Marine and Not Terrestrial Resources Support Nearshore Food Webs Across a Gradient of Glacial Watersheds in the Northern Gulf of Alaska

Abstract

Abstract Estuaries are among the most productive ecosystems on Earth, yet are especially at risk in high-latitude regions due to climate-driven effects on the connected terrestrial and marine realms. Warming in the Northern Hemisphere exceeds the global average and is a major cause of the rapid melting of glaciers. As a result, the timing and magnitude of freshwater discharge into estuaries is subject to increase during the peak in glacial meltwater, ultimately affecting the riverine flux of nutrients and organic matter (OM) from the land to coastal environments. Intertidal communities near the outflow of rivers often rely on supplementing autochtonous (marine) food sources with allochthonous (terrestrial) subsidies, even though terrestrial OM can be problematic for marine consumers to assimilate. We investigated if terrestrial matter subsidizes nearshore food webs in northern Gulf of Alaska watersheds, and if the relative proportion of terrestrial versus marine OM supporting these food webs differed with watershed glacial cover characteristics and with seasonal glacial discharge regimes. We employed a Bayesian stable isotope mixing model to determine the contribution of marine (phytoplankton, macroalgae) and terrestrial (vascular plant) sources to the diets of grazing/detritivore and filter/suspension-feeding coastal invertebrates at the outflows of watersheds of varying glacial influence and across various discharge periods. Additionally, we conducted a distance-based redundancy analysis to investigate the effects of watershed-characteristic sourcing and transport of terrestrial OM on nearshore consumer diets. The diets of both feeding groups were predominantly marine (> 90%) and varied little among sites or glacial discharge periods. However, consumers were depleted in 13C isotopes with increasing glacial cover; the significant watershed descriptors suggest that this change was more associated with discharge effects on marine primary production rather than consumption of terrestrial OM by the invertebrates. These results suggest that, while watershed exports may influence the stable isotope composition of OM sources, the diets of these feeding groups are mostly decoupled from terrestrial influence during the time of sampling. Our results suggest that marine OM availability in the study system is not limiting, and terrestrial OM subsidies in such productive systems are not needed to support nearshore food webs.