Spatial and temporal variability of dissolved aluminum and manganese in surface waters of the northern Gulf of Alaska

Abstract

The Northern Gulf of Alaska (NGA) shelf is a productive high-latitude environment where nutrient dynamics are greatly impacted by the seasonal variability in freshwater input and water column mixing. Iron is a key nutrient on the NGA shelf that directly modulates primary production, but inputs are difficult to quantify due to high biological uptake and control exerted by Fe-binding organic ligands. Other lithogenic elements such as aluminum and manganese have the same sources as iron (rivers and sediment) and similar abiotic removal via particle scavenging, but exhibit quasi- conservative behavior in seawater allowing for their use as tracers of these sources. Thus, Al and Mn distributions can help provide insight into iron inputs and the relative importance of various mechanisms influencing nutrient dynamics in the NGA. The data are derived from spring, summer, and fall NGA LTER (long term ecological research) cruises from 2018 and 2019 that included a focused five-day Copper River plume study, several surface transects from Kayak Island to Kodiak Island, and vertical profiles at several locations sparsely distributed throughout the shelf. We find that seasonal patterns in the surface concentrations of dMn and dAl mirrored annual glacial melt cycles, with the lowest values observed in spring and higher values in summer and fall. Spatial patterns were also apparent as both metals tended to be lower offshore than inshore, and were also lower overall (by 1-2 orders of magnitude) on transects further from the outflow of the Copper River, a major source of freshwater to the NGA. Extremely high concentrations in the Copper River plume (≤1395 nM dAl, ≤128 nM dMn) and strong correlations with salinity (p < 0.0001) highlight their quasi-conservative nature, and their usefulness as tracers of freshwater input, which helps inform iron inputs from this source. Enhanced dAl and dMn concentrations within nepheloid layers in subsurface waters indicate regions where a sedimentary source of iron is likely to be important. Residence times for dAl and dMn in surface waters over the NGA shelf were estimated to be 31 days (dAl) and 42 days (dMn) on average based on summer and fall data from both years.