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The number of trophic steps within a plankton food web plays an important role in determining the energy available to support higher-level consumers by affecting trophic transfer efficiency (TE): fewer steps can enhance TE by decreasing respiration and predation losses. In this study, trophic structure within the zooplankton community was investigated using stable isotopes in size-fractionated mesozooplankton, and related to 2 biomass proxies related to TE: the normalized biomass size spectra (NBSS) and the ratio of zooplankton:phytoplankton biomass (log 10 (zoo:phyto)). Four regions were compared: the California Current Ecosystem (CCE—most productive), the Equatorial Pacific (EqP), the Costa Rica Dome (CRD) and the North Pacific Subtropical Gyre (NPSG—least productive). Compound-specific isotope analysis of amino acids confirmed large differences (~3‰) in the isotopic baseline among ecosystems. EqP and NPSG had low and distinct source δ 15 N values, while CRD/CCE had high and overlapping values. Trophic differences indicated that the CCE had the lowest number (0) of trophic differences within the 4 zooplankton size classes; NPSG and EqP had the highest number (3), and CRD was intermediate (1). NBSS slopes confirmed the CCE and NPSG as extremes and statistically different from each other. TE patterns estimated from log 10 (zoo:phyto) suggested EqP was the least efficient, while the other 3 ecosystems (despite large ranges in zooplankton and phytoplankton biomass) had similar TEs. The inverse relationship between food chain length and system productivity, a paradigm originally formulated for microbial food webs, holds for the mesozooplankton assemblage at the productivity extremes. 

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. 

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.