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

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Exposure to solar ultraviolet radiation (UVR) is a fact of life for organisms living inthe photic zone of aquatic environments. It is increasingly evident that zooplankton,phytoplankton, protozoa, and bacterioplankton exhibit taxon-specific differences inphysiological and behavioral responses to UVR exposure. Although competition fornutrients is recognized as a critical factor in structuring plankton communitycomposition, nothing is known of the significance of UVR under natural conditions for theintensity of competition among plankton for limiting mineral nutrients. The proposedresearch will help to fill this gap in knowledge. We will examine the main andinteraction effects of the nutrient and UVR regime on the growth rates and competition forphosphorus of naturally occurring phytoplankton and bacterioplankton. A model ispresented and predictions made regarding the severity of nutrient competition under arange in conditions of nutrient (organic carbon and phosphorus) ratios and UVR exposure.We predict that, because of differential effects on bacterioplankton and phytoplanktongrowth rates, UVR will modify the severity of competition between phytoplankton andbacterioplankton for phosphorus. Using samples collected from a low-altitude reservoir innorth Mississippi, experiments will be conducted to test the model predictions. Thisstudy fulfills two needs. It will be the first to evaluate by experimentation usingnatural sunlight the interaction effects of UVR and the nutrient regime on growth rates ofautotrophic and heterotrophic plankton under various degrees of nutrient limitation.Second, it will provide knowledge of the influence of the UVR regime on the relativeimportance of autotrophic and bacterial heterotrophic production in a southern U.S.aquatic ecosystem, where, compared to higher latitudes, there has been very littleresearch on the ecological significance of UVR. Students will be involved in all aspectsof the project, from planning to sampling to data analysis to presentation. Withanticipated continued increases of UVR in temperate regions due to ozone depletion, theeffect of UVR on plankton trophic-level relationships constitutes a timely and importantproblem.