Seasonal fluctuation and estuarine removal of glacially-derived iron fluxes to the Gulf of Alaska

Iron (Fe) is thought to be a limiting nutrient for phytoplankton populations in much of the north Pacific and the Gulf of Alaska (GoA) in particular. The amount of freshwater entering the GoA from montane watersheds of coastal and interior Alaskan catchments is comparable to the discharge of the Mississippi River, ~50% of which is associated with glacial melt water. As such, the riverine flux of Fe to the GoA is an important source of this micronutrient that will evolve as regional glacial ice loss continues. While it is clear that riverine iron loads are an important nutrient source to the GoA, relatively little is known about the chemical nature and behavior of iron in glacierized river systems and estuaries. Here we present geochemical data from the Copper River watershed and estuary, the largest single point source of freshwater to the GoA. We utilize multiple time-series of water and suspended sediment samples from glacierized catchments to describe the evolution of the chemical nature(concentration, speciation, size partitioning) of riverine glacial iron loads over the year as hydrologic pathways and related iron sources change. We then present, to our knowledge, the first quantitative description of the chemical and physical removal of iron in an estuary dominated by glacially-derived iron input. Through the coupling of time-series riverine iron measurements and estuarine transect iron data with estimates of glacial melt water input to the GoA , we then generate glacially-derived iron fluxes to the GoA that include removal terms associated with estuarine processes. Through examination of these data in the context of an evolving high latitude landscape, we can speculate that continued ice loss and land cover change in the subarctic will influence both the flux and chemical nature of riverine-derived nutrient loads to GoA and other high latitude oceans.