Sea ice-associated algae and zooplankton fecal pellets fuel organic particle export in the seasonally ice-covered northwest Labrador Sea

Abstract. Ocean warming and Arctic sea ice decline are expected to affect biological pump efficiency by altering the timing, quantity, quality, and composition of export production. However, the origins and composition of sinking organic matter are still understudied for the oceans generally, and in ice-covered areas especially. Here we use compound-specific isotope analysis (CSIA) of amino acids (AAs) to investigate the sources and composition of exported organic matter from a sediment trap-derived time-series of sinking particles collected at depths of 469 m and 915 m at the edge of Saglek Bank in the northwest Labrador Sea from October 2017 to July 2019. The outer edge of Saglek Bank is located at the confluence of cold and fresh Arctic outflow and relatively warmer Atlantic waters. The area is subject to seasonal sea ice cover and is a biological hotspot for benthic organisms including deep-sea corals and sponges. Sea ice was present for ~50–60 % of the deployment days in both cycles. Phytoplankton blooms at our study site cooccurred with the onset of sea ice melt. Microalgal taxonomy indicated the presence of ice-associated diatoms in the sinking particles during the spring bloom in 2018, confirming that sea ice algae contributed to the organic particle export at our study site. Abundant copepods and copepod nauplii caught in the sediment traps was consistent with a high abundance of copepods in overlying epipelagic waters. Stable carbon isotopes (δ¹³C) of essential amino acids (EAAs) of the sinking particles revealed a potentially important contribution of sea ice algae as a carbon source at the base of the food web to sinking particles, with only minor modification by microbial resynthesis. Stable nitrogen isotopes (δ¹⁵N) of AAs of sinking particles provided independent evidence of the minor bacterial degradation and Bayesian mixing models based on normalized δ¹⁵N-AA values revealed dominant contribution of fecal pellets (76–96 %) to the sinking particles. Our study demonstrates the importance of sea ice algae and fecal pellets to the biological pump in the seasonally ice-covered northwest Labrador Sea, with sea ice algae exported either directly via passive sinking or indirectly via zooplankton grazing, with fecal pellets dominating the organic particle fluxes.