Glacier runoff as a source of labile dissolved organic matter for near-shore marine ecosystems in the Gulf of Alaska

Northern rivers transport large quantities of dissolved organic matter (DOM), however this organic material is typically thought to be refractory and therefore of little significance for the biogeochemistry of downstream marine ecosystems. Recent research in both the arctic and sub-arctic has shown that terrigenous DOM may be more bioavailable than was previously thought. These findings suggest that riverine DOM has the potential to support both heterotrophic metabolism and primary productivity in near-shore marine ecosystems. Along the Gulf of Alaska (GOA), the ongoing loss of glacier ice in coastal watersheds is altering the land-to-ocean transfer of freshwater and DOM. In particular, DOM derived from glacial runoff appears to be derived largely from microbial precursor material while DOM in watersheds with little or no glacier coverage is predominantly derived from terrestrial plants. The purpose of this study was to test the hypothesis that the character and bioavailability of riverine DOM being exported to the GOA will be altered as glaciers recede and contribute less to streamflow. We sampled rivers draining 12 coastal watersheds along a 500 mile stretch of the GOA during the peak glacier runoff season in the summer of 2008. The study watersheds were typical of the thousands of moderately sized (50-450 km2) watersheds draining the coast mountains along the GOA and ranged in watershed glacier coverage from 0 to >60%. Concentrations of DOC were relatively low in all 12 watersheds ranging from 0.6-2.2 mg C L-1. However, the chemical character of DOM varied widely across the watersheds. As watershed glacial coverage increased and glacier runoff comprised a large proportion of streamflow, riverine DOM became enriched in 13C-DOC and protein content as measured by fluorescence spectroscopy. These findings are consistent with the idea that DOM in pro-glacial streams is largely derived from sub-glacial microbial populations. Moreover, incubations of riverine DOM from all twelve sites that were inoculated with near-shore marine water from the GOA showed substantial losses of DOC (23-66%) over the course of a week. The percentage of bioavailable DOC was positively correlated (p=0.01) with the percentage of watershed covered by glacier ice suggesting that glacier runoff is an important source of labile DOM for heterotrophic bacteria in near-shore marine ecosystems around the GOA. Taken together, our results suggest that changes in watershed glacial coverage may alter the magnitude and bioavailability of riverine DOM entering productive near-shore ecosystems in the GOA.