Analysis of variable chlorophyll fluorescence in microphytobenthos assemblages: implications of the use of depth-integrated measurements

Measurements of in vivo chlorophyll a fluorescence made non-invasively on microphytobenthos samples represent the depth integration of the fluorescence emitted at different depths within the sediment. The effects of depth integration on fluorescence parameters and implications for the calculation and interpretation of indices used in quenching analysis were studied using a simulation model based on fluorescence light curves (Fs and Fm’ vs E curves), light attenuation coefficients, and microalgal biomass profiles. Results indicate that the use of depth-integrated fluorescence measurements leads to a significant light-dependent overestimation of the physiological value of the effective Photosystem II (PSII) quantum yield and of the relative electron transport rate, which may exceed 40% at saturating light levels. As a result, light curves derived from depth-integrated measurements appear to saturate at higher irradiances, or to be less photoinhibited when compared to the physiological response of the microalgae. Furthermore, they can vary due to changes in the vertical distribution of microalgal biomass not reflecting alterations in microalgal physiology. The results obtained from the numerical simulations were confirmed experimentally, and variations in fluorescence emission attributable to depth integration were also found in fluorescence light curves from the literature. A set of recursive equations was derived to allow the estimation of the physiological light response through the deconvolution of light curves computed from depth-integrated fluorescence measurements made on undisturbed samples.