Modelling the effects of chlorophyll fluorescence from subsurface layers on photosynthetic efficiency measurements in microphytobenthic algae

Irradiance is attenuated rapidly in benthic microalgal mats. This makes it difficult to quantify the relevant light field for the benthic community, which is a necessary parameter for accurate primary production estimates. Photosynthesis of microphytobenthos on estuarine mudflats can be measured using variable fluorescence techniques and can be expressed as relative linear photosynthetic electron transport (rETR) by multiplying the effective PSII photochemical efficiency (DeltaF/F-m') with the incident photon irradiance (E) impinging on the sediment surface. An assumption of the method is that the measured chlorophyll fluorescence signal is proportional to the inherent photosynthetic properties of the microalgal assemblage at any particular irradiance. However, contribution of upwelling fluorescence from algae present beneath the surface, originating at an irradiance lower than the surface irradiance, may distort the relationship. An optical model was constructed in order to test the relationship between inherent and apparent fluorescence responses from simulated intertidal sediments. The model uses a detailed description of microphytobenthic pigment-depth profiles in order to create a realistic simulation of the within-sediment irradiance field. The variable fluorescence yield measured by an instrument at the sediment surface overestimated the inherent yield by more than 20% at high irradiance. The largest overestimates of up to 60% were found in an extreme case of an algal cell layer positioned 100 to 150 mum below the sediment surface but the overestimation was, in most cases, in the range of 20 to 40%, with relatively little difference between sediments of different pigment profiles. Primary production was also estimated from the model using fluorescence-based parameters alone. For the sediment types most commonly found in the field situation, modeled production and calculated total production of the sediment were found to show a linear correlation, thus justifying the use of the variable fluorescence method in estimation of this important ecosystem function.