Effects of fish on emergent insects and their transport of methyl mercury from ponds

Abstract

Methyl mercury (MeHg) is an environmental contaminant affecting the health of wildlife. It was once thought that only aquatic consumers were at risk of MeHg contamination, but we now know emergent insects transport MeHg from aquatic systems to terrestrial food webs (MeHg flux). Factors regulating MeHg flux in emergent insects are currently unknown. This study tests the hypothesis that fish predation regulates insect emergence and flux of MeHg. The experiment utilized five ponds stocked with fish and five ponds without fish. Floating emergence traps were used to capture emergent insects. Fish significantly suppressed Hg flux in dragonflies and damselflies, significantly enhanced MeHg flux in caddisflies, and did not affect Hg flux in midges. Total MeHg flux was significantly greater in ponds without fish. This is the first study to show that fish have complex direct and indirect effects on insect emergence and the flux of MeHg out of ponds.