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

The maximum effective quantum yield of photosystem II was estimated from measurements of variable chlorophyll a fluorescence [(F'(m) - F'(o))/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 asirradiance 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 of 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 x 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 effects 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.