Natural Fe-binding organic ligands in Fram Strait and over the northeast Greenland shelf

There is a paucity of data on Fe-binding ligands in the Arctic Ocean. Here we investigate the distribution and chemical properties of natural Fe-binding ligands in Fram Strait and over the northeast Greenland shelf, shedding light on their potential sources and transport. Our results indicate that the main sources of organic ligands to surface waters of Fram Strait included primary productivity and supply from the Arctic Ocean. We calculated the mean total Fe-binding ligand concentration, [L_t], in Polar Surface Water from the western Fram Strait to be 1.65 ± 0.4 nM eq. Fe. This value is in between reported values for the Arctic and North Atlantic Oceans, confirming reports of north to south decreases in [L_t] from the Arctic Ocean. The differences between ligand sources in different biogeochemical provinces, resulted in distinctive ligand properties and distributions that are reflected in [L_t], binding strength (log K_Fe'L ^′) and competing strength (log α_Fe'L) of ligands. Higher [L_t] was present near the Nioghalvfjerdsfjorden (79 N) Glacier terminus and in the Westwind Trough (median of [L_t] = 2.17 nM eq. Fe; log K_Fe'L ^′ = 12.3; log α_Fe'L = 3.4) than in the Norske Trough (median of [L_t] = 1.89 nM eq. Fe; log K_Fe'L ^′= 12.8; log α_Fe'L = 3.8) and in Fram Strait (median of [L_t] = 1.38 nM eq. Fe; log K_Fe'L ^′ = 13; log α_Fe'L= 3.9). However, organic ligands near the 79 N Glacier terminus and in the Westwind Trough were weaker, and therefore less reactive than organic ligands in the Norske Trough and in Fram Strait. These weaker ligands, although more abundant than in the Fram Strait, reduce overall Fe solubility in waters transported from the 79N Glacier to Fram Strait. The lower ligand binding strength in the outflow results in a higher inorganic Fe concentration, [Fe´], which is more prone to precipitation and/or scavenging than Fe complexed with stronger ligands. Ongoing changes in the Arctic and sub-Arctic Oceans will influence both terrestrially derived and in-situ produced Fe-binding ligands, and therefore will have consequences for Fe solubility and availability to microbial populations and Fe cycling in Fram Strait.