Effects of nutrients and irradiance on PSII variable fluorescence of lake phytoplankton assemblages

Phytoplankton from Lake Ontario and six small Canadian lakes (Dorset Lakes) were supplemented with nitrogen (N) and phosphorus (P) to determine how nutrients affect Photosystem II (PSII) variable fluorescence and photoinhibition in natural freshwater communities. Susceptibility of PSII to photoinhibition by photosynthetically active radiation (PAR) and ultraviolet radiation (UVR), as well as recovery potential, was quantified using changes in variable fluorescence and compared between N- and P-supplemented (Nu+) and non-supplemented (Nu-) Lake Ontario phytoplankton. Nu+ communities exhibited slightly higher variable fluorescence than Nu-when dark-adapted (F-v:F-m) or under constant illumination (F-q':F-m'). Rates of relative electron transport (rETR) were greater for Nu+ than Nu- phytoplankton, with higher non-photochemical quenching (NPQ) by Nu-samples. The initial slope of the rETR-irradiance curve (a) did not differ significantly between nutrient treatments, but the saturation irradiance (E-K) was significantly higher for Nu+ samples than for Nu-samples. Nutrient supplementation increased rates of PAR- and UVR-dependent damage but also recovery, so that net PSII photoinhibition was equally severe as in the absence of added N and P. Additions of N, P, and N + P did not significantly alter F-v:F-m of Dorset Lakes phytoplankton. Compared to the range of variable fluorescence observable over the diel cycle of photoinhibition and recovery in Lake Ontario, the effects of supplemental nutrients observed in this study were minor.