Interaction Effects of Solar Ultraviolet Radiation and the Nutrient Environment on the Dynamics of Growth of Freshwater Bacteria and Phytoplankton

University Of Mississippi, University MS

Investigators

Abstract

Exposure to solar ultraviolet radiation (UVR) is a fact of life for organisms living in the photic zone of aquatic environments. It is increasingly evident that zooplankton, phytoplankton, protozoa, and bacterioplankton exhibit taxon-specific differences in physiological and behavioral responses to UVR exposure. Although competition for nutrients is recognized as a critical factor in structuring plankton community composition, nothing is known of the significance of UVR under natural conditions for the intensity of competition among plankton for limiting mineral nutrients. The proposed research will help to fill this gap in knowledge. We will examine the main and interaction effects of the nutrient and UVR regime on the growth rates and competition for phosphorus of naturally occurring phytoplankton and bacterioplankton. A model is presented and predictions made regarding the severity of nutrient competition under a range in conditions of nutrient (organic carbon and phosphorus) ratios and UVR exposure. We predict that, because of differential effects on bacterioplankton and phytoplankton growth rates, UVR will modify the severity of competition between phytoplankton and bacterioplankton for phosphorus. Using samples collected from a low-altitude reservoir in north Mississippi, experiments will be conducted to test the model predictions. This study fulfills two needs. It will be the first to evaluate by experimentation using natural sunlight the interaction effects of UVR and the nutrient regime on growth rates of autotrophic and heterotrophic plankton under various degrees of nutrient limitation. Second, it will provide knowledge of the influence of the UVR regime on the relative importance of autotrophic and bacterial heterotrophic production in a southern U.S. aquatic ecosystem, where, compared to higher latitudes, there has been very little research on the ecological significance of UVR. Students will be involved in all aspects of the project, from planning to sampling to data analysis to presentation. With anticipated continued increases of UVR in temperate regions due to ozone depletion, the effect of UVR on plankton trophic-level relationships constitutes a timely and important problem.