Phytoplankton Community Composition Research Articles

Temperature-enhanced effects of iron on Southern Ocean phytoplankton

Abstract. Iron (Fe) is a key limiting nutrient for Southern Ocean phytoplankton. Input of Fe into the Southern Ocean is projected to change due to global warming, yet the combined effects of a concurrent increase in temperature with dissolved Fe (dFe) addition on phytoplankton growth and community composition have not been extensively studied. To improve our understanding of how Antarctic phytoplankton communities respond to Fe and enhanced temperature, we performed four full factorial onboard bioassays under trace-metal-clean conditions with phytoplankton communities from different regions of the Weddell Sea and the Amundsen Sea in the Southern Ocean. Treatments consisted of 2 nM Fe addition with 2 °C warming (TF), Fe addition at in situ temperature (F) +2 °C warming with no Fe addition (T) and a control at in situ temperature with no Fe addition (control, C). Temperature had a limited effect by itself but boosted the positive response of the phytoplankton to Fe addition. Photosynthetic efficiency, phytoplankton abundances and chlorophyll a concentrations typically increased (significantly) with Fe addition (F and/or TF treatment), and the phytoplankton community generally shifted from haptophytes to diatoms upon Fe addition. The < 20 µm phytoplankton fraction displayed population-specific growth responses, resulting in a pronounced shift in community composition and size distribution (mainly towards larger-sized phytoplankton) for the F and TF treatments. Such a distinct enhanced impact of dFe supply with warming on Antarctic phytoplankton size, growth and composition will likely affect trophic transfer efficiency and ecosystem structure, with potential significance for the biological carbon pump.

Drivers of phytoplankton distribution, abundance and community composition off East Antarctica, from 55-80°E (CCAMLR Division 58.4.2 East)

Southern Ocean phytoplankton form the base of the Antarctic food web, influencing higher trophic levels through biomass and community structure. We examined phytoplankton distribution and abundance in the Indian Sector of the Southern Ocean during austral summer as part a multidisciplinary ecosystem survey: Trends in Euphausiids off Mawson, Predators and Oceanography (TEMPO, 2021). Sampling covered six meridional transects from 55-80°E, and from 62°S or 63°S to the ice edge. To determine phytoplankton groups, CHEMTAX analysis was undertaken on pigments measured using HPLC. Diatoms were the dominant component of phytoplankton communities, explaining 56% of variation in chlorophyll a (Chl a), with haptophytes also being a major component. Prior to sampling the sea ice had retreated in a south-westerly direction, leading to shorter ice-free periods in the west (< 44 days, ≤65°E) compared to east (> 44 days, ≥70°E), inducing a strong seasonal effect. The east was nutrient limited, indicated by low-iron forms of haptophytes, and higher silicate:nitrate drawdown ratios (5.1 east vs 4.3 west), pheophytin a (phaeo) concentrations (30.0 vs 18.4 mg m-2) and phaeo:Chl a ratios (1.06 vs 0.53). Biological influences were evident at northern stations between 75-80°E, where krill “super-swarms” and feeding whales were observed. Here, diatoms were depleted from surface waters likely due to krill grazing, as indicated by high phaeo:Chl a ratios (> 0.75), and continued presence of haptophytes, associated with inefficient filtering or selective grazing by krill. Oceanographic influences included deeper mixed layers reducing diatom biomass, and a bloom to the north of the southern Antarctic Circumpolar Current Front in the western survey area thought to be sinking as waters flowed from west to east. Haptophytes were influenced by the Antarctic Slope Front with high-iron forms prevalent to the south only, showing limited iron transfer from coastal waters. Cryptophytes were associated with meltwater, and greens (chlorophytes + prasinophytes) were prevalent below the mixed layer. The interplay of seasonal, biological and oceanographic influences on phytoplankton populations during TEMPO had parallels with processes observed in the BROKE and BROKE-West voyages conducted 25 and 15 years earlier, respectively. Our research consolidates understanding of the krill ecosystem to ensure sustainable management in East Antarctic waters.

Characteristics of Phytoplankton Community Structure in Lakes of Wetlands Dominated by Different Aquatic Plants and Its Driving Factors

In wetland ecosystems, small shallow lakes are critical transition zones of land and water, which are usually dominated by aquatic plants with different growth forms. However, the differences and key influencing factors of phytoplankton communities in shallow lakes dominated by different aquatic plants are unclear. On this basis, nine surveys were conducted at five sampling sites of three lakes in Zhangye National Wetland Park from June to November in 2022, which were respectively dominated by the emergent Phragmites australis （LL）, the submerged Potamogeton perfoliatus （CL）, and the floating-leaved Nymphaea tetragona （SL）. During the study period, the three lakes showed obvious habitat differences. A total of 237 species of phytoplankton in seven phyla and 93 genera were identified in the three lakes, including 189 species, 151 species, and 147 species in the LL, CL, and SL lakes, respectively. Among them, Ulnaria acus, Scenedesmus quadricauda, Achnanthidium minutissimum, Nitzschia stagnorum, Navicula radiosa, and Gymnodinium aeruginosum were shared dominant species of all three lakes, indicating that they had strong environmental adaptability, whereas Navicula lanceolala, Encyonopsis cesatii, and Eunotia diodon and Cymbella aequalis were only dominant in the CL, LL, and SL lakes, respectively. Simultaneously, these dominant algae appeared with obviously distinct statuses of niche width, niche overlap, and interspecific correlation among the three lakes. Using principal coordinate analysis （PCoA） and permutational multivariate analysis of variance （PERMANOVA）, significant differences were found in algal community composition among the three lakes （P<0.001）. Multiple regression on （dis）similarity matrices analysis （MRM） showed that the heterogeneity of phytoplankton communities among the three lakes was positively affected by NO3--N and pH and negatively affected by dissolved oxygen （DO） and was closely positively correlated with the abundance of six dominant species, namely, S. quadricauda, U. acus, N. stagnorum, Pseudoanabaena sp., Merismopedia punctata, and A. minutissimum. These results indicate that aquatic plants with different growth types could affect the composition, structure, and stability of phytoplankton communities in the same habitat with them by shaping their habitat heterogeneity. Therefore, selecting specific growth types of aquatic plants for aquatic ecosystem restoration in wetland construction and management will be conducive to regulate the state of water habitat and phytoplankton community structure effectively.

Response of dissolved organic matter to variation in nutrient availability and phytoplankton community composition in a mesotrophic, temperate-zone reservoir

Response of dissolved organic matter to variation in nutrient availability and phytoplankton community composition in a mesotrophic, temperate-zone reservoir

Hydrodynamics Mediates Biogeochemical Dynamics of Particulate Organic Matter in the Shelf of the Northern South China Sea During Summer

AbstractEnvironmental conditions, physiology and community composition of phytoplankton and the carbon and nitrogen isotope signature (δ13CPOC and δ15NPN) of particulate organic matter (POM) often covary across marine environments. However, little was known on the link of δ13CPOC and δ15NPN and the community and biochemical composition of phytoplankton. In this study, particulate organic carbon (POC) and nitrogen (PN), δ13CPOC, δ15NPN, phytoplankton community composition and biomass were determined during summer, along with environmental variables, in the shelf of the northern South China Sea influenced by the Pearl River plume, upwelling and anticyclonic eddy. Our results show that variability in δ13CPOC and δ15NPN along an environmental gradient is coupled with shifts in phytoplankton community composition and carbon to chlorophyll a (C:Chl a) ratio of phytoplankton. Low δ13CPOC values (−28.4 to −27.0‰) at nearshore stations (salinity &lt;21) were primarily due to terrestrial POM input. High δ13CPOC (&gt;−21.0‰) and δ15NPN (&gt;5.6‰) values are most likely attributed to high abundance of diatoms induced by riverine nutrients in the plume‐impacted waters with intermediate salinity (21&lt; salinity &lt;33). Low δ13CPOC (&lt;−22.0‰) and δ15NPN (−1.1–3.7‰) values are associated with high abundance of slow‐growing cyanobacteria in the oligotrophic area (salinity &gt;33), where the lowest δ15NPN is most likely attributed to high abundance of N2‐fixing Trichodesmium spp., due to the influence of the anticyclonic eddy. Therefore, hydrodynamics modulates the biochemical composition and community composition of phytoplankton, leading to changes in δ13CPOC and δ15NPN. Our findings advance our understanding of the coupling of physical and biogeochemical processes in marginal seas.

Negative effects of cigarette butt leachate on freshwater phytoplankton communities.

We experimentally tested the effects of different concentrations of cigarette butt leachate on freshwater phytoplankton chlorophyll-a, species richness, cell density, and community composition. For this, we sampled the phytoplankton from a eutrophic lake and acclimated it for 24 h in microcosms. We then conducted the experiment in microcosms maintained for 96 h. The experiment consisted of four treatments: control and leachate from 1 butt L-1 (T1), 5 butts L-1 (T5), and 10 butts L-1 (T10), which were prepared by diluting a stock solution of leachate from 50 butts L-1. We found that algal chlorophyll-a content was not affected by different leachate concentrations. In contrast, phytoplankton cell density decreased in a dose-dependent manner as concentrations of the leachate increased. Similarly, the number of species was highest in the control group relative to all other treatments, with T1 and T5 showing higher species richness than T10. Additionally, the exposition to different concentrations of the leachate impacted community composition across all treatments in comparison to the control group. Our results suggest that cigarette butt leachate alters the number of cells and species, as well as the distribution of abundance, without necessarily reducing chlorophyll-a concentrations. Our findings indicate that to gain a comprehensive understanding of the effects of cigarette butt leachate on freshwater ecosystems, it is essential to evaluate more realistic scenarios that incorporate aquatic communities, rather than isolated species.

Effect of Rice–Carp Coculture on Phytoplankton and Microzooplankton Community Composition in Paddy Water during Different Rice Growth Stages

Integrated rice–fish farming, an agricultural practice that combines cultivating rice and breeding fish in the same field, has attracted widespread attention. However, there is limited research on how the rice–carp coculture impacts the community structure of phytoplankton and microzooplankton in paddy water. This study employed eDNA metabarcoding sequencing to analyze the composition of phytoplankton and microzooplankton in a rice monoculture system (RM) and a rice–carp coculture system (RF). Following annotation, we identified 9 phyla, 89 families, 275 genera, and 249 species of phytoplankton, along with 20 phyla (or subphyla and classes), 85 families, 222 genera, and 179 species of microzooplankton. The alpha diversity indices revealed significantly higher richness, diversity, and evenness in the RF group compared to the RM group during grain-filling stage. Principal coordinates analysis (PCoA) demonstrated notable differences in the phytoplankton and microzooplankton compositions between the two groups across various rice growth stages. Composition analysis showed that rice–carp coculture increased the relative abundance of dominant phytoplankton phyla such as Bacillariophyta, Chrysophyta, and Euglenophyta while decreasing that of Cryptophyta. In microzooplankton, the coculture resulted in an increased abundance of Intramacronucleata (subphylum) and a decrease in Conoidasida (class). In conclusion, the rice–carp coculture enhances the diversity of plankton, particularly during the grain-filling stage, and simultaneously alters the composition and abundance of dominant plankton species in the paddy water. These findings enhance understanding of the broader impacts of integrated rice–carp farming on agricultural ecosystems, emphasizing alterations in the diversity and composition of aquatic microorganisms

An evaluation of cyanobacterial occurrence and bloom development in Adirondack lakes

Cyanobacterial harmful algal blooms (cyanoHABs) have occurred in many low nutrient (oligotrophic) lakes in the northeastern United States. The Adirondack Park in New York is a large, mountainous region with many low nutrient lakes. There is a gap in understanding regarding whether cyanoHAB reporting data are truly reflective of the susceptibility of lakes to develop bloom conditions. We evaluated lakes with and without documented cyanoHABs for cyanotoxin synthetase gene quantification, phytoplankton community composition, and akinete abundance to identify conditions associated with the observation of cyanoHABs. We analyzed (1) contributions of cyanobacteria to the overall phytoplankton community, (2) differences in cyanobacterial communities and the presence of cyanotoxin synthetase genes, and (3) lake physical and geomorphological attributes as drivers of differences in cyanobacteria occurrence. Two sample types (water and sediment) were collected from 2 sample locations (nearshore and open water) in 5 lakes in 2021. We found cyanobacteria in all lakes and sample locations. Phytoplankton biovolume and cyanotoxin synthetase genes differed among lakes and by cyanoHAB history. Samples from lakes with documented blooms were associated with marginally higher total phosphorus. Nonmetric multidimensional scaling was used to identify which environmental factors influenced community structure. Our study demonstrates the importance of multifaceted approaches to detect cyanobacteria that may only be apparent during ephemeral bloom events and the similarities among lakes with and without a history of bloom reports. This work contributes to a better understanding of cyanoHAB occurrence in Adirondack lakes, and of conditions that may cause low nutrient lakes to be susceptible to cyanoHABs.

Phytoplankton community composition as a driver of annual autochthonous organic carbon dynamics in the northern coastal Baltic Sea

Phytoplankton are the major primary producers in the pelagic system. They greatly influence biogeochemical cycles but little is known about the importance of shifting phytoplankton community composition for carbon dynamics. This study investigates the impact of seasonal changes in coastal phytoplankton communities on pelagic carbon fluxes. A field sampling campaign, covering an annual cycle in primary production, was conducted to assess the seasonal changes of phytoplankton communities and relevant organic carbon parameters in the coastal Baltic Sea. The monitoring frequency ranged from 1 to 3 wk, adapted to match the seasonal phytoplankton blooms. In addition, sediment traps were deployed to determine the particulate carbon and nutrient export to the seafloor in every season. We found that the phytoplankton biomass during the spring bloom was as high as 550 µg C l-1 and was dominated by diatom species (88% of total phytoplankton biomass). In comparison, the more species-rich summer bloom reached a combined maximum biomass of 236 µg C l-1. However, the highest export flux of particulate organic carbon was found in the middle of August (561 mmol C m-2 d-1) and, not as expected, around the spring bloom in May (226 mmol C m-2 d-1), suggesting a high potential for carbon recycling within the pelagic food web rather than being exported to the seafloor or advected laterally. Our study emphasizes the importance of keystone species and diversity for carbon transport processes in marine coastal ecosystems and highlights complex relationships between phytoplankton biomass production, community composition and carbon dynamics.

Seasonal trends in Subantarctic plankton δ13C and δ15N are driven by phytoplankton dynamics and nutrient preference

AbstractThe carbon and nitrogen isotope ratios (δ13C and δ15N) of marine plankton record biogeochemical processes at the base of the food web. In the Southern Ocean, such data, predominantly from summer, have been used to infer surface CO2 concentrations and the potential for biological carbon export. However, variability in plankton δ13C and δ15N remains poorly understood, with the lack of seasonal measurements from the Southern Ocean emerging as a particular limitation. Here, we investigate the δ13C and δ15N of suspended particulate matter (SPM) and zooplankton collected from the Subantarctic Ocean in winter, summer, and autumn. The low summertime δ13CSPM and δ15NSPM can be explained by strong diatom reliance on nitrate supplied during winter mixing. A subsequent increase in δ13CSPM and decrease in δ15NSPM by autumn is consistent with iron limitation in mid‐ to late‐summer favoring diatom succession by nano‐phytoplankton that consume mainly recycled ammonium. By winter, bacterial decomposition of biomass outpaces new biomass production, decreasing δ13CSPM and raising δ15NSPM. The δ13C and δ15N of contemporaneously sampled zooplankton generally reflect the variability in δ13CSPM and δ15NSPM, with one autumn mismatch suggesting that in situ SPM is not always the dominant zooplankton food source. Our study shows how nutrient dynamics and phytoplankton community composition shape the seasonality of the Subantarctic's isotopic baselines, emphasizing a key role for iron availability. This work has implications for isotope‐based food web studies, highlighting the need to consider seasonal variability in δ13CSPM and δ15NSPM, as well as the different turnover times of phytoplankton (i.e., SPM) vs. zooplankton biomass.

Stable phytoplankton community compositions in Lake Mead (Nevada-Arizona, USA) during two decades of severe drought

Lake Mead, a large reservoir on the Colorado River and a critical drinking water source for the southwestern United States, typically exhibits high water quality, characterized by low nutrient and chlorophyll-a concentrations. This stability persists despite the inflow of highly treated wastewater since the 1960s and significant water level declines since 2000, driven by the ongoing Megadrought and basin-wide consumptive use. Such environmental changes may alter phytoplankton communities, potentially leading to increased cyanobacteria abundance, which could negatively impact water quality and the aquatic ecosystem through harmful algal blooms and toxin production. Here we analyzed 17 years of phytoplankton community structure and chlorophyll-a concentrations in Lake Mead, alongside quantitative water quality data, including nutrients, temperature, and water clarity, to assess the effects of environmental changes on phytoplankton communities. Contrary to the hypothesis that cyanobacteria abundance would have increased throughout the reservoir, our results indicate that phytoplankton community structures have remained largely stable, except for shallow areas where increases in temperature or phosphorus levels were observed. Additionally, we evaluated machine learning models for predicting changes in phytoplankton community structures. While the models confidently predicted changes in total phytoplankton biovolume and chlorophyll-a concentrations within the input parameter boundaries, predictions of peak biovolume showed considerable uncertainty, emphasizing the importance of incorporating uncertainty analysis in forecasting and communicating results. This study underscores the current buffering capacity of large, oligotrophic reservoirs like Lake Mead to maintain stable phytoplankton communities despite environmental changes. However, it also highlights the potential for significant community shifts if this buffering capacity is exceeded.

Phytoplankton communities quantified from hyperspectral ocean reflectance correspond to pigment-based communities

Community detection analysis is a powerful tool to separate groups of samples that are similar based on their composition. Here, we use a paired global dataset of in-water hyperspectral remote sensing reflectance (Rrs) spectra and high-performance liquid chromatography (HPLC) pigment concentrations to investigate the similarity in phytoplankton composition of the communities detected from each method. Samples were separated into optical communities using network-based community detection analysis applied to the Rrs residual (δRrs), which is calculated by subtracting a modeled hyperspectral Rrs spectrum from a measured hyperspectral Rrs spectrum. The δRrs spectrum accentuates short spectral scale features (&lt;=10 nm) that should be related to phytoplankton pigment composition metricsTo test whether these optical communities correspond to phytoplankton communities, we also used network-based community detection analysis to separate HPLC pigment-based communities from twelve accessory pigment ratios to total chlorophyll-a. Our results demonstrate that three distinct phytoplankton communities can be separated from both hyperspectral Rrs data and HPLC pigment data and that a majority of these samples (74%) were assigned to the same communities. Differences in community assignment were also identified and potential sources for discrepancies were discussed. Importantly, the optical communities assigned here offer a new tool for assessing phytoplankton community composition on global scales using hyperspectral Rrs observations, such as those provided by the Ocean Color Instrument (OCI) on NASA’s new Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite.

Methylparaben changes the community composition, structure, and assembly processes of free-living bacteria, phytoplankton, and zooplankton

Parabens are common contaminants in river and lake environments. However, few studies have been conducted to determine the effects of parabens on bacteria, phytoplankton, and zooplankton communities in aquatic environments. In this study, the effect of methylparaben (MP) on the diversity and community structure of the aquatic plankton microbiome was investigated by incubating a microcosm with MP at 0.1, 1, 10, and 100 μg/L for 7 days. The results of the Simpson index showed that MP treatment altered the α-diversity of free-living bacteria (FL), phytoplankton, and zooplankton but had no significant effect on the α-diversity of particle-attached bacteria (PA). Further, the relative abundances of the sensitive bacteria Chitinophaga and Vibrionimonas declined after MP addition. Moreover, the relative abundances of Desmodesmus sp. HSJ717 and Scenedesmus armatus, of the phylum Chlorophyta, were significantly lower in the MP treatment group than in the control group. In addition, the relative abundance of Stoeckeria sp. SSMS0806, of the Dinophyta phylum, was higher than that in the control group. MP addition also increased the relative abundance of Arthropoda but decreased the relative abundance of Rotifera and Ciliophora. The β-diversity analysis showed that FL and phytoplankton communities were clustered separately after treatment with different MP concentrations. MP addition changed community assembly mechanisms in the microcosm, including increasing the stochastic processes for FL and the deterministic processes for PA and phytoplankton. Structural equation modeling analysis showed a significant negative relationship between bacteria richness and phytoplankton richness, and a significant positive relationship between phytoplankton (richness and community composition) and zooplankton. Overall, this study emphasizes that MP, at environmental concentrations, can change the diversity and structure of plankton microbial communities, which might have a negative effect on ecological systems.

Utilization of rice husk ash as a potential catalyst for diatom growth and adsorbent for heavy metals in aquaculture systems

Many aquaculture environments suffer from significant silicon deficiencies and severe heavy metal contamination, posing substantial risks to both production and safety in aquaculture. This study aimed to assess the release of silicate from rice husk ash (RHA) and its impact on phytoplankton abundance and community composition, which are key factors affecting shellfish growth and survival. Additionally, the capacity of RHA to adsorb mercury (Hg2+) was examined using different modification methods. The results demonstrated that the concentration of dissolved silicate in seawater increased with higher RHA doses, reaching a maximum of 189.77 ± 45.61 μmol L−1 in the high RHA treatment (HR) with 1.0 g L−1 of RHA. Notably, the HR treatment group exhibited a significant increase in chlorophyll-a concentration for nano-phytoplankton (2–20 μm), with an up to eightfold rise in abundance compared to the control group. Moreover, the HR treatment resulted in a maximum diatom abundance of 4217 ± 741.98 cells L−1, benefiting diatom species such as Coscinodiscus sp., Nitzschia sp., and Cyclotella sp. These findings underscore the potential of RHA as a catalyst for diatom growth, which in turn supports the growth of shellfish. Furthermore, no concentrations of mercury (Hg), lead (Pb), cadmium (Cd), arsenic (As), copper (Cu), zinc (Zn), and chromium (Cr) were detected in any of the silicate enrichment treatments, confirming the safety of the RHA. The treatment of RHA with NaOH achieved an impressive removal efficiency of Hg2+, reaching up to 97.41 ± 0.33 %. This study highlights the promising application of agricultural waste RHA as a catalyst for diatom growth and an adsorbent for heavy metals in water systems relevant to aquaculture practices.

Responses of coastal phytoplankton communities to seasonal herbicide inputs: Tolerance or degeneration?

Herbicide-induced phytoplankton inhibition threatens coastal biodiversity and ecosystem function. Although studies employing single-frequence exposure aid in understanding the phytoplankton community's responses to herbicides, it’s difficult to objectively assess their response to cyclic herbicide inputs (long-term low-dose and short-term high-dose) in marine ecosystems. Here, we analyzed the concentration and distribution of herbicides in global coastal waters and simulated this cyclic process through a two-phase atrazine exposure mesocosm experiment and laboratory tests. The results indicated that, the herbicide concentrations (0.82 nmol L−1, 95 % CI 0.55, 1.74) from May to August were significantly higher than that (0.14 nmol L−1, 95 % CI 0.02, 0.38) in the remainder months, and highest concentrations typically emerged in summer; the changes in phytoplankton community composition under environmental concentrations of triazine herbicides could recover in the short term, but sustained inhibition of biomass was produced; the dominant populations were more likely to develop tolerance through preexposure and recover from subsequent impulse of atrazine, but this process was accompanied by the loss of rare groups and a decrease in biodiversity, meanwhile, affected the bacterial community in phycosphere. Consequently, we considered that the cyclic herbicide inputs may cause more detrimental effects than single-frequence exposure, potentially leading to a large-scale decline in coastal primary productivity.

Microbial assessment of the ecological linkage between a red tide of Karenia brevis and bottom water anoxia off the coast of Fort Myers and Sanibel Island, Florida

Coastal ecosystem management necessitates a comprehensive understanding of the physical, chemical, and biological components and their interplay with socio-economic activities. Red tides of Karenia brevis reoccur almost annually on the West Florida Shelf and cause significant human and environmental health problems that often result in negative economic impacts. It is crucial that we understand the processes and dynamics of both phytoplankton and microbial communities as conditions necessary for the formation of hypoxia and anoxia in the coastal ecosystem. With the aim of identifying key microbial communities responsible for anoxia, we investigated the 2017–2018 Karenia bloom. Various water quality parameters, phytoplankton, and microbial community compositions were studied at eight sites on the West Florida Shelf. K. brevis densities were highest at the site of the plume of the Caloosahatchee River. At the site with the highest K. brevis abundance (6.3 ×106 cells/L), dissolved oxygen (DO) was supersaturated (303 %) at the surface, and the chlorophyll a concentration was 118 µg/L. The highest bacterial cell count (9.0 ×106 cells/mL) and the lowest bacterial diversity were also observed at this site. The bacterial community determined by high-throughput amplicon sequencing at this bloom site was unique and dominated by Bacteroidetes, which occupied 65 % of the relative abundance of the microbial community. The bottom water was generally hypoxic (mean ± SD: 0.98 ± 1.13 mg/L), with select stations having DO as low as 0.06 mg/L (anoxic). The anoxic bottom water harbored unique microbial taxa that were associated with the low oxygen conditions. Significant negative correlations were found between DO and the relative abundance of Deltaproteobacteria, Firmicutes, Spirochaetes, and unclassified Proteobacteria. Our investigation lays the groundwork for subsequent studies delving into the responses of phytoplankton and microbial communities to disturbances caused by Karenia blooms, as well as future strategies for mitigation and ecosystem restoration.

QPCR-based phytoplankton abundance and chlorophyll a: A multi-year study in twelve large freshwater rivers across the United States

Phytoplankton overgrowth, which characterizes the eutrophication or trophic status of surface water bodies, threatens ecosystems and public health. Quantitative polymerase chain reaction (qPCR) is promising for assessing the abundance and community composition of phytoplankton. However, applications of qPCR to indicate eutrophication and trophic status, especially in lotic systems, have yet to be comprehensively evaluated. For the first time, this study correlates qPCR-based phytoplankton abundance with chlorophyll a (the most widely used indicator of eutrophication and trophic status) in multiple freshwater rivers. From early summer to late fall in 2017, 2018, and 2019, we evaluated phytoplankton, chlorophyll a, pheophytin a, and the Trophic Level Index (TLI) in twelve large freshwater rivers in three regions (western, midcontinent, and eastern) in the United States. qPCR-based phytoplankton abundance had positive allometric correlations with chlorophyll a concentration (adjusted R2 = 0.5437, p-value < 0.001), pheophytin a concentration (adjusted R2 = 0.3378, p-value <0.001), and TLI (adjusted R2 = 0.4789, p-value < 0.001). Thus, a greater phytoplankton abundance suggests a higher trophic status. This work also presents the numerical values of qPCR-based phytoplankton abundance defining the boundaries among trophic statuses (e.g., oligotrophic, mesotrophic, and eutrophic) of freshwater rivers. The sampling sites in the midcontinent rivers were more eutrophic because they had significantly higher chlorophyll a concentrations, pheophytin a concentrations, and TLI values than the sites in the western and eastern rivers. The higher phytoplankton abundance at the midcontinent sites confirmed their higher trophic status. By linking qPCR-based phytoplankton abundance to chlorophyll a, this study demonstrates that qPCR is a promising avenue to investigate the population dynamics of phytoplankton and the trophic status (or eutrophication) of freshwater rivers.

Integrating pigment and fatty acid profiles for enhanced estimation of seston community composition

AbstractClimate change, nutrition pollution, and land use alterations influence the primary production of lakes. While light‐microscopy counting remains the standard for estimating phytoplankton community composition, its expense and time‐consuming nature necessitate cost‐effective alternatives for seston analysis. Furthermore, estimating the contribution of seston constituents other than primary producers, or non‐algal particles, is not possible with light‐microscopy counting. Biotracer approach using computational methods and chemotaxonomic biomarkers such as carotenoid pigments and fatty acids have been used as an alternative in seston analysis when species‐level taxonomy is not required. However, a comprehensive testing of how well carotenoid and fatty acids can be used in estimating a wide range of seston phytoplankton communities using different estimation methods is lacking. To assess the accuracy of a suite of state‐of‐the‐art biotracer‐based computational methods, namely CHEMTAX, FASTAR (Fatty Acid Source‐Tracking Algorithm in R), MixSIAR, and QFASA (Quantitative Fatty Acid Signature Analysis), lake water samples were collected in 2016, 2018, 2019, 2020, and 2021 for seston composition analysis in a boreal eutrophic lake with light‐microscopy counting serving as the reference for seston composition. Absolute errors between the biotracer‐based estimates were calculated to evaluate method performance. A small laboratory experiment to assess the reliability of estimating the contribution of non‐algal particles using the computational methods with fatty acids was also conducted. The closest alignment to light‐microscopy counting in terms of absolute error was achieved when both carotenoids and fatty acids were used together in the QFASA method. For CHEMTAX, FASTAR, and MixSIAR, using carotenoids alone produced the closest results. Additionally, the estimation methods accurately assessed the proportion of non‐algal particles in the seston when using fatty acid profiles, a capability not possible with light‐microscopy counting. Our findings demonstrate that the biotracer approach provides a viable and cost‐effective alternative to light‐microscopy counting when group‐level information of phytoplankton community composition suffices. Furthermore, we show that non‐algal particles can be effectively estimated together with phytoplankton when using fatty acids.

Differential association of key bacterial groups with diatoms and Phaeocystis spp. during spring blooms in the Southern Ocean.

Interactions between phytoplankton and heterotrophic bacteria significantly influence the cycling of organic carbon in the ocean, with many of these interactions occurring at the micrometer scale. We explored potential associations between specific phytoplankton and bacteria in two size fractions, 0.8-3 µm and larger than 3 µm, at three naturally iron-fertilized stations and one high nutrient low chlorophyll station in the Southern Ocean. The composition of phytoplankton and bacterial communities was determined by sequencing the rbcL gene and 16S rRNA gene from DNA and RNA extracts, which represent presence and potential activity, respectively. Diatoms, particularly Thalassiosira, contributed significantly to the DNA sequences in the larger size fractions, while haptophytes were dominant in the smaller size fraction. Correlation analysis between the most abundant phytoplankton and bacterial operational taxonomic units revealed strong correlations between Phaeocystis and picoeukaryotes with SAR11, SAR116, Magnetospira, and Planktomarina. In contrast, most Thalassiosira operational taxonomic units showed the highest correlations with Polaribacter, Sulfitobacteria, Erythrobacter, and Sphingobium, while Fragilariopsis, Haslea, and Thalassionema were correlated with OM60, Fluviicola, and Ulvibacter. Our in-situ observations suggest distinct associations between phytoplankton and bacterial taxa, which could play crucial roles in nutrient cycling in the Southern Ocean.

Nitrate Uptake and Primary Production Along the Amazon River Plume Continuum

AbstractThe Amazon River plume (ARP) has been shown to support high rates of nitrogen fixation and primary production. However, nitrogen fixation alone cannot account for total primary production determined in the region, hinting that other nitrogen uptake processes might play a role. For the first time, we measured nitrate uptake rates in the ARP during three cruises in May 2018, June 2019 and April/May 2021, along with primary production rates and an analysis of phytoplankton community composition via high performance liquid chromatography. Based on a classification according to the salt content the region was divided into estuarine (ES), mesohaline (MH) and oceanic (OC) stations. Primary production was light limited near the river mouth at ES stations and was maximal off the coasts of French Guiana and Suriname, where also nitrate uptake was highest with rates of 11.4 mmol m−2 d−1. The role of eddies pinching off a deflecting plume are discussed as possible reason for higher nutrient concentrations at the MH stations. Surprisingly, at most MH stations north of 5°N, nitrate uptake rates were low despite the presence of sufficient substrate concentration (up to 1.44 μM nitrate). Diatoms, dinoflagellates or Synechococcus sp. dominated phytoplankton communities. OC stations showed lowest productivity rates in accordance with oligotrophic conditions. However, rates seem to be sufficient to completely deplete the remaining riverine nitrate, preventing its export to the open ocean.