The influence of ultraviolet-B radiation on benthic communities in Rocky Mountain streams with different metal exposure histories

Abstract

Increased levels of atmospheric CO2 and associated global climate changes over the next 100 years are expected to have significant impacts on riparian vegetation, biogeochemical cycles and hydrologic processes in the Rocky Mountain region. While considerable research has been devoted to understanding these direct impacts of global warming, much less is known about the interactions between climate change and other forms of anthropogenic disturbance. Contamination from historic mining operations is common in many streams in Colorado and is recognized as a major environmental problem. Pollution from historic mining operations coupled with physicochemical characteristics of Rocky Mountain streams (e.g. shallow, low dissolved organic carbon, high elevation) that increase exposure of benthic communities to UVB (280 to 320 nm) provide an opportunity to examine how UVB interacts with metals contamination to structure benthic communities. I integrated a series of UVB addition experiments conducted in stream microcosms with a large-scale UVB removal experiment to test the hypothesis that the influence of UVB is greater on benthic communities from metal-polluted streams compared to reference streams. Fully understanding the influence UVB radiation in aquatic systems requires estimates of UVB dose. Although radiometric equipment is available for quantifying UVB dose, it is expensive and often impractical for continuous use at remote sites. I applied polysulfone (PSF) dosimetry to measure UVB at the water surface in conjunction with estimates of UVB attenuation and stream depth measurements to determine UVB dose to benthic communities in 12 Colorado Rocky Mountain streams over a 60-day period. Average cumulative UVB intensity near the water surface ranged between 12 J/cm2 to 66 J/cm2, while the average amount of UVB reaching the streambed ranged between 25 -71% of UVB measured at the water surface. Nearly 80% of the variation in the amount of UVB reaching the streambed was explained by dissolved organic carbon concentration (DOC). These results indicate that some benthic communities in high elevation Rocky Mountain streams are subject to levels of UVB that are over 70% of surface measurements. Microcosm experiments involved short-term exposure (7-10d) of natural benthic macroinvertebrate communities collected from reference and metal-contaminated sites to lamp-generated UVB. In all cases measures of abundance decreased in UVB-treated streams compared to controls. However, effects of UVB addition on the total abundance of mayflies, Heptageniidae, and Baetis bicaudatus was significantly greater in communities from metal-polluted sites compared to those from reference sites. The field experiment involved removing UVB for 60 days from portions of the streambed at 12 separate 1st - 4th order stream sites along a Zn gradient. Median Zn concentrations at these sites ranged from 5 and 530 μg/L. Results of the field experiment indicated that the removal of UVB significantly increased total abundance and the abundance of grazers, mayflies, caddisflies, Orthocladiinae midges, and the mayfly Baetis bicaudatus compared to controls. Although grazer abundance was significantly greater in UVB removal treatments, no treatment differences were found in algal biomass or Heptageniidae abundance. The metals gradient may have masked the chlorophyll a and Heptageniidae response to UVB removal, as these endpoints are influenced by metal concentration. Alternatively, increased grazing under the UVB removal treatments may have limited algae accrual. As with the microcosm experiments, effects of UVB removal were generally greater at metal-polluted sites than reference sites. This research incorporated the first large-scale field experiment to investigate the direct influence of UVB on benthic communities. Results from both laboratory and field experiments demonstrated that benthic communities in Rocky Mountain streams were negatively influenced by UVB radiation. Benthic response in both laboratory and field experiments was comparable to other studies, but our results demonstrate that benthic communities in Colorado Rocky Mountain streams are negatively influenced by UVB radiation and that communities subjected to long-term metal exposure are more sensitive to UVB than reference communities. As a consequence, the effects of increased UVB radiation reaching the earth's surface may be more severe than previously considered in systems receiving multiple stressors.