Studies investigating the role of species diversity in sustaining key ecosystem processes, such as primary production, have until now mainly focused on terrestrial plant and soil communities. Although the relationship remains controversial, most evidence suggests that decreases in species diversity adversely affect ecosystem functions. It is unclear, however, whether conclusions derived from terrestrial systems can be readily transferred to aquatic systems. In the present study, the relationship between the diversity of intertidal benthic diatom biofilms and their estimated net primary production (Pn) in the macrotidal Westerschelde estuary was investigated. Diversity measures were calculated on the basis of relative cell counts down to species level. Biomass was estimated as chlorophyll a, and Pn was modelled using a vertically resolved primary production model on the basis of measurements of photosynthetic activity, biomass and abiotic parameters. Species composition of benthic diatoms differed significantly between sites along the salinity gradient of the estuary. As epipelic species were strongly correlated with photosynthetically active surface biofilm biomass and, hence, also with primary productivity, we focused on the diversity of this functional group. The results indicate that (1) biomass appears to be inversely related to the diversity of the biofilms (Periods of low biomass did not show low diversity [as reported in phytoplankton], possibly because these events were driven by grazing pressure and not by nutrient stress) and (2) relationships between diversity (species richness and Shannon index) and Pn appeared to be site specific, with either a significant positive or a unimodal relationship between both parameters.
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