I study freshwater ecology, from the perspective of benthic invertebrates. Many of my research projects use chironomids, “non-biting midges”, as a model research system well-suited to studies of biodiversity. The Family Chironomidae includes thousands of species with an amazing range of ecological specialization. Chironomid larvae are abundant in many aquatic habitats, and thus are useful in studying the role of primary consumers in aquatic food webs. Three topics characterize my current research:
Species delineation and systematics of chironomids of the Genus Cricotopus, using analysis of DNA sequence data and morphological data: Worldwide, Cricotopus is a large genus with many undescribed and/or highly variable species. This project is being carried out in collaboration with Dr. Torbjørn Ekrem and Dr. Elisabeth Stur, of the Norwegian University of Science and Technology. We are using molecular genetic data to compare North American and European populations of nominal Cricotopus species, and to study their phylogenetic relationships.
Distribution of chironomid species along gradients of watershed urbanization: Collection of floating chironomid pupal exuviae (cast pupal exoskeletons) provides an effective means to compare species diversity among streams that are impacted by urban development. Since the chironomid species vary greatly in the sensitivity of their larvae to pollution, we are using samples of exuviae to develop statistical tools (metrics, tolerance values) which should provide improved resolution of changes in the ecological health of impacted streams, or streams undergoing restoration.
Periphyton (i.e., attached algae), provide important inputs to stream food webs, and are habitat for many aquatic invertebrates. Suspended silt-clay sediments are a widespread pollutant in streams and pose a major problem to the Chesapeake Bay. Production of stream algae is affected by grazing invertebrates and by fine silt-clay sediments, which are capable of binding and releasing phosphorus, a potentially limiting nutrient. We have developed laboratory microcosms to study the response of algal P-limitation and growth of chironomid larvae to different types and levels of suspended sediments.
S.E. Gresens, E. Stur & T. Ekrem. 2012. Phenotypic and genetic variation within the Cricotopus sylvestris species-group (Diptera, Chironomidae), across a Nearctic- Palaearctic gradient. Fauna Norvegica 31:137-149.
S.E. Gresens. 2011. Response of Chironomidae to multiple gradients of urban impact.
Proceedings of the XVIIth Symposium on Chironomidae. X. Wang (ed.), Nankai University, Tianjin, China.
Stranko, S.A.., S.E. Gresens, R.J. Klauda, J.V. Kilian, P.J. Ciccotto, M.J. Ashton & A.J. Becker. 2010. The differential effects of urbanization and non-natives on imperiled stream species. Northeastern Naturalist 17:593-614.