Effects of Fish on Emergent Insect-Mediated Flux of Methyl Mercury across a Gradient of Contamination Brent N. Tweedy, † Ray W. Drenner, † Matthew M. Chumchal,* ,† and James H. Kennedy ‡ † Department of Biology, Texas Christian University, Winton Scott Room 401, 2800 South University Drive, Fort Worth, Texas 76129, United States ‡ Department of Biology, University of North Texas, 1155 Union Circle No. 310559, Denton, Texas 76203, United States * S Supporting Information ABSTRACT: We examined the effects of fish predation on emergent insect-mediated methyl mercury (MeHg) flux across a gradient of MeHg contamination in experimental ponds. Emergent insects were collected from ponds with (n = 5) and without fish (n = 5) over a six week period using floating emergence traps. We found that the potential for MeHg flux increased with Hg contamination levels of the ponds but that the realized MeHg flux of individual insect taxa was determined by fish presence. Fish acted as size-selective predators and reduced MeHg flux by suppressing emergence of large insect taxa (dragonflies and damselflies) but not small insect taxa (chironomids and micro- caddisflies). MeHg flux by small insect taxa was correlated with concentrations of MeHg in terrestrial spiders along the shorelines of the study ponds, demonstrating for the first time the cross-system transport of MeHg by emergent insects to a terrestrial spider. ■ INTRODUCTION The fate and transfer of the methylated form of Hg (MeHg) in the environment is of particular concern to ecotoxicologists because MeHg readily bioaccumulates in the tissues of biota and is extremely toxic, negatively affecting the health of wildlife. 1 Worldwide anthropogenic emissions of inorganic forms of Hg coupled with a dynamic, global atmospheric cycle have resulted in contamination of most ecosystems on Earth with levels of Hg that exceed preindustrial baselines. 2 Because the conversion of inorganic forms of Hg to MeHg is a process primarily mediated by aquatic microbes, 2 MeHg was previously thought to only threaten aquatic biota and consumers of aquatic organisms. However, recent studies have found elevated concentrations of MeHg in terrestrial consumers. 3-6 Contaminants, such as MeHg, that enter aquatic food webs have the potential to be transferred to terrestrial food webs when aquatic insects that spend part of their life cycle in aquatic ecosystems emerge as adults. 6-10 Although it is recognized that emergent aquatic insects provide critical energy subsidies to terrestrial food webs adjacent to aquatic systems, 11-16 their role as biovectors of aquatic contaminants to terrestrial ecosystems is just beginning to be understood. 6-9,17 Factors that control the magnitude of insect-mediated MeHg flux have rarely been studied, 18 and the relationship between insect-mediated MeHg flux and MeHg concentration in terrestrial predators has not been examined. In this study, we assess insect-mediated MeHg flux across a gradient of Hg contamination and determine how MeHg flux is affected by fish predation. We hypothesized that fish predation can reduce insect-mediated contaminant flux out of aquatic ecosystems by reducing aquatic insect biomass and altering insect community structure. 18-23 Here, we use experimental pond ecosystems to test two hypotheses: (H 1 ) the potential for insect-mediated MeHg flux from waterbodies is positively related to the overall level of food web contamination, but the realized MeHg flux is regulated by fish predation on emergent aquatic insects; and (H 2 ) the MeHg flux of emerging insects is correlated with MeHg concentrations in terrestrial long-jawed orb weaver spiders (Tetragnathidae: Tetragnatha sp.). Spiders that live on the shorelines of aquatic ecosystems are predators of aquatic insects 7-9,24-27 and play a key role in mediating contaminant flux from aquatic to terrestrial ecosystems because they are themselves consumed by terrestrial predators such as birds. 4,8 ■ METHODS Study Site. We conducted the present study in 10 experimental ponds in Fort Worth, Texas, USA. The experimental ponds are whole ecosystems with earthen bottoms that contain complex communities of macrophytes, benthic invertebrates, and herptiles. Ponds are large and range in size from 0.23 to 0.54 ha with maximum and average depths of 1.2 and 0.6 m, respectively. Macrophyte communities were variable between ponds and were composed of several species of emergent and submerged taxa. An image from the Received: August 16, 2012 Revised: December 21, 2012 Accepted: January 3, 2013 Published: January 3, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 1614 dx.doi.org/10.1021/es303330m | Environ. Sci. Technol. 2013, 47, 1614-1619