Trophic structure of rocky intertidal communities in contrasting high-latitude environments

Abstract

Coastal ecosystems in high latitudes are increasingly impacted by glacial melt and river discharge due to climate change. One way to understand ecosystem responses to these stressors is assessing trophic relationships. The goal of this study was to better understand how hydrographic conditions influence trophic structure in high-latitude rocky intertidal systems. I compared the trophic structure of rocky intertidal assemblages in two regions in the northern Gulf of Alaska that comprise the same regional species pool but are hydrographically distinct, one glacially influenced and one primarily marine influenced. Common macroalgal and invertebrate taxa, as well as particulate organic matter (POM) were sampled at three rocky intertidal communities in each region in 2017 and 2018. Food web structure was compared using trophic metrics based on the distribution of shared taxa in isotopically-derived (delta¹³C and delta¹⁵N) trophic niche space. I found that trophic niche space in the glacially-influenced rocky intertidal system was larger, driven by larger ranges in both carbon and nitrogen stable isotope values. In particular, the lower delta¹³C values in Kamishak Bay suggest that an additional food source, most likely terrestrial organic material from glacial discharge, was incorporated into the food web. This supported the hypothesis that patterns in food web structure differed based on hydrographic influences to maintain overall stability. Isotopic evenness, i.e., the distribution of the shared taxa within trophic space, did not differ between the two regions. Macroalgae were overall a more important food source than POM in both regions, and even more so in the glacially-influenced region, where macroalgae may be an energetically beneficial food source compared to silt-laden glacial inputs. This study showed that common intertidal taxa have high tropic flexibility enabling them to respond to variable food sources under a variety of environmental conditions. As hypothesized, taxa in food webs occupying a larger trophic niche space engaged in more heterogeneous trophic pathways and used diverse resources depending on local environmental conditions, making these systems more stable to perturbations affecting a single resource. The common taxa of high-latitude rocky intertidal systems studied here are responding to current levels of glacial input with more diverse resource use, but it remains to be seen how well those systems are suited to maintain trophic stability with an expected increase in glacial stress from climate warming.