Communities in freshwater coastal rock pools of Lake Superior, with a focus on chironomidae (Diptera)

Abstract

Exposed freshwater shores have received little ecological attention. At Isle Royale, an archipelago in Lake Superior, coastal areas have a moderated climate due to the large, deep lake. This project was instigated because 1) little was known about coastal ecosystems at Isle Royale, 2) unique aquatic species were expected in this habitat, 3) potentially disastrous impacts from shipping accidents could cause population losses of sensitive species, and 4) long-term shifts in climate could influence communities via warming conditions and erratic precipitation. Focal habitats were pools on the open shore, formed in bedrock depressions from precipitation, ground water, overland flow, and wave wash. The focal study taxon was Chironomidae, a diverse family of aquatic flies, well-known for variable responses to pollution and ecological gradients. Results showed a surprisingly species-rich (n = 102) and diverse assemblage. Moderate to extreme range expansions and novel habitat use were documented. Distinct communities were observed between two vertical zones, based on distance from the lake. Differences in seasonal emergence occurred close to the lake, but emerging assemblages were similar across seasons above lake influence. Some evidence supported both biogeographic community variation based on distances between islands and control of emergence events by water temperature. However, there was no evidence that pool depth or potential for desiccation influenced chironomid occupancy. Nutrients (phosphorus, nitrogen, and carbon) were the most important factors driving chironomid assemblage differences. Pools higher on the shore showed nutrient input from upland habitats, while pools lower on the shore appeared to have nutrients flushed out by wave action and replaced with low-nutrient water. Comprehensive habitat mapping revealed high pool densities, with an extraordinary number on Passage Island, arguably the most threatened locality to shipping pollution. Also, Passage Island had the highest density of Pseudacris triseriata, a frog species with a remarkably narrow habitat use at Isle Royale. Overall, a diverse community was revealed in what superficially looks like unremarkable habitat. Apparent similarities to northern and western shores of Lake Superior suggest this study of basic aquatic ecology and physical structure describes reference conditions for coastal management across much of the lake.