conditions than TD based on identities. As expected, indices based on biomass are better related to the environment than their counterpart based on identities.