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. Iron is a key nutrient on the NGA shelf that directly modulates primary production, and freshwater is a major input of iron to the NGA shelf. However, variability in the input of dissolved iron from freshwater sources is obscured by high biological uptake and by the concentration of organic ligands, which bind iron and help maintain it in solution. Riverine inputs of other lithogenic elements such as aluminum and manganese are expected to behave quasi-conservatively on the NGA shelf and could help provide insight into the variability of iron input in this region.Here we present the seasonal (spring, summer, and fall) distribution of dissolved aluminum (dAl) and manganese (dMn) observed during 2018 and 2019 in the NGA region. Data were obtained during the NGA Long Term Ecological Research (LTER) site cruises and include several surface transects from Kayak Island to Kodiak Island, vertical profiles at several locations sparsely distributed throughout the shelf, and surface information from a Copper River plume study. 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. Sharp cross-shelf gradients were observed for both metals particularly in summer and fall. Surface concentrations decreased (by 1–2 orders of magnitude) away from the outflow of the Copper River, the major point source of freshwater within the NGA LTER site. Extremely high concentrations in the Copper River plume (≤1395 nM dAl, ≤128 nM dMn) and strong correlations with salinity (p = 7.9E-14 for dAl, 1.4E-14 for dMn) highlight the quasi-conservative nature of these metals within the plume. Enhanced dAl and dMn concentrations within nepheloid layers in subsurface waters indicate regions where a sedimentary source of iron could also 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.