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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.

The 2014–2015 warm anomaly (aka “the Blob”), the largest of periodic and intensifying marine heat wave (MHW) perturbations in the northeast Pacific, may provide some insight about the future warmer ocean. Here, we use mixed‐layer carbon estimates for total phytoplankton, major size classes and functional groups from 45 CalCOFI cruises to: (1) compare 2014–2015 MHW impacts in the southern California Current System to baseline estimates from 2004 to 2013 and (2) to test a space‐for‐time exchange hypothesis that links biomass structure to variability of nitracline depth (NCD). Seasonal and inshore‐offshore analyses from nine stations revealed almost uniform 2°C MHW warming extending 700 km seaward, fourfold to sixfold declines in nitrate concentration and 18‐m deeper NCDs. Phytoplankton C decreased 16–21% compared to 45–65% for Chla, with the threefold difference due to altered C : Chla. Among size classes, percent composition of nanoplankton decreased and picophytoplankton increased, driven by higherProchlorococcusbiomass, whileSynechococcusand picoeukaryotes generally declined. Diatom and dinoflagellate C decreased in both onshore and offshore waters. Seasonally, the MHW delayed the normal winter refresh of surface nitrate, resulting in depressed stocks of total phytoplankton and nanoplankton,Synechococcusand picoeukaryotes during winter. Consistent with the space‐for‐time hypothesis, biomass variations for baseline and MHW cruises followed similar (not significantly different) slope relationships to NCD. All biomass components, exceptProchlorococcus, were negatively related to NCD, and community biomass structure realigned according to regression slopes differences with NCD variability. Empirically derived biomass‐NCD relationships could be useful for calibrating models that explore future food‐web impacts in this coastal upwelling system.

Bluefin tuna spawn in restricted areas of subtropical oligotrophic seas. Here, we investigate the zooplankton prey and feeding selectivity of early larval stages of Atlantic bluefin tuna (ABT, Thunnus thynnus) in larval rearing habitat of the Gulf of Mexico. Larvae and zooplankton were collected during two multi-day Lagrangian experiments during peak spawning in May 2017 and 2018. Larvae were categorized by flexion stage and standard length. We identified, enumerated and sized zooplankton from larval gut contents and in the ambient community. Ciliates were quantitatively important (up to 9%) in carbon-based diets of early larvae. As larvae grew, diet composition and prey selection shifted from small copepod nauplii and calanoid copepodites to larger podonid cladocerans, which accounted for up to 70% of ingested carbon. Even when cladoceran abundances were <0.2 m−3, they comprised 23% of postflexion stage diet. Feeding behaviors of larvae at different development stages were more specialized, and prey selection narrowed to appendicularians and primarily cladocerans when these taxa were more abundant. Our findings suggest that ABT larvae have the capacity to switch from passive selection, regulated by physical factors, to active selection of presumably energetically optimal prey.

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.