Estimating primary production rates from photosynthetic electron transportin estuarine microphytobenthos

Microphytobenthos primary production was measured using C fixation and comp ared with production estimates based on measurements on intact cores using variable chlorophyll fluorescence with a PAM (pulse-amplitude-modulated) fl uorometer at a number of different sites on intertidal flats in the Westers chelde and Oosterschelde estuaries (The Netherlands). We observed a linear relationship between the minimal fluorescence, F-0, and the chlorophyll a c ontent of the sediments, but the relationship was quite noisy, which was pa rtly due to a vertical scale mismatch between the depth of sampling (1 mm) and the measuring depth the of PAM fluorometer (100 to 200 mum) On most sit es, C fixation was a linear function of photosystem II electron transport ( ETR, obtained from PAM measurements), although for Stn B, a relatively silt y site on the Molenplaat, an intertidal flat in the Westerschelde, the agre ement between C fixation and ETR was often non-linear at irradiances satura ting C fixation. This was sometimes observed for some of the other stations as well. The differences at high irradiance might be related to vertical m igration of epipelic diatoms, optical properties of the sediment or alterna tive electron accepters. A conversion factor was calculated, allowing estim ation of C fixation from ETR measurements. This conversion factor was not d ifferent for the different stations and no clear seasonal influence was obs erved, although the conversion factor could vary within hours. The seasonal ly averaged conversion factor for all stations and each site was used to ca lculate areal rates of production and compared with primary production esti mates obtained from the C fixation measurements. It appeared that the PAM-b ased estimates gave a good prediction of the (potential) C fixation, with t he exception of a number of dates for Molenplaat Stn B. This demonstrates t hat the variable fluorescence technique can become, after further calibrati on with C-14 for Other shallow sediments, a very useful tool in production studies, and can be instrumental in obtaining a good spa; tial coverage of primary production, which remains very difficult due to the patchiness in m icrophytobenthos biomass and production.