Pelagic-benthic coupling, food webs, and organic matter degradation in the Chukchi Sea: Insights from sedimentary pigments and stable carbon isotopes

Abstract

An understanding of the origin and fate of organic matter deposited on the seabed in the Chukchi Sea is critical to our understanding of how regional changes in climate will affect this highly productive ecosystem. We measured the concentrations of chlorophyll a, pheopigments, and accessory pigments in sediments at 39 stations using high-performance liquid chromatography (HPLC) to investigate the degradation pathways of chlorophyll-containing organic matter. Chlorophyll a concentrations were among the highest ever reported in marine sediments (up to 665 mg m-2). Measurements of pheopigments, including pheophytin a, pheophorbide a, and pyropheophorbide a, revealed substantial degradative processing of organic matter that began ∼ 2 weeks after ice breakup. However, the ratio of chlorophyll a to total pheopigments (Chl a : pheo) was > 1 in most stations, indicating that chlorophyll a was preserved despite grazing and senescent degradation pathways that produced accumulated pheopigments. Increased δ13C values of bulk sediment organic matter was also positively correlated to concentrations of pheophytin a (p < 0.01, r=0.46), and total pheopigment concentration was correlated to the δ13C values of the epibenthic scavenging snow crab Chionoecetes opilio (p < 0.01, r = 0.92) and the surface deposit feeding bivalves Macoma spp. (p < 0.01, r = 0.69). Based on these observations, we hypothesize that organic matter degraded by microbial and herbivorous grazing pathways produces an enriched 13C signal that is transferred to macrofaunal consumers, providing a critical source of carbon to a rich and diverse benthic food web.