Estimating primary production rates from photosynthetic electron transport in estuarine microphytobenthos

Microphytobenthos primary production was measured using C fixation and compared with production estimates based on measurements on intact cores using variable chlorophyll fluorescence with a PAM (pulse-amplitude-modulated) fluorometer at a number of different sites on intertidal flats in the Westerschelde and Oosterschelde estuaries (The Netherlands). We observed a linear relationship between the minimal fluorescence, F-0, and the chlorophyll a content of the sediments, but the relationship was quite noisy, which was partly 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 sites, C fixation was a linear function of photosystem II electron transport (ETR, obtained from PAM measurements), although for Stn B, a relatively silty site on the Molenplaat, an intertidal flat in the Westerschelde, the agreement between C fixation and ETR was often non-linear at irradiances saturating C fixation. This was sometimes observed for some of the other stations as well. The differences at high irradiance might be related to vertical migration of epipelic diatoms, optical properties of the sediment or alternative electron accepters. A conversion factor was calculated, allowing estimation of C fixation from ETR measurements. This conversion factor was not different for the different stations and no clear seasonal influence was observed, although the conversion factor could vary within hours. The seasonally averaged conversion factor for all stations and each site was used to calculate areal rates of production and compared with primary production estimates obtained from the C fixation measurements. It appeared that the PAM-based estimates gave a good prediction of the (potential) C fixation, with the exception of a number of dates for Molenplaat Stn B. This demonstrates that the variable fluorescence technique can become, after further calibration 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 microphytobenthos biomass and production.