Influence of Total Organic Carbon and UV-B Radiation on Zinc Toxicity and Bioaccumulation in Aquatic Communities DONNA R. KASHIAN,* ,† BLAIR A. PRUSHA, ‡ AND WILLIAM H. CLEMENTS † Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, Colorado 80523, and North Carolina Department of Environment and Natural Resources, Division of Water Quality, Raleigh, North Carolina 27699 The effects of total organic carbon (TOC) and UV-B radiation on Zn toxicity and bioaccumulation in a Rocky Mountain stream community were assessed in a 10-d microcosm experiment. We predicted that TOC would mitigate Zn toxicity and that the combined effects of Zn and UV-B would be greater than Zn alone. However, TOC did not mitigate Zn toxicity in this study. In fact, treatments with TOC plus Zn had significantly lower community respiration as compared with the controls and Zn concentrations associated with the periphyton increased in the presence of TOC. UV-B had no additive effect on periphyton Zn accumulation or community respiration. Heptageniid mayflies (Ephemeroptera) were particularly sensitive to Zn, and reduced abundances were observed in all Zn treatments. UV-B did not additionally impact Heptageniid abundances; however UV-B did have a greater effect on macroinvertebrate drift than Zn alone. Ephemeroptera, Plecoptera, and Trichoptera (groups typically classified as sensitive to disturbance) were found in highest numbers in the drift of UV-B + Zn treatments. Measures of Zn accumulation in the caddisfly Arctopsyche grandis, periphyton biomass, and total macroinvertebrate abundance were not sufficiently sensitive to differentiate effects of TOC, UV-B, and Zn. These results indicate that UV-B and TOC affect Zn bioavailability and toxicity by impacting species abundance, behavior, and ecosystem processes. Introduction Most ecosystems are subjected to multiple perturbations whether natural or human induced. Paine et al. (1) provides a compelling argument that more serious ecological con- sequences result from multiple disturbances as opposed to single perturbations. Metal pollution from historic mining operations in Colorado has been a major concern in Rocky Mountain streams for the last century (2, 3). In addition, these high elevation streams may be particularly susceptible to UV-B (280-320 nm) because solar intensity increases with elevation and because these streams generally have naturally low levels of organic matter, which reduces UV-B exposure (4, 5). Although UV-B is a natural stressor in these ecosystems, anthropogenic release of chlorofluorocarbons and ozone depletion have resulted in a 10-20% increase in UV-B per decade at the earth surface in the Northern Hemisphere (6). UV-B exposure can cause cellular damage to organisms (7), resulting in adverse impacts on community structure and ecosystem function (8, 9). Recent studies suggest that UV-B may influence the distribution and abundance of freshwater organisms (8-10), potentially disrupting basic ecosystem functions such as respiration, primary productiv- ity, nutrient cycling, and food web dynamics (11). Several studies also document aquatic macroinvertebrate behavioral changes associated with UV-B exposure. Donahue and Schindler (12) found that UV-B influences the diel drift of simuliidae larvae. Similarly, Kiffney et al. (13) documented an increase in drifting stream macroinvertebrates associated with elevated UV-B exposure. In contrast, other investigators have found that ambient UV-B exposure has no inhibitory effect on stream benthic algae or invertebrates (14, 15). The depth to which UV-B penetrates aquatic systems is influenced by the quantity of natural organic matter (NOM) present in the system (16). The composition of NOM is variable but typically contains a large proportion of humic substances together with a host of other polyelectrolyte macromolecules (17, 18). NOM plays an important role in aquatic photochemistry (14) and provides protection to aquatic communities from UV-B. However, UV-B degrades NOM into low molecular weight compounds through a process known as photobleaching (19, 20). Because of photodegradation of NOM (21), UV-B penetration and exposure may increase in shallow, high elevation streams. Previous research has demonstrated that metal impacts can structure benthic macroinvertebrate communities. Clem- ents et al. (2) found that metal concentration was the most important predictor of benthic community structure in a study investigating 95 sites in the Rocky Mountains of Colorado. Adverse physiological and individual responses to metal exposure have been observed in algae and mac- roinvertebrates, including reduced growth rates (22-24) and increased mortality (24). Changes in behavior associated with metal exposure have also been documented. Balch et al. (25) found a correlation between high Zn body burdens and the production of abnormal food capture nets in the filter-feeding caddisfly larvae (Hydropsyche betteni). Clements (26) reported an increase in drifting stream macroinvertebrates associated with metal exposure. In addition to concerns over metal pollution and increases in UV-B radiation, recent evidence has indicated that these streams may experience changes in natural dissolved organic material due to changes in hydrologic regime associated with climate change (27). Organic matter plays an important role in both metal bioavailability and UV-B exposure. Interactions among metals, organic matter, and UV-B are complex, and the ability of organic matter to bind metals and attenuate light varies with source (e.g., allochthonous versus autoch- thonous) and chemical composition. Organic matter forms complexes with heavy metals in aquatic ecosystems, thereby reducing metal bioavailability and toxicity (21, 28). Prusha and Clements (29) found an inverse relationship between dissolved organic carbon (DOC), which typically comprises 50% of the mass of NOM (30), and Zn body burdens in the filter-feeding caddisfly Arctopsyche grandis in metal-con- taminated streams. On the other hand, NOM may also increase metal bioavailability. Parent et al. (31) found that the total amount of aluminum associated with the algae was * Corresponding author phone: (970)491-5563; fax: (970)491-5091; e-mail: dkashian@cnr.colostate.edu. † Colorado State University. ‡ North Carolina Department of Environment and Natural Re- sources. Environ. Sci. Technol. 2004, 38, 6371-6376 10.1021/es049756e CCC: $27.50  2004 American Chemical Society VOL. 38, NO. 23, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6371 Published on Web 09/02/2004