The influence of vertical mixing on the photoinhibition of variable chlorophyll a fluorescence and its inclusion in a model of phytoplankton photosynthesis

The maximum active quantum yield of photosystem II was estimated from measurements of variable chlorophyll a fluorescence [(F' m - F' 0 )/F' m = F' v ,/F' m ] in samples of phytoplankton collected from various depths in Chaffey Reservoir, Australia. During stratified conditions, F' v /F' m showed depth-dependent decreases as irradiance increased during the morning, and increases as irradiance reduced in the afternoon. Wind-induced mixing disrupted the diel pattern, but even under well-mixed conditions a vertical gradient in F' v /F' m remained. Differences in F' v /F' m values between samples incubated at fixed depths and unconstrained lake samples enabled identification of the phytoplankton mixing depth. Recovery of F' v /F' m was modelled as a function of time and the degree f F' v /F' m inhibition, while damage was considered a function of photon dose. A combined, numerical model was fitted to diel sequences of F' v /F' m to estimate rate constants for damage and repair Recovery rate constants ranged from r = 0.7 × 10 -4 to 9.1 X 10 -4 s -1 , while damage rate constants ranged from k = 0.03 to 0.22 m 2 mol photon -1 . A fluorescence-based model of photosynthesis was used to investigate the sects of wind speed, euphotic depth and mixed layer depth on photoinhibition. At different mixing rates, depth-integrated photosynthesis was enhanced by up to 16% under the conditions tested, while increases of 9% occurred between phytoplankton with different measured damage and repair characteristics.

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