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

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research Centre<br><br>MDFRC item.<br><br>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 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.