Bacterioplankton and phytoplankton production rates compared at different levels of solar ultraviolet radiation and limiting nutrient ratios

In aquatic environments in which nutrient concentrations are low, bacteria and phytoplankton may compete for the same nutrient resources. The degree and outcome of this interaction will depend partly on the relative efficiencies with which co-occurring organisms assimilate the limiting nutrient and produce new biomass. Any factor that affects differentially the production rates of competing organisms could influence the competitive outcome. One such factor, which is likely to be relevant in sunlit environments, is ultraviolet radiation (UVR). We conducted a lake water mesocosm experiment to test the effects of UVR in environments of varying carbon + nitrogen (C + N) and phosphorus + nitrogen (P + N) concentrations and ratios on volumetric production of bacterioplankton and phytoplankton. Over the course of the 7 day experiment, only with P + N addition did UVR have a significant effect on bacterioplankton, reducing bacterioplankton production by 21%. UVR also significantly reduced phytoplankton production with P + N addition, by up to 45%. For both bacterioplankton and phytoplankton production, the stimulatory effect of P + N addition was more than 2-fold inhibition due to UVR exposure. Interaction effects of UVR and the nutrient environment were evident for both bacterioplankton and phytoplankton. Overall, the negative effect of UVR on bacterioplankton production was less than that on phytoplankton production in mesocosms in which production was stimulated by nutrients. These results suggest that, at the level of the system, with nutrient addition, UVR provided a competitive advantage to bacterioplankton. Assessments of current or future effects of UVR on aquatic ecosystems require that combined effects of UVR exposure and nutrient availability on ecological interactions be fully considered.