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

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.