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1. Phytoplankton group-specific and community responses to co-limiting nutrients in a managed reservoir: Lake Murray, South Carolina

   https://doi.org/10.1093/plankt/fbaf003  

   Durbin, Haley; Cain, Ainsley; Madgett, Gavin; Knotts, Eilea; Pinckney, James (February 2025, Journal of Plankton Research)

   Beisner, Beatrix E (Ed.)

   Abstract Eutrophication is increasingly becoming a problem for freshwater lakes. We evaluated the effects of additions nitrate (N as NO3−) and phosphate (P as PO43−) on phytoplankton in a temperate lake reservoir (Lake Murray, South Carolina). High-performance liquid chromatography and ChemTax were used to measure concentrations of microalgal groups in the lake in 2021–2023 and bioassays. The phytoplankton community during the summer months consisted of green algae (37%), diatoms (27%), cryptophytes (20%), cyanobacteria (11%) and dinoflagellates (4%). Bioassays of N (20-μM NaNO3), P (10-μM KH2PO4) and N + P additions were conducted monthly from April to October 2023. All microalgal groups, except cyanobacteria, exhibited nutrient co-limitation with N as the primary limiting nutrient. Similarly, cyanobacteria exhibited co-limitation, but with P as the primary limiting nutrient. Nutrient additions of N + P (but not N or P singularly) also resulted in significant community shifts, with a strong response by green algae. The management implications for this study are that increases in N and P loading and ratio changes in the lake may result in major phytoplankton community changes toward dominance by green algae. However, increasing P loading relative to N may promote cyanobacterial growth over other phytoplankton groups in this lake system. 

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2. Low diversity of a key phytoplankton group along the West Antarctic Peninsula

   https://doi.org/10.1002/lno.11765  

   Brown, Michael S.; Bowman, Jeff S.; Lin, Yajuan; Feehan, Colette J.; Moreno, Carly M.; Cassar, Nicolas; Marchetti, Adrian; Schofield, Oscar M. (May 2021, Limnology and Oceanography)

   The West Antarctic Peninsula (henceforth “Peninsula”) is experiencing rapid warming and melting that is impacting the regional marine food web. The primary phytoplankton groups along the Peninsula are diatoms and cryptophytes. Relative to diatoms, there has been little focus on regional cryptophytes, and thus our understanding of their diversity and ecology is limited, especially at the species level. This gap is important, as diatoms and cryptophytes play distinct roles in the regional marine food web and biogeochemistry. Here, we use a phylogenetic placement approach with 18S rRNA gene amplicon sequence variants to assess surface ocean cryptophyte diversity and its drivers at a high taxonomic resolution along the Peninsula. Data were collected over 5 years (2012–2016) during the regional research cruises of the Palmer Long‐Term Ecological Research program. Our results indicate that there are two major cryptophyte taxa along the Peninsula, consisting of distinctGeminigeraspp., which in aggregate always comprise nearly 100% of the cryptophyte community (indicating low taxa evenness). The primary taxon dominates the cryptophyte community across all samples/years, which span a broad range of oceanographic conditions. A shift in cryptophyte community composition between a lower (higher) primary (secondary) taxon percentage is associated with distinct oceanographic conditions, including lower (higher) temperature, salinity, nutrients, and cryptophyte relative abundance (phytoplankton biomass and diatom relative abundance). These results emphasize the need for a full characterization of the ecology of these two taxa, as it is predicted that cryptophytes will increase along the Peninsula given projections of continued regional environmental change. 

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3. Shore and mid-channel surveys reveal distinct phytoplankton-bacterial population associations along an urban estuary

   https://doi.org/10.3354/ame02012  

   Humphries, GE; Ambrosone, M; Roldan-Ayala, Z; Brown, M; Greenfield, DI (September 2024, Aquatic Microbial Ecology)

   A growing body of literature has highlighted the importance of phytoplankton-bacterial associations to marine and estuarine ecological and biogeochemical function, but their population linkages remain sparsely characterized within urban estuaries. Since many developed coastlines are heavily impacted by anthropogenic nutrient inputs, elucidating their phytoplankton-bacterial dynamics provides insight into nutrient cycling, productivity, and can help inform water quality management. This study compared surface (0.5 m depth) physical water quality, cell abundances of major phytoplankton taxa and bacteria, as well as concentrations of chlorophylla(chla) and dissolved organic matter (DOM) in the nitrogen (N)-enriched Western Long Island Sound (WLIS), USA, between mid-channel and shore sites (in 2020 and 2021). Shore bacterial and phytoplankton abundances as well as DOM concentrations (primarily dissolved organic N and carbon [DOC]), were significantly higher than mid-channel, especially during summer, indicative of terrestrial loading influencing microbial assemblages as well as N and C cycling. Abundances of key phytoplankton taxa were better indicators of bacterial abundances than chla, as bacterial abundances positively and significantly correlated with those of dinoflagellates, especially the most common generaProrocentrum(mid-channel, shore) andHeterocapsa(shore only), but not with diatoms. However, pennate diatom abundances negatively and significantly correlated with DOC concentrations in the mid-channel. Results highlight the impact of terrestrial inputs on WLIS microbial assemblage dynamics, presumably by favoring bacteria and dinoflagellate population coupling, as well as shed new ecological insight into how phytoplankton and bacterial communities respond to nutrient loadings in urban estuaries. 

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4. The interaction of physical and biological factors drives phytoplankton spatial distribution in the northern California Current

   https://doi.org/10.1002/lno.11431  

   Krause, Jeffrey W.; Brzezinski, Mark A.; Largier, John L.; McNair, Heather M.; Maniscalco, Michael; Bidle, Kay D.; Allen, Andrew E.; Thamatrakoln, Kimberlee (February 2020, Limnology and Oceanography)

   Abstract Transitions in phytoplankton community composition are typically attributed to ecological succession even in physically dynamic upwelling systems like the California Current Ecosystem (CCE). An expected succession from a high‐chlorophyll (~ 10μg L⁻¹) diatom‐dominated assemblage to a low‐chlorophyll (< 1.0μg L⁻¹) non‐diatom dominated assemblage was observed during a 2013 summer upwelling event in the CCE. Using an interdisciplinary field‐based space‐for‐time approach leveraging both biogeochemical rate measurements and metatranscriptomics, we suggest that this successional pattern was driven primarily by physical processes. An annually recurring mesoscale eddy‐like feature transported significant quantities of high‐phytoplankton‐biomass coastal water offshore. Chlorophyll was diluted during transport, but diatom contributions to phytoplankton biomass and activity (49–62% observed) did not decline to the extent predicted by dilution (18–24% predicted). Under the space‐for‐time assumption, these trends infer diatom biomass and activity and were stimulated during transport. This is hypothesized to result from decreased contact rates with mortality agents (e.g., viruses) and release from nutrient limitation (confirmed by rate data nearshore), as predicted by the Disturbance‐Recovery hypothesis of phytoplankton bloom formation. Thus, the end point taxonomic composition and activity of the phytoplankton assemblage being transported by the eddy‐like feature were driven by physical processes (mixing) affecting physiological (release from nutrient limitation, increased growth) and ecological (reduced mortality) factors that favored the persistence of the nearshore diatoms during transit. The observed connection between high‐diatom‐biomass coastal waters and non‐diatom‐dominated offshore waters supports the proposed mechanisms for this recurring eddy‐like feature moving seed populations of coastal phytoplankton offshore and thereby sustaining their activity. 

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5. Whole-Ecosystem Experiments Reveal Varying Responses of Phytoplankton Functional Groups to Epilimnetic Mixing in a Eutrophic Reservoir

   https://doi.org/10.3390/w11020222  

   Lofton, Mary; McClure, Ryan; Chen, Shengyang; Little, John; Carey, Cayelan (February 2019, Water)

   Water column mixing can influence community composition of pelagic phytoplankton in lakes and reservoirs. Previous studies suggest that low mixing favors cyanobacteria, while increased mixing favors green algae and diatoms. However, this shift in community dominance is not consistently achieved when epilimnetic mixers are activated at the whole-ecosystem scale, possibly because phytoplankton community responses are mediated by mixing effects on other ecosystem processes. We conducted two epilimnetic mixing experiments in a small drinking water reservoir using a bubble-plume diffuser system. We measured physical, chemical, and biological variables before, during, and after mixing and compared the results to an unmixed reference reservoir. We observed significant increases in the biomass of cyanobacteria (from 0.8 ± 0.2 to 2.4 ± 1.1 μg L−1, p = 0.008), cryptophytes (from 0.7 ± 0.1 to 1.9 ± 0.6 μg L−1, p = 0.003), and green algae (from 3.8 to 4.4 μg L−1, p = 0.15) after our first mixing event, likely due to increased total phosphorus from entrainment of upstream sediments. After the second mixing event, phytoplankton biomass did not change but phytoplankton community composition shifted from taxa with filamentous morphology to smaller, rounder taxa. Our results suggest that whole-ecosystem dynamics and phytoplankton morphological traits should be considered when predicting phytoplankton community responses to epilimnetic mixing. 

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