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1. Seasonal variability of the microzooplankton biomass and community composition on the Northeast US Shelf

   https://doi.org/10.1093/plankt/fbag018  

   Lopez, Frankie; Marrec, Pierre; Menden-Deuer, Susanne (February 2026, Journal of Plankton Research)

   Dam, Hans (Ed.)

   Abstract Grazing on phytoplankton by microzooplankton is a critical process in making energy available to higher trophic levels, shaping marine food webs, and biogeochemical cycling. Despite its key role, the drivers of grazing remain poorly constrained due to the concurrent influences of biotic and abiotic factors. To elucidate the impact of microzooplankton biomass and community composition on grazing rates, we measured ciliate and dinoflagellate biomass in boreal summer and winter from 2018-2023, at three stations spanning the Northeast US Shelf from coastal Rhode Island, USA to the shelf break. This record of microzooplankton biomass encompasses the seasonal and spatial variability from shore to shelf. Dinoflagellate biomass showed a clear seasonal shift from larger (>20 μm) cells in winter to smaller ones in summer, while ciliate biomass lacked seasonal trends. Anomalies in community composition coincided with disruptions in the typical seasonal phytoplankton size structure, suggesting that microzooplankton size structure was linked to the size of available prey. In winter, microzooplankton biomass was positively correlated with microzooplankton grazing, indicating that the mechanism moderating the planktonic food web structure changes seasonally. These results inform our understanding of planktonic food webs by identifying the linkage between grazing and seasonal changes in phytoplankton community structure that impact energy transfer to higher trophic levels. 

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2. Microbial food web dynamics in tropical waters of the bluefin tuna spawning region off northwestern Australia

   https://doi.org/10.1016/j.dsr2.2025.105547  

   Landry, Michael R; Stukel, Michael R; Yingling, Natalia; Selph, Karen E; Kranz, Sven A; Fender, Christian K; Swalethorp, Rasmus; Bhabu, Ria I (December 2025, Deep Sea Research Part II: Topical Studies in Oceanography)

   Whereas recruitment success for many fisheries depends on coincident timing of larvae with abundance peaks of their prey, less can be more in the tropical/subtropical spawning areas of bluefin tunas if lower but steady food resources are offset by reduced larval vulnerability to pelagic predators. To understand larval habitat characteristics for Southern Bluefin Tuna (SBT), we quantified microbial community carbon flows based on growth and grazing rates from depth profiles of dilution incubations and carbon biomass assessments from microscopy and flow cytometry (FCM) during their peak spawning off NW Australia (Indian Ocean) in February 2022. Two Chla-based estimates of phytoplankton production gave differing offsets due to cycling or mixotrophy, exceeding 14C net community production on average (677 ± 98 versus 447 ± 43 mg C m−2 d−1). Productivity was higher than in the Gulf of Mexico spawning area for Atlantic Bluefin Tuna but less than similar studies of oceanic upwelling regions. Microzooplankton grazing averaged 482 ± 63 mg C m−2 d−1 (71 ± 13 % of production). Two measurement variables for Prochlorococcus gave average production and grazing rates of 282 ± 36 and 248 ± 32 mg C m−2 d−1 (86 ± 6 % grazed). Prochlorococcus comprised almost half of production and grazing fluxes in the upper (0–25 m) euphotic zone where SBT larvae reside. Prochlorococcus declined and eukaryotic phytoplankton and heterotrophic bacteria increased in relative importance in the lower euphotic zone. These results describe relatively classic open-ocean oligotrophic conditions as the food web base for nutritional flows to SBT larvae. 

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3. Size‐specific grazing and competitive interactions between large salps and protistan grazers

   https://doi.org/10.1002/lno.11770  

   Stukel, Michael R.; Décima, Moira; Selph, Karen E.; Gutiérrez‐Rodríguez, Andres (January 2021, Limnology and Oceanography)

   null (Ed.)

   We investigated competition between Salpa thompsoni and protistan grazers during Lagrangian experiments near the Subtropical Front in the southwest Pacific sector of the Southern Ocean. Over a month, the salp community shifted from dominance by large (>100-mm) oozooids and small (<20-mm) blastozooids to large (~60-mm) blastozooids. Phytoplankton biomass was consistently dominated by nano- and microphytoplankton (>2 µm cells). Using bead-calibrated flow-cytometry light scatter to estimate phytoplankton size, we quantified size-specific salp and protistan zooplankton grazing pressure. Salps were able to feed at a >10,000:1 predator:prey size (linear-dimension) ratio. Small blastozooids efficiently retained cells >1.4-μm (high end of picoplankton size, 0.6-2 µm cells) and also obtained substantial nutrition from smaller bacteria-sized cells. Larger salps could only feed efficiently on >5.9-μm cells and were largely incapable of feeding on picoplankton. Due to the high biomass of nano- and microphytoplankton, however, all salps derived most of their (phytoplankton-based) nutrition from these larger autotrophs. Phagotrophic protists were the dominant competitors for these prey items and consumed approximately 50% of the biomass of all phytoplankton size classes each day. Using a Bayesian statistical framework, we developed an allometric-scaling equation for salp clearance rates as a function of salp and prey size: Clearance(ESD)=φ∙〖TL〗^ψ × min⁡((ESD⁄(θ×〖TL〗^γ ))^2/(0.16+(ESD⁄(θ×〖TL〗^γ )) ),1)× 〖Q_10〗^(((T-12°C))⁄10) where ESD is prey equivalent spherical diameter, TL is Salpa thompsoni total length, φ = 5.6×10-3 ± 3.6×10-4, ψ = 2.1 ± 0.13, θ = 0.58 ± 0.08, and γ = 0.46 ± 0.03. We discuss the biogeochemical and food-web implications of competitive interactions between salps, krill, and protozoans. 

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4. Diversity of Microbial Eukaryotes Along the West Antarctic Peninsula in Austral Spring

   https://doi.org/10.3389/fmicb.2022.844856  

   Grattepanche, Jean-David; Jeffrey, Wade H.; Gast, Rebecca J.; Sanders, Robert W. (May 2022, Frontiers in Microbiology)

   During a cruise from October to November 2019, along the West Antarctic Peninsula, between 64.32 and 68.37°S, we assessed the diversity and composition of the active microbial eukaryotic community within three size fractions: micro- (> 20 μm), nano- (20–5 μm), and pico-size fractions (5–0.2 μm). The communities and the environmental parameters displayed latitudinal gradients, and we observed a strong similarity in the microbial eukaryotic communities as well as the environmental parameters between the sub-surface and the deep chlorophyll maximum (DCM) depths. Chlorophyll concentrations were low, and the mixed layer was shallow for most of the 17 stations sampled. The richness of the microplankton was higher in Marguerite Bay (our southernmost stations), compared to more northern stations, while the diversity for the nano- and pico-plankton was relatively stable across latitude. The microplankton communities were dominated by autotrophs, mostly diatoms, while mixotrophs (phototrophs-consuming bacteria and kleptoplastidic ciliates, mostly alveolates, and cryptophytes) were the most abundant and active members of the nano- and picoplankton communities. While phototrophy was the dominant trophic mode, heterotrophy (mixotrophy, phagotrophy, and parasitism) tended to increase southward. The samples from Marguerite Bay showed a distinct community with a high diversity of nanoplankton predators, including spirotrich ciliates, and dinoflagellates, while cryptophytes were observed elsewhere. Some lineages were significantly related—either positively or negatively—to ice coverage (e.g., positive for Pelagophyceae, negative for Spirotrichea) and temperature (e.g., positive for Cryptophyceae, negative for Spirotrichea). This suggests that climate changes will have a strong impact on the microbial eukaryotic community. 

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5. Resilience of plankton trophic structure to an eddy-stimulated diatom bloom in the North Pacific Subtropical Gyre

   https://doi.org/10.3354/meps13333  

   Décima, M; Landry, MR (June 2020, Marine Ecology Progress Series)

   null (Ed.)

   We investigated the response of an open-ocean plankton food web to a major ecosystem perturbation event, the Hawaiian lee cyclonic eddy Opal, using compound-specific isotopic analyses of amino acids (CSIA-AA) of individual zooplankton taxa. We hypothesized that the massive diatom bloom that characterized Opal would lead to a shorter food chain. Using CSIA-AA, we differentiated trophic position (TP) changes that arose from altered transfers through protistan microzooplankton, versus metazoan carnivory, and assessed the variability at the base of the food web. Contrary to expectation, zooplankton TPs were higher in the eddy than in ambient control waters (up to 0.8 trophic level), particularly for suspension feeders close to the food-web base. Most of the effect was due to increased trophic transfers through protistan consumers, indicating a general shift up, not down, of grazing and remineralization in the microbial food web. Eucalanus sp., which was 15-fold more abundant inside compared to outside of the eddy, was the only taxon observed to be a true herbivore (TP = 2.0), consistent with a high phenylalanine (Phe) δ 15 N value indicating feeding on nitrate-fueled diatoms in the lower euphotic zone. Oncaea sp., an aggregate-associated copepod, had the largest (1.5) TP difference, and lowest Phe δ 15 N, suggesting that detrital particles were local hot spots of enhanced microbial activity. Rapid growth rates and trophic flexibility of protistan microzooplankton apparently allow the microbial community to reorganize to bloom perturbations, as microzooplankton remain the primary phytoplankton grazers—despite the dominance of large diatoms—and are heavily preyed on by the mesozooplankton. 

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