Food Web Structure in the Mariana and Kermadec Trenches from Stable Isotope Analysis

Andrew Tokuda, University of Hawaii at Manoa, Department of Oceanography, Honolulu, United States, Mackenzie Gerringer, State University of New York at Geneseo, HI, United States, Brian N Popp, University of Hawaiʻi at Mānoa, Department of Earth Sciences, Honolulu, United States, Natalie Wallsgrove, University of Hawaii at Manoa, Department of Earth Sciences, Honolulu, United States, Eleanna Grammatopoulou, University of Aberdeen, Aberdeen, United Kingdom, Daniel Mayor, National Oceanography Center, Soton, Southampton, United Kingdom and Jeffrey Drazen, University of Hawai'i at Mānoa, Department of Oceanography, Honolulu, United States
Abstract:
Hadal trenches may serve as sites of increased and high community activity because their V-shaped topography concentrates food. Despite previous work, trench food web structure, which could provide insight into sources of organic matter used by trench fauna remains unknown. We evaluated food web structure, nutritional sources, and animal trophic levels in the Kermadec and Mariana trenches using carbon and nitrogen stable isotope analysis (δ15N and δ13C values) of organism bulk tissues and individual amino acids (AA-CSIA). In the Kermadec Trench, bulk δ15N values ranged from 5.8‰ in trench sediment to 17.5‰ in the supergiant amphipod, Allicela gigantea, tissues. Unexpectedly, δ15N values of detritovores were much higher (6.0‰ more) than those to sediment, their putative food source. The δ13C values ranged from -21.4‰ in sediments to -17.3‰ in the brittle star, and did not co-vary with δ15N values. In the Mariana Trench, fishes, amphipods, and sediments had similar δ15N values to those from the Kermadec Trench (sediment δ15N values averaged 6.8‰ and the supergiant amphipod values averaged 15.0‰). The substantial 15N enrichment in sediments relative to detritivores and the variability in δ13C values suggest multiple food inputs to the system. Based on AA-CSIA, trophic levels in the Kermadec Trench ranged from three for detritivores to five for fishes. The δ15N values of source amino acids increase with depth for fish (r2=0.57) suggesting a depth related shift in sources of nutrition between abyssal and hadal species. However, δ15N values of source amino acids were variable across crustaceans (r2<0.01), suggesting a variety of food sources including carrion or refractory organic matter moved downslope by turbidity flows. Overall, our study suggests that processes of nutritional input are similar in the two trenches while interactions within each trench are more complicated than previously described.