Mercury Transport by Aquatic Insect Emergence from Permanent and Temporary Ponds

Abstract

There are millions of small man-made ponds throughout the Great Plains. These ponds exist across a permanence gradient. Permanent ponds never go dry and can support fish populations while temporary ponds dry periodically and cannot support fish populations. These ponds are all contaminated with a toxic aquatic contaminant, methylmercury (MeHg). Most mercury (Hg) in the environment is released as a result of human activities such as burning coal for energy production and gold mining. These activities release Hg into the atmosphere, and it is eventually deposited into watersheds. Mercury is converted to MeHg in water bodies by aquatic bacteria. Methylmercury moves into food chains and eventually to high level consumers such as insects and fish. Some insects emerge from the water and thereby transport MeHg from aquatic to terrestrial systems, a process known as insect-mediated MeHg flux. Little is known about insect-mediated MeHg flux in ponds and how it is affected by pond permanence and the presence of fish. In our study, we used floating-emergence traps to collect four taxa of emerging insects from five permanent and five temporary ponds at the TCU experimental pond facility from February to May of 2015. Methylmercury flux increased from winter to spring, becoming highest from March through May. Average MeHg flux from temporary ponds was greater than from permanent ponds because fish suppressed the emergence of Chaoboridae, an insect taxa known to be vulnerable to fish predation. Our study suggests insect-mediated MeHg flux from ponds is seasonal and varies depending on pond permanence. Highest MeHg flux will be found in spring in temporary ponds without fish.