In situ UVA exposure modulates change in the uptake of radiophosphate in size-fractionated plankton assemblages following UVR exposure.

We investigated the effect of ultraviolet radiation (UVR) on the uptake and partitioning of radiophosphate ((33)PO (4) (3-) ) in size-fractionated plankton assemblages (0.2-0.8, 0.8-2.0 and >2.0 μm) collected from nine freshwater lakes located in Saskatchewan, Canada. A significant (p < 0.05) reduction in (33)PO (4) (3-) uptake by plankton was observed in seven of the nine lakes. Plankton >2.0 μm were generally unaffected by UVR, whereas the 0.2-0.8 μm size fraction exhibited severe photoinhibition. The effect of UVR on the 0.8-2.0 μm size fraction was variable, ranging from significant reductions to significant increases in (33)PO (4) (3-) uptake. The >2.0 μm size fraction was composed of a diversity of phytoplankton genera, suggesting that P uptake mechanisms for a range of phytoplankton are resistant to UVR. Our ability to detect a UVR effect on specific plankton size fractions was confounded by the resolution of the analysis. That is, only examining the <2.0 and >2.0 μm size fractions concealed the effect of UVR on plankton <0.8 μm. The magnitude of decrease in P uptake by plankton <0.8 μm was significantly and negatively correlated with in situ UVA exposure. Our results underscore the need for studies to consider both the size resolution of their analysis (i.e., the size of target organisms) and the ambient light conditions under which organisms may have acclimated before generalizing results across limnetic systems.