The influence of glaciers on coastal marine ecosystems in the Gulf of Alaska

Abstract

Glacier runoff (i.e., meltwater and rainwater discharged at the glacier terminus) provides about half of the freshwater discharge into coastal margins of the Gulf of Alaska, where contemporary glacier melting rates are among the highest in the world. Roughly 410 billion metric tons of glacier runoff enter the Gulf of Alaska each year. With freshwater discharge volumes of that magnitude, I hypothesized that glacier runoff has both direct and indirect effects on the receiving coastal marine ecosystems that support rich food webs, abundant and diverse marine communities, commercial fisheries and tourism industries. To examine the influence of glacier runoff on coastal marine ecosystems, I focused on three questions: 1) How does the marine food web respond to physical and biological gradients induced by glacier runoff? 2) What is the contribution of riverine organic matter (OM) and ancient carbon resources in glacier runoff to marine food webs from plankton to seabirds? and 3) How does the influence of glaciers on coastal marine ecosystems differ at small to large spatial and temporal scales? I measured physical, chemical and biological indices within four fjord systems along the eastern Gulf of Alaska coast. In chapter one I used geostatistics as well as parametric and non-parametric models to demonstrate a strong influence of glacier runoff on ocean conditions and coastal food webs across all the fjord systems. In chapter two I used isotopes (δ2H, δ13C, δ15N, and Δ14C) to trace riverine OM and ancient carbon resources into the marine food web. This work included the development of a novel multi-trophic level 3-isotope Bayesian mixing model to estimate the proportion of allochthonous resources in animal tissues. Mean estimates from 14 species groups spanning copepods to seabirds ranged from 12 – 45 % riverine OM source assimilation in coastal fjords, but ancient carbon use by marine food webs was low. In the third chapter I synthesized information on the scale-dependent influence of glaciers on lower-trophic level productivity, predator-prey interactions and ways that humans may be affected by anticipated changes in glacier coverage. This contemporary understanding of glacier influence on coastal ecosystems aligns with paleoenvironmental evidence suggesting that over geological time scales glaciers have and will continue to shape marine ecosystems in the Gulf of Alaska.