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

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Abstract
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 × 10−4 to 9.1 × 10−4 s−1, while damage rate constants ranged from k = 0.03 to 0.22 m2 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.

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