Photophysiological responses of intertidal benthic microalgal communities to in situ light environments: Methodological considerations

Photosynthesis vs. irradiance (P-l) curves usually express production in terms of production per unil biomass. However, for benthic microalgal communities, the depth of the sediment photic zone as well as the amount of biomass exposed to light varies as a function of incident irradiance. We examined the photosynthetic responses of intertidal benthic microalgal communities from different light environments with traditional depth-integrated methods as well as the percent P,,, and fixed-depth interval methods. P-lcurve parameters (a, I,, I,,,) change depending on the method used. With the depth-integrated biomass-specific method, microalgae in unshaded mudflats have a higher (Y (0.261 kO.016) than microalgae in shaded tall Spartina habitats (0.16 1 +_O.O 14), suggesting that mudflat microalgae are more shade-accli-mated than tall Spartina microalgae. However, if curves are constructed,with measurements obtained at a lixed depth (surface) in the sediment, tall Spartina microalgae exhibit a significantly higher (Y (0.260-1-0.020) than mudllat microalgac (0.175 kO.006). With the latter method, tall Spartina surface microalgae appear more shade-acclimated than mudflat surface microalgae. We conclude that the fixed-depth interval method provides a more realistic representation of the photophysiological responses of benthic microalgal com-munities