The impact of glacier runoff on the biodegradability and biochemical composition of terrigenous dissolved organic matter in near-shore marine ecosystems

The processing of terrigenous dissolved organic matter (DOM) by aquatic food webs modifies its biochemical composition from riverine to coastal ecosystems. We used parallel factor analysis (PARAFAC) of fluorescence excitation–emission matrices (EEMs) and biodegradable dissolved organic carbon (BDOC) incubations to investigate changes in the biochemical composition and lability of terrigenous DOM in three estuaries of coastal southeastern Alaska: 1) a watershed with high glacial coverage, 2) low glacial coverage, and 3) low glacial coverage and high wetland coverage. Laboratory BDOC incubations were conducted for each site by inoculating filtered river water with microbial inocula collected from four different salinities (0, 2, 10 and 25) along the estuarine transect. The percent BDOC for all three sites ranged from 22 to 44% for the 28-day incubations and was greatest in the estuary draining the highly glaciated watershed. Moreover, percent BDOC was greatest for river water samples inoculated with marine compared to freshwater bacteria suggesting marine bacterioplankton were able to utilize a larger fraction of the terrigenous DOM pool than riverine microbes. PARAFAC modeling of fluorescence EEMs showed non-conservative estuarine mixing behavior for DOM including removal at low salinities and addition at mid-high salinities for all three sites. For example, tyrosine-like fluorescence decreased dramatically between salinity values 0 and 0.5 and was undetectable by salinity 2 for all three estuaries. However, humic-like C4 (correlated with aliphatic carbon content) and tryptophan-like fluorescence increased non-conservatively during estuarine mixing, likely associated with an increase in bacterioplankton growth. These results indicate that terrigenous DOM, particularly from glacial runoff, is an important source of carbon and nutrients to near-shore coastal zones of southeast Alaska.