One of the most challenging questions in environmental biology is how human-accelerated changes in multiple environmental variables influence 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 exposureof 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 UV-lakes project examined 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 regulates 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) 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 study integrates a carefully focused set of analytical and experimental approaches and applies them across a wide range of pelagic organisms. During the first two years the project investigators looked 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. The wavelength-dependence of these responses and the indirect effects of UV photolysis of CDOM on pelagic food webs also was investigated. A novel phototron apparatus and protocol were 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 were examined, with particular attention to the importance of nutrient limitation and acclimation.
A series of in situ mesocosm experiments, combined with direct-effect bioassays like those in years 1 and 2, examined mechanisms that underlie community and ecosystem level responses. The mesocosms and associated bioassays were designed to 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 were used to directly quantify molecular-level DNA damage and repair in the experiments as well as in nature during the project period. The research focused 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 was 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 research goal is to advance our understanding of how pelagic communities are likely to respond to future changes in UV related to climate change and ozone depletion.
The broader impacts of the research include the close integration of students to provide them with a diverse range of quantitative skills, and simultaneously introduce them to some of the complex issues that environmental biologists face in this period of human-accelerated environmental change. The UV-lakes project directly interfaced with the Lacawac Sanctuary Foundation, a regional non-profit dedicated to preservation, education, and research, and contributed to field station infrastructure by providing an active summer internship program and associated research and educational programs to interface with the foundation's public programs. The project also helped build bridges between major scientific labs that have traditionally used diverse approaches to studying environmental UV effects, including careful inter-comparisons among techniques across a wide variety of taxa. The results of the research were disseminated through a pre-planned set of workshops and symposia that inviteed input from other lead scientists in the field as well as a variety of peer-reviewed publications. The potential benefit to society is an increased understanding of the interaction of multiple stressors on freshwater ecosystems that provide us with water, one of our most valuable resources.
last modified on Feb 12, 2009