Benthic trophic sensitivity to on-going changes in Pacific Arctic seasonal sea ice cover – Insights from the nitrogen isotopic composition of amino acids

Abstract

We studied trophic relations among benthic species collected in the northern Bering and Chukchi seas, specifically in biologically productive “hotspots” within the Distributed Biological Observatory (DBO) program. The stable nitrogen isotope composition of compound-specific amino acids (δ15NAA) was used as an approach to assess how several benthic species in this Arctic ecosystem are responding to the earlier timing of sea-ice melt and associated shifts to the onset of the annual production season. The trophic level of 16 common benthic taxa with different feeding behaviors were collected along a south-to-north latitudinal gradient extending from the northern Bering Sea, south of St. Lawrence Island, to the northern Chukchi Sea, including the Barrow Canyon undersea feature, and examined with respect to the available food sources. The δ15NAA values of source and trophic amino acids varied among studied regions and among species. δ15NAA-based trophic level estimations, using δ15N values in both source and trophic amino acids and the measure of heterotrophic resynthesis (ΣV parameter), provided new insights on trophic levels and feeding behaviors of different taxonomic groups that were not apparent from more commonly used isotopic measurements of bulk tissues. The compound-specific amino acid isotope compositions varied geographically, but not necessarily by latitude. This indicates that a simple time for space replacement perspective extending from south to north over the study area and reflecting variable persistence of seasonal sea ice, is inadequate to explain food web complexity. Highest bulk δ15N and δ15NAA values were observed in depositional zones (near and within the St. Lawrence Island Winter-Spring Polynya, in particular). The δ15NAA values also suggested high feeding plasticity in benthic species in the northern Bering and Chukchi seas hotspots. Benthic consumers most likely can change feeding behavior (e.g. suspension versus deposit feeding) in response to different environmental conditions and resulting changes in the quality of organic matter reaching the sea floor. We conclude that the sensitivity of productive arctic benthic ecosystems to climate related change, i.e. earlier sea-ice melt and related onset of primary production, might be mitigated by robust capabilities to adapt to changes in food quality and supply.