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1. Trophic flows to mesozooplankton support the conventional paradigm of pelagic food web structure in ocean ecosystems

   https://doi.org/10.1093/icesjms/fsae131  

   Landry, Michael_R ; Décima, Moira ; Stukel, Michael_R ; Gutiérrez-Rodríguez, Andrés ; Andersen, ed., Ken ( September 2024 , ICES Journal of Marine Science)

   The current conventional paradigm of ocean food web structure inserts one full level or more of microzooplankton heterotrophic consumption, a substantial energy drop, between phytoplankton and mesozooplankton. Using a dataset with contemporaneous measurements of primary production (PP), size-fractioned mesozooplankton biomass, and micro- and mesozooplankton grazing rates from 10 tropical to temperate ocean ecosystems, we examined whether the structural inefficiencies in this paradigm allow sufficient energy transfer to support active metabolism and growth of observed zooplankton standing stocks. Zooplankton carbon requirements (ZCR) were determined from allometric equations that account for ecosystem differences in temperature and size structure. ZCRs were relatively low (∼30% of PP or less) for both oligotrophic systems and bloom biomass accumulation in eutrophic coastal waters. Higher relative ZCRs (>30% PP) were associated with elevated mesozooplankton grazing scenarios (bloom declines, abundant salps), advective subsidies, and open-ocean upwelling systems. Microzooplankton generally dominated as grazers of PP but were equal or secondary to direct herbivory as nutritional support for mesozooplankton in five of eight regional studies. All systems were able to satisfy ZCR within the conventional food-web interpretation, but balanced open-ocean upwelling systems required the most efficient alignments of contributions from microzooplankton grazing, direct herbivory, and carnivory to do so.

    

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2. Mesozooplankton biomass, grazing and trophic structure in the bluefin tuna spawning area of the oceanic Gulf of Mexico

   https://doi.org/10.1093/plankt/fbab008  

   Landry, Michael R ; Swalethorp, Rasmus ( March 2021 , Journal of Plankton Research)

   null (Ed.)

   Abstract We investigated size-fractioned biomass, isotopes and grazing of mesozooplankton communities in the larval habitat of Atlantic bluefin tuna (ABT) in the oceanic Gulf of Mexico (GoM) during the peak spawning month of May. Euphotic-zone biomass ranged from 101 to 513 mg C m−2 during the day and 216 to 798 mg C m−2 at night. Grazing varied from 0.1 to 1.0 mg Chla m−2 d−1, averaging 1–3% of phytoplankton Chla consumed d−1. Carnivorous taxa dominated the biomass of > 1-mm zooplankton (78% day; 60% night), while only 13% of smaller zooplankton were carnivores. δ15N enrichment between small and large sizes indicates a 0.5–0.6 trophic-step difference. Although characteristics of GoM zooplankton are generally similar to those of remote oligotrophic subtropical regions, zooplankton stocks in the ABT larval habitat are disproportionately high relative to primary production, compared with HOT and BATS averages. Growth-grazing balances for phytoplankton were resolved with a statistically insignificant residual, and trophic fluxes from local productivity were sufficient to satisfy C demand of suspension feeding mesozooplankton. While carnivore C demand was met by local processes in the central GoM, experiments closer to the coastal margin suggest the need for a lateral subsidy of zooplankton biomass to the oceanic region. 

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3. Seasonal dynamics of midwater zooplankton and relation to particle cycling in the North Pacific Subtropical Gyre

   https://doi.org/https://doi.org/10.1016/j.pocean.2020.102266  

   Hannides, CCS ; Popp, BN. ; Close, HG ; Benitez-Nelson, CR ; Ka’apu-Lyons, CA ; Gloeckler, K ; Wallsgrove, N ; Uhau, B ; Palmer, E ; Drazen, JC ( January 2020 , Progress in oceanography)

   Midwater zooplankton are major agents of biogeochemical transformation in the open ocean; however their characteristics and activity remain poorly known. Here we evaluate midwater zooplankton biomass, amino acid (AA)-specific stable isotope composition (δ15N values) using compound-specific isotope analysis of amino acids (CSIA-AA), trophic position, and elemental composition in the North Pacific Subtropical Gyre (NPSG). We focus on zooplankton collected in the winter, spring, and summer to evaluate midwater trophic dynamics over a seasonal cycle. For the first time we find that midwater zooplankton respond strongly to seasonal changes in production and export in the NPSG. In summer, when export from the euphotic zone is elevated and this ‘summer pulse’ material is transported rapidly to depth, CSIA-AA indicates that large particles (> 53 μm) dominate the food web base for zooplankton throughout the midwaters, and to a large extent even into the upper bathypelagic zone. In winter, when export is low, zooplankton in the mid-mesopelagic zone continue to rely on large particle basal resources, but resident zooplankton in the lower mesopelagic and upper bathypelagic zones switch to include smaller particles (0.7–53 μm) in their food web base, or even a subset of the small particle pool. Midwater zooplankton migration patterns also vary with season, with migrants distributed more evenly at night through the euphotic zone in summer as compared to being more compressed in the upper mixed layer in winter. Deeper zooplankton migration within the mesopelagic zone is also reduced in late summer, likely due to the increased magnitude of large particle material available at depth during this season. Our observed seasonal change in activity and trophic dynamics drives modestly greater biomass in summer than winter through the mesopelagic zone. In contrast midwater zooplankton carbon (C), nitrogen (N), and phosphorus (P) composition does not change with season. Instead we find increasing C:N, C:P, and N:P ratios with greater depths, likely due to decreases in proteinaceous structures and organic P compounds and increases in storage lipids with depth. Our study highlights the importance and diversity of feeding strategies for small zooplankton in NPSG midwaters. Many small zooplankton, such as oncaeid and oithonid copepods, are able to access small particle resources at depth and may be an important trophic link between the microbial loop and deep dwelling micronekton species that also rely on small particle-based food webs. Our observed midwater zooplankton trophic response to export-driven variation in the particle field at depth has important implications for midwater metabolism and the export of C to the deep sea. 

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4. 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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5. Nitrogen and Isotope Flows Through the Costa Rica Dome Upwelling Ecosystem: The Crucial Mesozooplankton Role in Export Flux

   https://doi.org/10.1029/2018GB005968  

   Stukel, Michael_R ; Décima, Moira ; Landry, Michael_R ; Selph, Karen_E ( December 2018 , Global Biogeochemical Cycles)

   The Costa Rica Dome (CRD) is an open‐ocean upwelling ecosystem, with high biomasses of picophytoplankton (especiallySynechococcus), mesozooplankton, and higher trophic levels. To elucidate the food web pathways supporting the trophic structure and carbon export in this unique ecosystem, we used Markov Chain Monte Carlo techniques to assimilate data from four independent realizations of δ¹⁵N and planktonic rate measurements from the CRD into steady state, multicompartment ecosystem box models (linear inverse models). Model results present well‐constrained snapshots of ecosystem nitrogen and stable isotope fluxes. New production is supported by upwelled nitrate, not nitrogen fixation. Protistivory (rather than herbivory) was the most important feeding mode for mesozooplankton, which rely heavily on microzooplankton prey. Mesozooplankton play a central role in vertical nitrogen export, primarily through active transport of nitrogen consumed in the surface layer and excreted at depth, which comprised an average 36–46% of total export. Detritus or aggregate feeding is also an important mode of resource acquisition by mesozooplankton and regeneration of nutrients within the euphotic zone. As a consequence, the ratio of passively sinking particle export to phytoplankton production is very low in the CRD. Comparisons to similar models constrained with data from the nearby equatorial Pacific demonstrate that the dominant role of vertical migrators to the biological pump is a unique feature of the CRD. However, both regions show efficient nitrogen transfer from mesozooplankton to higher trophic levels (as expected for regions with large fish, cetacean, and seabird populations) despite the dominance of protists as major grazers of phytoplankton.

    

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