The impacts of waterbird-mediated elemental enrichment on chironomid assemblages from island ponds in Lake Ontario

Abstract

Ring-billed gulls (Larus delawarensis) and double-crested cormorants (Phalacrocorax auritus) have nested in dense colonies across the Laurentian Great Lakes since their rapid population expansions during the early- and mid-20 th century, respectively. Dated sediment cores from ponds located near nesting sites have been used to track the history of these large colonies on islands in eastern Lake Ontario. However, less is known about avian-mediated biotransport of contaminants to these nesting sites and its effects on aquatic biota at mid-trophic levels, such as chironomids. Here, we used a paleolimnological approach to determine the extent of possible avian- mediated metal(loid) contamination of ponds in nesting sites compared to reference sites. Water from avian-im- pacted ponds had higher concentrations of Al, Cd, Cu, Fe, Pb, and Se than the reference ponds, and exceeded the Canadian Council of Ministers of the Environment (CCME) guidelines for the protection of aquatic life at some sites. Down-core geochemical analysis of elements in sediments showed that concentrations of Cd, Se, and Zn were significantly higher in impacted ponds than in reference sites and correlated significantly with stable nitrogen isotopes, a marker of avian-derived nitrogen. In sites impacted by ring-billed gulls, Cd and Zn concentrations in sediments exceeded or approached those at which frequent adverse biotic effects are expected. Metal contamination was linked to altered chironomid assemblages, with subfossil remains in the avian-impacted sites including more taxa tolerant of heavy metal contamination than reference sites. Notably, chironomids were largely absent from the sites with the highest metal concentrations. These findings have implications for understanding and managing contaminant cycling in the Great Lakes, as waterbirds that nest in dense colonies can act as biovectors that transport metals to new and unexpected locations.