Abstract
Although gelatinous zooplankton are key members of marine ecosystems and food webs, their trophic ecology is poorly described across the deep pelagic. We used stable carbon (bulk tissue) and nitrogen (bulk tissue and amino acid) isotope analysis to estimate the trophic positions (TPs) of abundant gelatinous zooplankton (chaetognaths, cnidarians, ctenophores, mollusks, pelagic tunicates) across depth habitats. We collected gelatinous zooplankton from a range of feeding guilds, sampling animals from 0 to 3000 m on four cruises across the years 2020 to 2023 within the southern California Current Ecosystem. Within taxonomic groups, bulk carbon (delta 13C) and nitrogen isotope (delta 15N) values were similar, regardless of animal size. Gelatinous zooplankton spanned 2.4 trophic levels. Trophic positions were highest for animals that consume other gelatinous zooplankton (e.g., Aegina spp., TP = 3.8) and lowest for grazers (e.g., Pyrosoma atlanticum, TP = 1.9). Trophic positions based on delta 15N values of amino acids often agreed with diets reported from the literature, where available, while TPs calculated using bulk tissue delta 15N values were often lower. Animal delta 15N (bulk and source amino acid) values increased with increasing depth, suggesting that shallow and deep-pelagic taxa rely on distinct basal food resources. Medusae residing from 600 to 1000 m had less variable bulk tissue delta 13C and delta 15N values across locations and seasons than ctenophores, medusae, and pelagic tunicates collected from 0 to 600 m, suggesting that deep-pelagic taxa rely on stable food resources at depth. Animal delta 15N and delta 13C values were relatively consistent between seasons, potentially suggesting stability in the trophic structure of the jelly web.