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1. Zooplankton trophic processes in the eastern Indian Ocean off northwest Australia

   https://doi.org/10.1016/j.dsr2.2026.105592  

   Décima, Moira; Swalethorp, Rasmus; Cawley, Grace F; Traboni, Claudia; Davies, Claire H; Landry, Michael R (April 2026, Deep Sea Research Part II: Topical Studies in Oceanography)

   The region off northwestern Australia in the eastern Indian Ocean is an oligotrophic open-ocean ecosystem characterized by low nutrients and high stratification, and is the only known spawning ground for Southern Bluefin Tuna (SBT). We conducted four multi-day Lagrangian experiments (cycles) and transect sampling to investigate plankton trophic flows during the peak SBT spawning season (Jan–Feb 2022). These cycles were carried out after recent storm mixing and subsequent warming and stratification of the water column. We quantified mesozooplankton size-fractionated biomass and grazing, targeting processes that coincide with SBT larval habitat (upper 30 m) and the full euphotic zone (150 m). Higher grazing occurred in the upper 30 m at the start of our study, coinciding with the deeper mixed layer. Euphotic-zone integrated grazing peaked later, coinciding with higher stratification and increased abundance of diatoms at depth. Mesozooplankton grazing impact averaged 0.11 ± 0.01 d-1 for the region, equivalent to 21.2 % of phytoplankton growth. Combined with microzooplankton grazing removals, phytoplankton growth and total zooplankton grazing losses were approximately in balance for the euphotic zone. While direct consumption of phytoplankton only supported respiratory needs of a portion of the assemblage in two of the four experimental cycles, respiratory and production needs were largely met by including the consumption of microzooplankton production. Thus, the production of mesozooplankton prey required to sustain SBT larvae in this nutrient-poor environment was obtained by both high grazing and efficient trophic coupling. This study highlights the role of metazoan zooplankton in warm oligotrophic waters of the eastern tropical Indian Ocean, and indicates that efficient ecosystem transfer can be achieved even under conditions assumed to severely limit production and biomass accumulation. 

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2. 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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3. Mesozooplankton biomass and temperature-enhanced grazing along a 110°E transect in the eastern Indian Ocean

   https://doi.org/10.3354/meps13444  

   Landry, MR; Hood, RR; Davies, CH (September 2020, Marine Ecology Progress Series)

   null (Ed.)

   Low-latitude waters of the Indian Ocean are warming faster than other major oceans. Most models predict a zooplankton decline due to lower productivity, enhanced metabolism and phytoplankton size shifts that reduce trophic transfer efficiency. In May-June 2019, we investigated mesozooplankton biomass and grazing along the historic 110°E transect line from the International Indian Ocean Expedition (IIOE) of the 1960s. Twenty sampling stations from 39.5 to 11.5°S spanned latitudinal variability from temperate to tropical waters and a pronounced 14°C gradient in mean euphotic zone temperature. Although mesozooplankton size structure was similar along the transect, with smaller (<2 mm) size classes dominant, total biomass increased 3-fold (400 to 1500 mg dry weight m -2 ) from high to low latitude. More dramatically, gut-fluorescence estimates of grazing (total ingestion or % euphotic zone chl a consumed d -1 ) were 14- and 20-fold higher, respectively, in the low-latitude warmer waters. Biomass-normalized grazing rates varied more than 6-fold over the transect, showing a strong temperature relationship (r 2 = 0.85) that exceeded the temperature effects on gut turnover and metabolic rates. Herbivory contributed more to satisfying zooplankton energetic requirements in low-chl a tropical waters than chl a -rich waters at higher latitude. Our unexpected results are inconsistent with trophic amplification of warming effects on phytoplankton to zooplankton, but might be explained by enhanced coupling efficiency via mixotrophy. Additional implications for selective herbivory and top-down grazing control underscore the need for rigorous field studies to understand relationships and validate assumptions about climate change effects on the food webs of tropical oceans. 

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4. 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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5. Microbial food web dynamics in the oceanic Gulf of Mexico

   https://doi.org/10.1093/plankt/fbab021  

   Landry, Michael R; Selph, Karen E; Stukel, Michael R; Swalethorp, Rasmus; Kelly, Thomas B; Beatty, Jennifer L; Quackenbush, Cameron R (April 2021, Journal of Plankton Research)

   null (Ed.)

   Abstract Phytoplankton growth and microzooplankton grazing rates were measured in repeated profiles of dilution experiments incubated in situ on a drift array in order to assess microbial production and food web characteristics in the oligotrophic bluefin tuna spawning habitat of the Gulf of Mexico (May peak spawning seasons, 2017–2018). Grazing often exceeded growth with the processes more balanced overall in the surface mixed layer, but biomass accumulated in the mid-euphotic zone. Community production estimates (260–500 mg C m−2 day−1) were low compared to similar open-ocean studies in the Pacific Ocean. Prochlorococcus was a consistent major contributor (113–204 mg C m−2 day−1) to productivity, while diatoms and dinoflagellates (2–10 and 4–13 mg C m−2 day−1, respectively) were consistently low. Prymnesiophytes, the most dynamic component (34–134 mg C m−2 day−1), co-dominated in 2017 experiments. Unexpected imbalances in grazing relative to production were observed for all picoplankton populations (Prochlorococcus, Synechococcus and heterotrophic bacteria), suggesting a trophic cascade in the absence of mesozooplankton predation on large microzooplankton. Study sites with abundant larval tuna had the shallowest deep chlorophyll maxima and significant net positive phytoplankton growth below the mixed layer. 

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