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UV Lakes Project

One of the most challenging questions in environmental biology is how human-accelerated changes in multiple environmental variables are influencing natural communities and ecosystems. One of the more pervasive of these changes is the increase in ultraviolet radiation (UV) related to stratospheric ozone depletion. In aquatic ecosystems, changes in chromophoric (color-absorbing) dissolved organic matter (CDOM) related to changes in climate, land use, and hydrology, are of key importance in regulating changes in the exposure of aquatic organisms to UV. Complex environmental gradients in temperature and UV combine with concerns about future trends toward increasing regional and global UV and temperature to argue for the need to understand the interactive effects of temperature and UV in natural ecosystems. The fact that DNA is the primary target for UV damage argues for the need for studies of UV effects to be integrated across levels of biological organization from the molecular to the ecosystem level.

The project examines the regulation of UV impacts on pelagic food webs with an integrated study of two opposing, but interrelated hypotheses - regulation of UV impacts at the molecular level (photoprotection vs DNA repair) versus regulation at the ecosystem level (CDOM-mediated attenuation of UV and associated indirect effects). It is argued that climate change will regulate UV damage at both the molecular level (by altering temperature), and at the ecosystem level (by altering CDOM). More specifically it is hypothesized that environments with high UV and low temperature (high UV:T ratio) will favor more autotrophic, phytoplankton-based food webs while environments with low UV:T ratios will favor more microbial, bacteria-based food webs with consequently different zooplankton grazers.

The proposed study will integrate a carefully focused set of analytical and experimental approaches and apply them across a wide range of pelagic organisms. During the first two years the project will look at UV damage and the effectiveness of different molecular mechanisms of UV defense in phytoplankton, bacteria, protozoa, zooplankton, and fish at a range of temperatures. It will also look at the wavelength-dependence (Biological Weighting Functions) of these responses and the indirect effects of UV photolysis of CDOM on pelagic food webs. A novel UV exposure apparatus and protocol will be used to estimate temperature-dependent changes in UV tolerance and its components: photorepair, dark repair, and photoprotection. The effects of temperature on UV impacts on primary productivity and bacterial productivity will be examined, with particular attention to the importance of nutrient limitation and acclimation.

During years 3 and 4 a series of in situ mesocosm experiments, combined with direct-effect bioassays like those in years1 and 2, will examine mechanisms that underlie community and ecosystem level responses. The mesocosms and associated bioassays will test whether the strong effects often observed in past small-scale bag and bottle experiments actually translate to meaningful changes at the community and ecosystem level in the surface mixed layer of lakes. Molecular photoproducts will be used to directly quantify molecular-level DNA damage and repair in the experiments as well as in nature during all four years. The research will center on four low elevation lakes in Pennsylvania and in high elevation lakes in the Beartooth Mountains of Wyoming where very high UV:T ratios are observed. CDOM from these systems will be exposed to UV to characterize the biolability and changing optical signatures of the CDOM (spectral slope, DOC-specific absorbance). By applying a diverse set of experimental and analytical tools to multiple trophic levels within the same study, the proposed research will advance our understanding of how pelagic communities are likely to respond to future changes in UV related to climate change and ozone depletion.

The project takes in a broad scope and involves investigators at seven collaborating institutions. The specific contribution of the SERC program is to measure wavelength dependence (Biological Weighting Functions) to characterize the direct effects of UV on metabolism of phytoplankton, bacteria, protozoa, and growth and survivorship of zooplankton, and fish. These functions are used to infer the extent and nature of acclimation over trophic levels and as basis for modeling direct effects in the mesocosm experiments. Incident UV radiation in the lakes is monitored using the SR18.
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