Using stable isotopes to describe the trophic structure of gelatinous zooplankton across the deep pelagic

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 (δ 13 C) and nitrogen isotope (δ 15 N) 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 δ 15 N values of amino acids often agreed with diets reported from the literature, where available, while TPs calculated using bulk tissue δ 15 N values were often lower. Animal δ 15 N (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 δ 13 C and δ 15 N 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 δ 15 N and δ 13 C values were relatively consistent between seasons, potentially suggesting stability in the trophic structure of the jelly web.