OOS 37-6 - Glacial influence on nearshore communities in high latitude estuaries

Background/Question/Methods
The climate is warming rapidly, triggering numerous dramatic impacts including widespread glacial recession in high latitudes. Correspondingly, climate warming and variability is influencing the freshwater and material fluxes from glaciers, streams, and land to coastal areas. As the climate continues to warm and shift, we anticipate changes to the structure and function of terrestrially influenced marine environments. EPSCoR Fire and Ice is conducting a five-year natural space-for-time substitution experiment where it is quantifying multiple environmental and biological parameters to determine how varying amounts of glacial cover and discharge influence the structure and function of nearshore communities. From 2019-2022, we surveyed ten watersheds in two regions in the Gulf of Alaska (GoA), five in Kachemak Bay (northern Gulf of Alaska) and five in Lynn Canal (southeast Alaska). The GoA’s coastline extends almost 50,000 km and covers an area of 1,533,000 km2. Glaciers cover about 20% of this coast, and approximately 50% of the freshwater that drains into the Gulf comes from glaciers. Glacier mass loss in Alaska is among the highest globally (~75 + 11 Gt/year).

Results/Conclusions
Environmental parameters such as salinity, temperature, and turbidity varied along the gradient due to differences in freshwater discharge and oceanic influence. For example, nearshore salinity decreased with increasing glacial coverage consistently within the more oceanic Kachemak Bay while the inland Lynn Canal was variable through time. In more glaciated watersheds, rocky intertidal communities were more dominated by macroalgae than filter feeders. Laboratory experiments showed that macroalgae activate an array of photobiological and metabolic mechanisms that allows them to thrive under turbid conditions typical of glaciated systems. Additional experiments with intertidal herbivores revealed higher consumption but lower preference for macroalgae from less saline sites suggesting they may be of lower quality. Macroalgae were a larger diet component of nearshore invertebrates before the summer discharge period but decreased in importance over the summer season. Terrestrial matter derived from freshwater discharge was not a significant food source in these nearshore food webs. While fish communities did not significantly vary along glacial gradients, prey composition of resident gunnels differed between the two most glaciated and the other, less glaciated sites. As our climate continues to shift, we expect further changes and differences in the structure and function of high latitude terrestrially influenced marine environments.