Does feeding type matter? Contribution of organic matter sources to benthic invertebrates on the Arctic Chukchi Sea shelf

Abstract Benthic communities play an important role in the nutrient cycling of settling organic matter and provide an energy link to higher trophic levels. These benthic communities are highly dependent on the food sources provided through sinking organic material, with pelagic-benthic coupling particularly strong on Arctic shelf systems. Arctic shelves, however, are experiencing shifts in the timing of sea ice breakup that can have severe implications on the amount and composition of organic matter from different primary production sources supplied to the benthos. The role of benthic invertebrates in processing organic matter is typically classified by feeding types. The goal of this study was to evaluate if benthic invertebrate feeding types are a useful concept for understanding how organic matter from various biosynthetic sources is used by benthic invertebrates across the Arctic Chukchi Sea. We employed essential amino acid (EAA) specific stable isotope fingerprinting to identify proportional contributions of three EAA sources (from bacterial, phytoplankton, and terrestrial production) as a proxy of carbon sources to the diets of benthic consumers. When grouped by feeding types, the proportional contributions of the three EAA sources were similar among suspension, deposit feeders and predators/scavengers. Different genera within the same feeding type, however, showed significant differences among the EAA sources, indicating that EAA use is genus-specific rather than feeding type-specific. We discuss characteristics other than feeding mode among genera that could account for different EAA use, including mobility, selectivity of available EAA in sediments, and other trophic aspects such as assimilation efficiency. These characteristics provide useful additional considerations when grouping organisms by feeding types. High proportions of terrestrial EAA were found in the majority of benthic genera across all feeding types and characteristics in this study, reflecting the high proportions of this organic matter source in sediments and confirming that this source is being used by benthic consumers in the Arctic. EAA contributions to various benthic genera across the Chukchi Sea shelf had only weak correlations with environmental variables. While the distinctly different uses of different EAA sources by benthic-feeding genera but not by feeding types may represent high trophic plasticity in Arctic benthic invertebrates, expected climate change-driven shifts in the supply of EAA sources to the Chukchi Sea shelf benthos may still have implications for the composition of benthic communities and energy flow through the benthic food web, including energy pathways that support current top predators.