Factors controlling primary production and photosynthetic characteristics of intertidal microphytobenthos

Microphytobenthic primary production and biomass were studied in an intertidal flat located in the Westerschelde (SW Netherlands) with a vertical resolution of 1 mm. Short-term variability of primary production and photosynthetic characteristics were compared during low tide in 2 types of sediment: sandy and muddy sands. The changes observed were compared with abiotic factors and the biomass abundance, which was dominated by benthic diatoms. ¹⁴C uptake values obtained from incubations in a photosynthetron were used for the construction of P-I (photosynthesis-irradiance) curves. Annual averages indicate that both sediments were equally productive (34.5 ± 23.6 mg C m^-2 h^-1 and 41.1 ± 11.6 mg C m^-2 h^-1 for the sands and muddy sands respectively), but production rates were highly variable on monthly time scales and were regulated by different mechanisms. Light and temperature played an important role in determining the production rates, especially in the muddy sediments, where changes in I(k) (light saturation) were correlated with temperature. I(k) showed seasonal changes, suggesting that algae adapted to the seasonal light conditions but there was not a significant correlation between the I(k) and PAR (photosynthetic available radiation) at any of the stations. Vertical migration of the algae, as followed by spectroradiometric measurements, probably accounted for a general absence of photoinhibition. In the sandy sediments, production appeared to be limited by the low biomass of algae, due to resuspension and export. The fact that gross oxygen production rates measured on intact cores using microelectrodes were not lower than potential production obtained from ¹⁴C fixation suggests that short-term limitation of production due to nutrients and/or carbon is not frequent in the microphytobenthos of the Westerschelde.