Phytoplankton Composition Research Articles

African desert dust influences migrations and fisheries of the Atlantic skipjack-tuna

Atmospheric deposition of desert dust and other aerosols influence on the open ocean ecosystem and climate. These aerosols provide iron, phosphorus and bio-essential trace elements, which affect the composition and growth of phytoplankton, generating new organic matter that is distributed across the food web. Although this process has an impact on upper trophic levels and fisheries, direct evidence is lacking. Skipjack tropical tuna (Katsuwonus pelamis) is the most important commercial tuna in the Atlantic, main stocks occur in the tropical and subtropical North-East Atlantic. We found that the migrations and fisheries of skipjack are connected to Saharan dust variability. From boreal winter to summer, skipjack performs an Atlantic-Saharan migration, from equatorial (0-5°N) to subtropical waters of the North-East Atlantic (regularly reaching open waters off Mauritania ∼20°N and the Canary Islands ∼28°N), tracking the seasonal shift of dust deposition in the North-East Atlantic. The observed long-term associations of skipjack catch with the seasonal cycles, anomalies and meridional variability of dust over the North-East Atlantic, shows that along the year skipjack catches mainly occur in waters affected by massive dust deposition linked (i) to dust wet deposition in tropical waters and (ii) to dust dry deposition in the Saharan desert-dust outflow. Atmospheric deposition of dust in the open ocean and in upwelling waters contributes to support zooplankton-rich areas, which are optimal for feeding small fish, molluscs and cephalopods, and suitable for feeding large predators, as skipjacks, where they are caught in abundance. The most important fishing area of Atlantic skipjack tuna is located off North West Africa in the waters affected by massive dust deposition that underly the dusty airstream from the North Africa desert. The role of dust as fertilizing and supporter of phytoplankton and zooplankton rich areas also has implications for other species of fishing interest, including other tropical tuna (e.g. bigeye and yellowfin), which are often exploited simultaneously with skipjack by the same fisheries. As far as we know this is the first study showing the connexions between atmospheric inputs of desert dust to ocean and the migrations and fisheries of marine species. These results have important implications on our understanding on the influence of atmospheric dust on marine ecosystems and on the management of fisheries.

Metaphyton contributes to open water phytoplankton diversity

In contrast to pelagic and benthic realms of the aquatic ecosystems, studies on the metaphytic habitats remain underrepresented in the literature. However, this realm may have a potential impact on composition and diversity of the open water assemblages through metacommunity processes (source-sink dynamics, mass-effect) especially in small ponds with extended littoral zone. Using a limnocorral experiment we studied how metaphyton affects diversity and composition of open water phytoplankton in a small eutrophic pond in the vegetation period. The three habitats (metaphyton, isolated and non-isolated open water) showed considerable differences in their taxa and functional group composition. Abundance-based diversity measures did not reveal remarkable differences among the assemblages of the three habitats. However, taxonomic and functional richness of the metaphyton and the non-isolated part of the pelagial significantly exceeded that of the limnocorral. Incidence-based similarity index values also showed closer resemblance of the metaphyton and plankton samples compared to that of the limnocorral. In the case of several functional groups, their functional redundancy in the metaphyton exceeded that in the open water areas. These results suggest that the metaphyton provides a refuge for several euplanktic elements that survive in the littoral and occasionally enrich the phytoplankton of the open water areas, representing that a within–lake metacommunity processes shape the composition and functioning of the open water areas in standing waters.

Wind‐driven currents and water masses shape spring phytoplankton distribution and composition in hydrologically complex, productive shelf waters

AbstractThe Northern Patagonian shelf (35–42°S SW South Atlantic) is a transitional area between the subtropical and subantarctic domains, where the shelf shifts from source to sink of atmospheric CO2. In the southeast portion (39–42°S, 61–55°W) of this area, winds interact with different water masses and picture a complex circulation that supports two productive frontal systems: the Mid‐Shelf Front (MSF) and the Patagonian Shelf‐break Front. We combined remote sensing (e.g., monthly surface chlorophyll a over the period: September–December 2002–2021), reanalysis (winds and surface currents), and in situ measurements from three cross‐shelf cruises in spring 2016, 2017, and 2019, to assess the underlying drivers of phytoplankton distribution. The anomalous high chlorophyll concentration in the MSF in spring 2016 was related to enhanced water mass retention by N‐NE winds. Contrasting phytoplankton assemblages emerged related to stratified waters in the shelf influenced by the Patagonian Current and vertically mixed waters at the continental edge (Malvinas Current). Large diatoms prevailed in the outer shelf followed by dinoflagellates, while nanoflagellates and microzooplankton dominated in the mid‐shelf. Furthermore, thickness of the subsurface chlorophyll maximum was broader in the early spring than in a more advanced state of the bloom where the coccolithophore Emiliania huxleyi was abundant. We showed that different time‐scale physical processes—winds, surface currents and vertical mixing—are underpinning the development of spring blooms with different taxa composition in the Patagonian shelf, where different water masses define the hydrological complexity that led to patchy phytoplankton.

Phytoplankton composition in intensive shrimp ponds in Bac Lieu province, Vietnam

Phytoplankton composition in intensive shrimp ponds in Bac Lieu province, Vietnam

Interannual and decadal variabilities of phytoplankton community in the Bering Sea and the Arctic Ocean: a case study of relationship with ENSO and Arctic Oscillation abnormity

We used molecular biomarkers (brassicasterol, dinosterol and C37 alkenones) measured from the surface sediments of the Bering Sea, the Chukchi Sea and the western Arctic Ocean in years of 1999 and 2010 to represent the groups of phytoplankton (diatoms, dinoflagellates and coccolithophores) and reconstruct the phytoplankton composition structure and biomass. The distribution of concentrations of three biomarkers were compatible to previous studies on measured phytoplankton, which showed that the phytoplankton biomass was most abundant in the Chukchi Sea, followed by the Bering Sea and the western Arctic Ocean, and diatoms were the dominant group. It suggests that molecular biomarkers are the suitable indicators of phytoplankton composition, structure and abundance. A record of biomarkers in a sediment core (NB01) collected at the Bering Sea in 2010 presented that the biomass of three phytoplankton groups increased in past 107 years, and their variation patterns were consistent. The synchronous increase of concentrations of phytoplankton biomarkers and cholesterol, a proxy of zooplankton biomass, implied that the primary production increased intensely in the last hundreds of years, corresponding to the trend of sea-ice melting as a result of global warming. Moreover, our results suggested that the combined effect of the anormal interannual changes of the El Niño-Southern Oscillation (ENSO) and the Arctic Oscillations (AO) played a key role in regulating the interannual and decadal variations of phytoplankton biomass and community composition, giving us an insight into the impact of atmospheric circulation on phytoplankton production and carbon flux in the Arctic seas.

Local environmental variables outperform spatial and land use pattern in the maintenance and assembly of phytoplankton communities in the wetland cluster

Phytoplankton is a crucial primary producer in wetland ecosystems and is highly sensitive to changes in water quality. It has long been considered one of the most important biological indicators for monitoring water pollution. However, our understanding of the various aspects of phytoplankton communities assembly and the factors driving them remains limited. In this study, we conducted collection and analysis of phytoplankton samples from a temperate wetland cluster encompassing both lake and marsh types under the backdrop of industrial pollution from the petroleum industry. Our aim was to investigate the influence of spatial factors, land use patterns, and local environmental variables on the taxonomic and functional structure of the phytoplankton community, utilizing structural equation modeling (SEM) and variation partitioning. Our results revealed that environmental filtering exerts a greater relative impact on shaping the taxonomic composition of phytoplankton compared to mass effects. Human activities, such as industrial and agricultural production, can lead to landscape modifications that increase water nutrient levels and consequently influence the community composition. Taxonomic beta diversity is determined by both mass effects and environmental filtering. In contrast, functional beta diversity responds more to local environmental gradients, while the land use pattern was not important for facets of beta diversity in the wetland cluster. These results validate the concordance and complementarity between functional and taxonomy-based biome structures, highlighting the significance of integrating multiple ecological drivers. The results of this study contribute to a deeper understanding of wetland community assembly and the processes of biogeography, and are of crucial significance for the biological monitoring and management of freshwater ecosystem environmental pollution.

Phytoplankton diversity explained by connectivity across a mesoscale frontal system in the open ocean

Phytoplankton community composition is important in establishing ecosystem structure and function. Intuitively, we recognize that water movements must be important for modifying spatial gradients and plankton diversity. However, identifying boundaries and exchange between habitats in the open ocean is not straightforward. Here, we use the abundance of nine phytoplankton species closely sampled in a mesoscale frontal system in the northeastern North Sea as a proxy for community composition and explore the relationship between phytoplankton biogeography and transport patterns. Subsurface community distributions could be related to modeled patterns in water movement. A methodology for analyzing pelagic diversity that includes a representation of plankton community composition and an Eulerian connectivity tracer was developed, and the relative importance of connectivity and geographical distance for phytoplankton species composition analyzed. The connectivity tracer identifies timescales and dispersal barriers in the open ocean. Connectivity was found to be superior in explaining pelagic plankton diversity and found to be a prerequisite for understanding the pelagic phytoplankton composition. This approach is a valuable tool for establishing the link between ocean transports, ecosystem structure and biodiversity and for informing the placement of marine protected areas.

Production, composition and preservation of phytoplankton on the northwest shelf of Australia through the Pliocene-Pleistocene period

In this study, we applied steroid records at IODP Site U1461 to reconstruct long-term variations of production and composition of phytoplankton, and the microbial degradation on phytoplankton-derived organic carbon (OC) on the northwest shelf of Australia through the Pliocene-Pleistocene period. We aim to probe whether and how these behaviors are impacted by strengths of the Indonesian Throughflow (ITF) and the Leeuwin Current (LC), sea level variation, and continental material input. The results indicate that production of phytoplankton generally increased during 6–1.2 Ma, and decreased after ∼1.2 Ma, due to combination effect from intensified upwelling strength by pronounced Pliocene-Pleistocene cooling and low-salinity, tropical nutrient-deficient surface water input by the ITF and LC. The results also demonstrate increasing contributions of diatoms at ∼5.2 Ma, relatively higher contributions of dinoflagellates during 3.8–1.7 Ma, and another increasing contribution of diatoms after ∼1.7 Ma. We suggest that stronger ITF during ∼6.0–3.8 Ma and stronger LC during ∼1.2–0 Ma both induced higher contributions of diatoms to total phytoplankton, while increased terrestrial soil input in the mid-Pleistocene might be the potential reason why the timing of increasing contribution of diatoms (∼1.7 Ma) is slightly ahead of the intensified LC condition (∼1.2 Ma). Compared with the Pliocene, the microbial degradation on phytoplankton-derived organic carbon was more active in the mid-late Pleistocene, which could be related to terrestrial soil input, subsurface currents, and sea level variation. With synchronously lowered bioproductivity, phytoplankton-derived organic carbon burial capacity on the northwestern shelf of Australia lowered since ∼1.7 Ma.

Phytoplankton and pigment composition during an Emiliania huxleyi bloom in the Black Sea

Alterations in composition and carbon biomass of phytoplankton community are closely related to seasonal variations, anthropogenic inputs and ultimately climatic changes in a longer perspective. Accuracy of the collected data is important to determine changes occurring in phytoplankton community composition. An intercomparison exercise of haptophyte abundance obtained by microscopy was performed by three research laboratories based on samples taken from 2 to 11 June 2016, in the western and central Black Sea. Total carbon biomass of phytoplankton groups was estimated by one of the three laboratories based on microscopy. Phytoplankton group composition was also determined with marker pigments. Carbon biomass of large sized diatoms and dinoflagellates was correlated with CHEMTAX derived chlorophyll a concentrations (Chl a) of these groups (r2=0.44-0.60, p<0.05). According to microscopy, carbon biomass of dinoflagellates was generally higher than that of diatoms and haptophytes, while based on HPLC-CHEMTAX analysis haptophytes and diatoms dominated in the region. Abundance of Emiliania huxleyi obtained by three laboratories were significantly different from each other (One way ANOVA) and abundance of only one of the laboratories was correlated with 19’-hexanoyloxyfucoxanthin concentration. Difference in abundance of this nanoplankton species among different laboratories appears to be related to both methodology in counting the cells and fixation of samples.

Assessing the effects of silicate addition on phytoplankton composition and copepod production in an inorganic fertilization system

Assessing the effects of silicate addition on phytoplankton composition and copepod production in an inorganic fertilization system

Co-Limitation of Phytoplankton by N and P in a Shallow Coastal Lagoon (Ria Formosa): Implications for Eutrophication Evaluation

AbstractThe Ria Formosa coastal lagoon is a highly productive shallow ecosystem in southern Portugal, subjected to nutrient inputs from anthropogenic and natural sources. Nutrients are major abiotic drivers of phytoplankton in this system, but their effects on phytoplankton assemblages and the occurrence of nutrient limitation are still poorly understood. The main goal of this study was, thus, to evaluate the occurrence, type, and effects of nutrient limitation on phytoplankton community and specific functional groups in the Ria Formosa coastal lagoon. We conducted nutrient enrichment experiments with factorial additions of nitrogen (N) and phosphorus (P) using natural phytoplankton assemblages from distinct locations in the Ria Formosa, throughout a yearly cycle. Phytoplankton composition and abundance were evaluated using inverted and epifluorescence microscopies, and spectrophotometric methods were used for biomass. Limitation was defined as higher phytoplankton growth following enrichment with a particular nutrient in relation to the non-enriched control. The most common type of phytoplankton limitation was simultaneous co-limitation by N and P; diatoms, as r-strategists, were the most frequently limited group. Single N and P limitation, and serial P limitation were also observed, as well as negative responses to nutrient enrichment. Group-specific responses to nutrient enrichment were not reflected in the relative abundance of phytoplankton groups within the whole assemblage, due to the numerical dominance of pico-sized groups (cyanobacteria and eukaryotic picophytoplankton). Ambient nutrient ratios and concentrations did not predict phytoplankton nutrient limitation, given the different nutrient utilisation traits among phytoplankton functional groups. Therefore, nutrient ratios should not be used as indicators of nutrient limitation in eutrophication assessment.

Spatiotemporal variation of phytoplankton communities and water quality among seaweed, shellfish and cage fish culture systems

Various marine aquaculture systems have different impacts on the environment, but few assessments were focused on the environmental impact by different systems in the same region. To study the effects of various aquaculture systems on phytoplankton community structure and water properties, 5 surveys were carried out in seaweed (Gracilaria lemaneiformis, GL), shellfish (Mytilus coruscus, MC) and cage fish (Larimichthys crocea, LC) mariculture areas in Dongji island, Zhejiang, China from June to September 2020. Significant differences were observed in some environmental parameters and phytoplankton communities among three aquaculture systems. The dissolved oxygen concentrations and Secchi depth in the surface waters in GL area were relatively higher than in the blank and other areas. As for nutrients concentration, LC and MC areas had higher concentrations than blank area, while GL area was the lowest. Though Chlorophyll-a concentration displayed fluctuations, relatively lower concentrations were found in GL area. Shannon diversity index was found to be relatively constant and higher in GL area. The Non-metric multidimensional scaling results revealed that phytoplankton composition had a distinct pattern among sampling times. The correlations and Redundancy analysis showed that total nitrogen, salinity and transparency were primary environmental factors associated with phytoplankton composition. Our study confirmed that different marine aquaculture systems can cause environmental fluctuations. Among the three systems, seaweed cultivation can bring multiple positive effects by improving surrounding water quality and increasing the phytoplankton composition. G. lemaneiformis culture in summer has great positive impact on seawater environment as it can maintain the ecological balance and reduce the risk of harmful algal blooms (HABs), and therefore, it is strongly recommended more G. lemaneiformis cultivation in this region in summer.

A size‐based perspective on the decoupling between compositional and functional changes in planktonic communities

AbstractRecent studies have shown a decoupling in the way community composition and functions respond to environmental changes. A common pattern observed is that aggregated functions at the community level are more stable than community composition, which is likely the result of functional compensatory dynamics driven by interspecific differences in response to environmental change. However, the mechanisms by which these patterns emerge remain largely unexplored. Here we investigated in a mesocosm experiment for four weeks the compositional and functional responses of edible phytoplankton (&lt;64 μm) and cladoceran zooplankton communities to climate warming (a constant increase of +3.5°C plus heat wave) and eutrophication (nutrient additions) from a size‐based perspective. Our results show that warming increases small‐sized taxa and decreases large‐sized taxa within both phytoplankton and zooplankton community composition. We found that such opposite responses of different‐sized taxa contributed to the stability of planktonic community functions and thereby resulted in a decoupling between compositional and functional changes. We also found that nutrient additions increased the abundance of all‐sized algal taxa, while phytoplankton community function remained stable. Nutrient additions did not alter the zooplankton community, neither compositionally nor functionally. Under the combined stress of warming and nutrient additions, the compositional and functional responses of planktonic communities were mainly driven by warming. In a broader perspective, our findings reveal a size‐dependent compensation mechanism and suggest that functional stability relies on compensatory effects among different‐sized taxa, and it is therefore important that communities host a large range of taxa differing in size to withstand an increasingly more variable environment in the future.

Phytoplankton abundance and diversity in the Northern Banda Sea

Although phytoplankton of the Banda Sea has been studied since 1970s, its information is still limited. In order to provide a better understanding of phytoplankton diversity, during the Expedition of Jala Citra 2 Banda 2022, phytoplankton sampling was done in the surface waters at five stations in the northern Banda Sea. The purpose of this study was to analyze the abundance and species composition of phytoplankton during the southeast monsoon season. Phytoplankton samples were taken at different mixing layers, kept them into sample bottle and were identify in the laboratory of Deep Sea Research Centre BRIN Ambon. Chemical and physical parameters were measured simultaneously. A total of 64 species of phytoplankton were found and they composed of diatom which the most diversified group with 48 species, followed by dinoflagellate with 16 species. Among diatoms, most dominant species were contributed by Thalassionema nitzschiodes (3640-1,266,573 cell/m3) and Thalassionema fraunfeldii (2800-288,570 cell/m3) followed by Chaetoceros sp and Skeletonema costatum. These dominant species composition were different from previous reports. Most of these species were found in a chain. For the dinoflagellates Ceratium longipes (280-5,231 cell/m3) dan Ceratium vulvur (280-1.960 cell/m3) were identified as the dominant species. The cell count of total phytoplankton varied from 280-1,266,573 cell/m3. Spesies diversity index was calculated using Shannon’s formula (H’) and dominance index (D) range from 1.089-2.552 and 0.117-0.418, respectively. Water temperature, salinity, pH and dissolved oxygen ranged from 26.7-29.5°C, 33-34 ppt, 7.85-8.14, and 5.57-7.03 mg/l, respectively.

Assessment of Physico-Chemical Properties and Plankton Composition of Ajiwa Reservior in Katsina State, Nigeria

Plankton composition and physicochemical properties of Ajiwa water reservoir were assessed over a twelve (12) month period. The physicochemical and biological parameters were determined using conventional methods and procedures at three sampling points. The outcome was revealed. The phytoplankton composition was Chlorophyta (57.66%), Bacillariophyta (25.70%), Cyanophyta (14.73%), and Dinophyta (1.91%), with Rotifera (30.55%), Copepoda (29.33%), Protozoa (22.27%), and Cladocera (17.85%) being the least. The temperature fluctuated with a mean SE value of (23.08 0.8OC); the pH fluctuated with a mean SE value of 6.8 0.1; and the turbidity fluctuated with a mean SE value of 99.3 3.6NTU. DO readings range from 3.8mgL-1 to 7.9mgL-1, with a mean SE of 6.6 0.3mgL-1. BOD showed monthly fluctuation, with a mean SE value of 3.2 0.4mgL-1. The electrical conductivity ranged from 102.4 µS/cmto 105.1S/cm, with a mean SE of 129.9 4.1µS/cm. The higher chlorophyta and rotifer composition in the reservoir indicated that the water quality is good, but increased human activities such as runoffs of inorganic fertilisers and pesticides may influence change in water quality over time. The reservoir water is suitable for irrigational and domestic purposes in terms of most of the physicochemical and biological parameters analysed. As a result, an effective anthropogenic inputs control strategy in the reservoir is required.

Lipids of different phytoplankton groups differ in sensitivity to degradation: Implications for carbon export.

The future of life on Earth depends on how the ocean might change, as it plays an important role in mitigating the effects of global warming. The main role is played by phytoplankton. Not only are phytoplankton the base of the oceans' food web, but they also play an important role in the biological carbon pump (BCP), the process of forming organic matter (OM) and transporting it to the deep sea, representing a sink of atmospheric CO2 . Lipids are considered important vectors for carbon sequestration. A change in the phytoplankton community composition as a result of ocean warming is expected to affect the BCP. Many predictions indicate a dominance of small at the expense of large phytoplankton. To gain insight into interplay between the phytoplankton community structure, lipid production and degradation, and adverse environmental conditions, we analyzed phytoplankton composition, particulate organic carbon (POC) and its lipid fraction in the northern Adriatic over a period from winter to summer at seven stations with a gradient of trophic conditions. We found that at high salinity and low nutrient content, where nanophytoplankton prevailed over diatoms, the newly fixed carbon is substantially directed toward the synthesis of lipids. Lipids produced by nanophytoplankton, coccolithophores, and phytoflagellates, are more resistant to degradation than those produced by diatoms. The difference in lipid degradability is discussed as a difference in the size of the cell phycosphere. We hypothesize that the lipids of nanophytoplankton are less degradable due to the small phycosphere with a poorer bacterial community and consequently a lower lipid degradation rate compared with diatoms. The lipid chemical composition of the different phytoplankton groups could have a different susceptibility to degradation. Results suggest a successful lipid carbon sink of nanophytoplankton and, thus, a negative feedback on global warming.

Complex drivers of primary production along an anthropised estuary (Seine estuary—France)

The Seine estuary is a typical model of a system in which phosphorus (P) inputs have been considerably reduced to reduce past eutrophication, with a parallel decrease in phytoplankton biomass. However, reducing P alone while concentrations of nitrate (N) remains high led to a dystrophic nutrient balance in the estuary (high N/P and N/Si). To identify the drivers of primary production and phytoplankton communities in the highly anthropised Seine estuary, sampling was performed along a 110-km stretch over a period of 3 years. Photosynthetic parameters were measured with a single turnover active fluorimeter and the phytoplankton community was assessed using a fluoroprobe and flow cytometry. The results revealed an annual primary production of 33 g C.m−2.y−1 which was largely controlled by light availability (turbidity) but also by nutrients in late spring and summer period. Massive blooms, not seen since 2002, were observed in a specific area of the estuary. None of the nutrients measured explained the particular location of the blooms but phytoplankton production and productivity indicators were higher in this specific zone excluding a biomass accumulation phenomenon. The local effect of tide slowdown increasing water residence time in this exact part of the estuary could explain the bloom area. The question thus arises: does phosphate play a role as a limiting nutrient? The diatom bloom led to total depletion of Si, but no clear limitation of P was observed. Nevertheless, the decrease in Si appears to have played a key role by triggering a succession from diatoms (microphytoplankton) to chlorophytes (nanophytoplankton) and hence affecting phytoplankton composition. The N/P balance appeared to be more important than the absolute concentration of P to explain the phytoplankton dynamics and the biomass decrease observed the last decades.

СОСТОЯНИЕ ПЛАНКТОННЫХ СООБЩЕСТВ НИКОЛЬСКОГО УСТЬЯ РЕКИ СЕВЕРНАЯ ДВИНА В ОСЕННИЙ ПЕРИОД 2022 ГОДА

The work is part of the environmental monitoring of the Nikolskaya estuary. Severnaya Dvina, carried out in 2022, with the aim of comprehensive research of aquatic biological resources and their environment. Information was obtained on the species diversity and structure of plankton communities of the studied object belonging to the drainage basin of the Severnaya Dvina River. The taxonomic composition of phytoplankton and zooplankton in the navigation period of 2022 is shown. Species diversity was assessed using the Shannon index, based on the relative abundance of species, the Menhinik, index, which determines species richness, the average abundance and biomass of phytoplankton of the Nikolsky Estuary were determined. The Pantle-Bukka saprobity index was calculated to assess the pollution of natural waters. According to observations, 53 algae taxa with a rank below the genus and 21 zooplankton organisms have been registered. It was found that the basis of algoflora was diatoms. Copepoda copepods formed the basis of the zooplankton community. New data on the state of phytoplankton and zooplankton communities can serve as an informational and methodological basis for the development of environmental monitoring programs for aquatic ecosystems in the region.

Nitrogen Reduction Causes Shifts in Winter and Spring Phytoplankton Composition and Resource Use Efficiency in a Large Subtropical Lake in China

Nitrogen Reduction Causes Shifts in Winter and Spring Phytoplankton Composition and Resource Use Efficiency in a Large Subtropical Lake in China

Seasonal variation of phytoplankton in My Thanh River, Mekong delta, Vietnam

A study on the seasonal variation of phytoplankton composition was conducted at the upper, middle, and lower parts of the My Thanh River, which supplies an important source of water for aquaculture. Qualitative and quantitative samples of phytoplankton were collected monthly at both high and low tide. The results showed that a total of 171 phytoplankton (algae) species were recorded, belonging to 59 genera and 5 phyla. Diatoms were the most abundant group with the highest species number, followed by green algae. The other phyla possessed a lower number of species. The species composition was more diverse in the rainy season and at high tide at most of the sampling sites. The mean density of algae varied from 30,900-43,521 ind.L-1. The density of diatoms was higher in the middle and lower parts. At the same time, euglenoids displayed the highest density in the upper part, showing a difference in the dominant algae group under the influence of salinity. Salinity was found to be significantly positively correlated (p&lt;0.01) with diatoms, whereas it was negatively correlated (p&lt;0.05) with blue-green algae and euglenoids. The algae composition was quite diverse, with the H’ index ranging from 2.0-3.3, showing the water quality was slightly to moderately polluted.