Trophic relationships along a bathymetric gradient at the deep-sea observatory HAUSGARTEN

Abstract Deep-seafloor communities, especially those from the ice-covered Arctic, are subject to severe food limitation as the amount of particulate organic matter (POM) from the surface is attenuated with increasing depth. Here, we use naturally occurring stable isotope tracers ( δ 15 N) to broaden our rudimentary knowledge of food web structure and the response of benthic organisms to decreasing food supplies along the bathymetric transect (∼1300–5600 m water depth) of the deep-sea observatory HAUSGARTEN. Encompassing five trophic levels, the HAUSGARTEN food web is among the longest indicating continuous recycling of organic material typical of food-limited deep-sea ecosystems. The δ 15 N signatures ranged from 3.0‰ for Foraminifera to 21.4‰ (±0.4) for starfish ( Poraniomorpha tumida ). The majority of organisms occupied the second and third trophic level. Demersal fish fed at the third trophic level, consistent with results from stomach contents analysis. There were significant differences in the δ 15 N signatures of different functional groups with highest δ 15 N values in predators/scavengers (13.2±0.2‰) followed by suspension feeders (11.2±0.2‰) and deposit feeders (10.2±0.3‰). Depth (=increasing food limitation) affected functional groups in different ways. While the isotopic signatures of predators/scavengers did not change, those of suspension feeders increased with depth, and the reverse was found for deposit feeders. In contrast to the results of other studies, the δ 15 N signatures in POM samples obtained below 800 m did not vary significantly with depth indicating that changes in δ 15 N values are unlikely to be responsible for the depth-related δ 15 N signature changes observed for benthic consumers. However, the δ 15 N signatures of sediments decreased with increasing depth, which also explains the decrease found for deposit feeders. Suspension feeders may rely increasingly on particles trickling down the HAUSGARTEN slope and carrying higher δ 15 N signatures than the decreasing POM supplies, which elevates the δ 15 N value of their tissues. Our results imply that a depth-stratified approach should be taken to avoid a misinterpretation of data obtained at different depths.