Glacier Extent and Subsurface Flow Path Dominate Seasonal Dissolved Organic Matter Character Across Glacierized Watersheds

Coastal alpine glaciers across the Gulf of Alaska are receding at an accelerated rate due to climate change, and glacial meltwater is supplying downstream environments with significant inputs of dissolved organic matter (DOM). DOM plays a critical role in aquatic ecosystems by fueling microbial metabolism and influencing nutrient cycling and trace metal transport. In this study, we investigated seasonal DOM composition across four watersheds spanning a 30 km transect, with glacial coverages ranging from 0 to 60%. Stream and shallow groundwater samples were collected in July and September 2023 to capture DOM seasonality, from peak summer glacier ice melt to the onset of autumn. Using ultra-high-performance liquid chromatography–Orbitrap mass spectrometry, we found that watersheds with large glacial coverage were associated with elevated relative abundances of aliphatic, carbohydrate-, and lipid-like DOM, classes providing an estimate for the bioavailability present in streams. While dissolved organic carbon concentrations were low in these watersheds, the estimated yield of molecularly labile carbon exported to coastal margin ecosystems increased with watershed glaciation. In a glacier-containing watershed, shallow groundwater DOM exhibited declining putative bioavailability with increasing distance from a proximal glacial stream along the aquifer flow path from the stream to the coast. Across the four stream systems, stream water DOM composition showed less variability between sites during peak ice melt in July, when molecularly labile DOM export from glaciers was hypothesized to be the highest. These findings pair with existing hydrogeochemical studies in the region to build a defined framework of seasonal nutrient flux to nearshore marine environments and create a baseline for future DOM studies in the Gulf of Alaska and beyond.