Benthic mineralization and nutrient retention in fjord systems with marine- and land-terminating glaciers (Southwest Greenland)

Abstract

Ongoing glacial melt is rapidly altering Arctic and sub-Arctic fjord systems, with uncertain implications for their biogeochemical functioning. Yet, data on marine benthic nutrient recycling in such fjords remain sparse. This study investigates the sediment biogeochemistry of two neighboring sub-Arctic fjords in southwest Greenland characterized by different surface productivities related to their glacial regimes: high primary productivity in Nuup Kangerlua influenced by marine-terminating glaciers, and lower surface productivity in Ameralik impacted only by a land-terminating glacier. We assessed benthic mineralization rates by measuring total oxygen uptake and nutrient fluxes (NH4+, NO3−, PO43−), along with porewater profiles (including dissolved Fe and Mn), to evaluate whether fjord sediments act as sources or sinks of key dissolved nutrients. Total oxygen uptake varied markedly between fjords and seasons: in Ameralik the range was 3.55–9.69 mmol O2 m−2 d−1 in spring and 5.27–13.34 mmol O2 m−2 d−1 in late summer, whereas Nuup Kangerlua showed comparable spring and summer values, spanning 2.83–5.48 mmol O2 m−2 d−1 across both seasons. Despite the approximately 3-fold higher annual primary productivity in Nuup Kangerlua, sediment mineralization rates were highest in Ameralik, particularly in its outer basin. Sediments in both fjords acted as sinks for specific nutrients. In Nuup Kangerlua, sedimentary ammonium and phosphate retention dominated near glacial discharge zones, likely due to rapid burial and dilution of organic matter, and adsorption to metal oxides. Meanwhile, the entire Ameralik basin acted as a sink for both ammonium and nitrate as reflected in low effluxes and ratios of dissolved inorganic nitrogen to dissolved inorganic phosphorus well beneath Redfield. These findings underscore the need to consider organic matter preservation conditions, geomorphology, and particle reactivity in addition to glacial discharge regime when assessing the biogeochemical response of Arctic fjords to continued climate-driven glacier retreat.