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

MDFRC item.

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.