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Vital rates, including growth responses to environmental variability, are poorly characterized for the diverse taxa of heterotrophic bacteria (HBact) in marine ecosystems. Here, we evaluated the potential for combining molecular analyses with dilution experiments to assess taxon-specific growth (cell division) and net growth rates of HBact in natural waters. Two-treatment dilution experiments were conducted within situincubations under 3 environmental conditions in the California Current Ecosystem, at offshore and inshore sites during a warm upwelling-suppressed year (2014) and for normal inshore upwelling, representing a 33-fold primary production range. Relative sequence reads from 16S rRNA metabarcoding were normalized to total HBact counts from flow cytometry for community abundance and rate calculations. Composition varied from dominance of Alphaproteobacteria (56%) in oligotrophic offshore (SAR11) and mesotrophic inshore waters (SAR11 and Rhodobacteria) to Bacteriodes/Flavobacteria dominance (64%) and mixed sub-taxon importance (Polaribacter, Rhodobacteria,Formosa) during upwelling. Net growth rates in bottles, validated by comparison to ambient community net growth following a satellite-tracked drifter, varied from near steady state for offshore and inshore conditions to dynamic community changes during upwelling. Mean growth rates doubled (0.33 to 0.62 d^-1) over the productivity range, and taxon estimates varied from -0.17 d^-1(Formosa, offshore) to 1.53 d^-1(SAR11, upwelling). Increasing growth of Flavobacteria and Rhodobacteria paralleled their abundance and dominance increases with productivity. SAR11 growth remained higher than average with increasing production, despite declining abundances. We highlight possible PCR or 16S rRNA gene copy biases of growth rate estimates as research needs for further applications of this approach. 

We investigated how the network of food-web flows in open-ocean systems might support high rates of mesozooplankton respiration and production by comparing predicted rates from empirical relationships to independently determined solutions from an inverse model based on tightly constrained field-measured rates for the equatorial Pacific. Model results were consistent with estimates of gross:net primary production (GPP:NPP), bacterial production:NPP, sinking particulate export, and total export for the equatorial Pacific, as well as general literature values for growth efficiencies of bacteria, protozooplankton, and metazooplankton. Mean rate estimates from the model compared favorably with the respiration predictions from Ikeda (1985; Mar Biol 85:1-11 ) (146 vs. 144 mg C m -2 d -1 , respectively) and with production estimates from the growth rate equation of Hirst & Sheader (1997; Mar Ecol Prog Ser 154:155-165 ) (153 vs. 144 mg C m -2 d -1 ). Metazooplankton nutritional requirements are met with a mixed diet of protozooplankton (39%), phytoplankton (36%), detritus (15%), and carnivory (10%). Within the food-web network, NPP of 896 mg C m -2 d -1 supports a total heterotrophic carbon demand from bacteria, protozoa, and metazooplankton that is 2.5 times higher. Scaling our results to primary production and zooplankton biomass at Stn ALOHA suggests that zooplankton nutritional requirements for high growth might similarly be met in oligotrophic subtropical waters through a less efficient trophic structure. Metazooplankton production available to higher-level consumers is a significant contributor to the total export needed for an overall biogeochemical balance of the region and to export requirements to meet carbon demand in the mesopelagic depth range. 

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. 

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.