Effects of Nutrients, Photoinhibition & Photoacclimation on Photosystem II Function of Freshwater Phytoplankton Communities
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Electron flow through Photosystem II (PSII) is essential to all life on Earth. The efficiency of this process in freshwater phytoplankton can be depressed by nutrient limitation, ultraviolet radiation (UVR), and excessive photosynthetically active radiation (PAR). The effects of nutrients and radiation on PSII function of natural communities were assessed using changes in the variable fluorescence of PSII (PSII VF), as determined by a pulse amplitude modulation (PAM) fluorometer. The net susceptibility to photoinhibition of PSII depends upon the sensitivity of the assemblage to photodamage and the efficiency of recovery. Damage and recovery rates were quantified by fitting changes in FV:FM during experimental spectral exposures to the model of Kok (1956). Phytoplankton from deep chlorophyll maxima were found to exhibit much higher photosynthetic impairment under UVR exposure than epilimnetic communities in two lakes, due to differences in both damage and recovery rates. In six temperate lakes of the Dorset-Haliburton region, the susceptibility to UVR-induced photoinhibition of PSII was found to be a function of the water transparency (dissolved organic carbon content) of the systems from which the plankton were isolated, with no obvious taxonomic pattern to the responses. Nutrient (nitrogen & phosphorus) supplementation of communities from the Dorset Lakes and from Lake Ontario did not have strong effects on PSII VF, and did not alter the response of Lake Ontario phytoplankton to spectral irradiance. Diurnal changes in FV:FM of Lake Ontario phytoplankton were modeled and average values for the upper half of the water column showed reasonable agreement with observed data; however it is suggested that the addition of a model to simulate vertical mixing could improve the depth-specific accuracy of the predictions. It is concluded that the light history (photoacclimation status) of phytoplankton is the major determinant of the sensitivity of PSII to UVR, and that the nutrient status and taxonomic composition of phytoplankton communities have a comparatively minor influence.