Seasonality of dissolved organic matter in lagoon ecosystems along the Alaska Beaufort Sea coast

Seasonal variations in dissolved organic matter (DOM) fluxes from land to sea have been well documented in the Arctic, yet remarkably little is known about how DOM varies seasonally within Arctic coastal waters. This is particularly true for shallow inshore environments. Here we document seasonality of DOM in lagoons along the eastern Alaska Beaufort Sea coast during the spring ice break-up (June), summer open-water (August), and late winter ice-covered (April) periods. We measured dissolved organic carbon (DOC) concentrations, C:N ratios, and chromophoric DOM (CDOM) characteristics (SUVA(254) and S275-295). On average, lagoon DOC was highest during spring (similar to 200 mu M) and lowest during winter (similar to 120 mu M). C:N and SUVA(254) also differed markedly between spring (C:N similar to 28; SUVA(254) similar to 3.5 L mg C-1 m(-1)) and winter (C:N similar to 16; SUVA(254) similar to 2.2 L mg C-1 m(-1)). Average S275-295 values (similar to 14-17 x 10(-3) nm(-1)) were not significantly different among seasons, but variability in S275-295 was much lower in spring. This was attributed to terrestrial runoff based on its relatively uniform composition during the spring freshet. Seasonal variations in lagoon DOM were broadly correlated with variations in salinity (i.e., lagoon water transitioned from largely fresh in spring to hypersaline in winter), but a mixing analysis revealed strong evidence of processing and net DOC generation in summer. We also found evidence of DOC processing and build-up under the ice over the winter. This baseline understanding of DOM seasonality in Arctic lagoons is essential for evaluating climate-linked impacts on Arctic coastal ecosystems and broader feedbacks associated with carbon cycling.