Dietary resource use of nearshore fishes among estuaries that differ in glacial cover

Abstract

Glaciers have a major impact on the way climate change manifests in aquatic ecosystems. When glaciers are present, rising air temperatures associated with climate change can decrease water temperatures and increase freshwater flux due to glacier melt compared to the opposite pattern in neighboring watersheds and estuaries without glaciers. This study considers whether differences in glacier cover of watersheds upstream of nearshore estuaries affect fish food web dynamics, because fish are important integrators of energy flow and ecosystem processes within estuaries. Dietary resources of common nearshore fish species were compared among estuaries with a gradient of upstream glacial cover (0 - 60% watershed cover) and across discharge periods (pre-peak, peak, post-peak) in the Northern Gulf of Alaska (Kachemak Bay) using stomach content and stable carbon and nitrogen isotopes. Comparison among estuaries and all three discharge periods focused on crescent gunnels (Pholis laeta), because they are unlikely to move among estuaries and, therefore, are likely represent local conditions. Discharge period had greater influence on the diet composition and trophic niche breadth of crescent gunnels than glacial coverage. Resource use of more mobile staghorn sculpin (Leptocottus armatus) and starry flounder (Platichthys stellatus) were also considered as additional indicator species within the post-peak discharge period. Staghorn sculpin and starry flounder exploited a larger prey base compared to crescent gunnels, as expected for more mobile fishes. Although fishes differed in their dietary resource use among estuaries, there was no obvious pattern associated with the glacial coverage in any fish species examined. Diet seasonality was apparent with a larger niche breadth in crescent gunnel stomach contents during the post-peak discharge period compared to pre-peak and peak periods. Increasing discharge appears to be associated with a shift in prey composition compared to pre-peak and peak discharge periods. Higher degrees of freshwater input during periods with higher discharge volume may change the organic matter pathways that support lower trophic level prey, resulting in lower trophic position of nearshore fishes over the course of the summer. The results presented here suggest that glacier cover alone is not a major driver of estuarine food webs. It appears that environmental conditions associated with glacial cover during our study period (2020-2021) were within a range that allow nearshore fishes to have similar energy pathways and prey bases across these estuaries. The influence of glaciers on estuarine food webs may become more apparent in years of extreme warmth or drought.