Glacier shrinkage impacts dissolved and particulate organic carbon export from coastal temperate forested watersheds

Riverine transport of organic carbon (OC) to the coastal ocean is a key contributor to the global carbon cycle because river networks transport, mineralize, and bury significant amounts of OC in estuarine and shelf sediments. The impact of riverine OC on atmospheric CO₂ is influenced by its size fraction, particulate (POC) versus dissolved (DOC), and the extent to which POC is sourced from the lithosphere versus the terrestrial biosphere. Here, we collected streamwater weekly during the main glacial runoff season (May through October) to evaluate seasonal sediment and OC budgets for four watersheds in southeast Alaska that vary in glacier coverage from 0 to 48%. Samples for ∆¹⁴C-POC were also collected monthly from all sites during this same time period to evaluate how the source of POC changed across the watershed glacierization gradient. Concentrations of DOC were similar to POC in the two more heavily glacierized watersheds (0.2-4 mg C L^-1) but DOC was higher by an order of magnitude in the non-glacierized forested watershed. Across the watershed gradient, differences in OC concentrations and discharge resulted in POC yields increasing by ~2.5x and DOC yields decreasing by ~7x between the least and most heavily glacierized watersheds. In the non-glacierized watershed, the average ∆¹⁴C-POC (-43‰) and low sediment yields are consistent with a predominance of biospheric POC derived from recently fixed carbon in C3 plant biomass. However, average ∆¹⁴C-POC values in the three glacierized watersheds exceeded -100‰ indicating a shift towards aged biogenic or petrogenic sources of POC. Our findings show that future changes in glacier coverage will shift the size fraction of riverine OC and potentially the source of POC, with implications for the export of biolabile OC from heavily glacierized regions. We suggest that climate warming will result in greater changes in the magnitude and size fractionation of riverine OC in glacier watersheds compared to non-glacierized forested watersheds.