Impact of fiddler crab activity on microphytobenthic communities in a South African mangrove forest

Abstract

Microphytobenthos (MPB) are important primary producers in mangroves, where they are an important source of food for fiddler crabs. The St Lucia mangroves are currently disconnected from the sea and thus considered a non-tidal mangrove ecosystem. Fiddler crabs occur close by, along with sympatric sesarmid crabs. This study examined the foraging effects of the fiddler crab Austruca occidentalis on MPB biomass and composition. The effects of two dominant sesarmids, Chiromantes eulimene and Neosarmatium africanum, on A. occidentalis were also investigated as sesarmids are known to interfere with fiddler crab surface activities such as foraging and burrow maintenance.In situ experimental plots contained fiddler crabs at three different densities, combinations of fiddler crabs and sesarmids, and controls with no crabs. The biomass of three main microalgal groups were measured (i.e. diatoms, cyanobacteria and green algae) using a fluoroprobe and overall MPB biomass was measured using acetone extraction analysis.Austruca occidentalis affected cyanobacteria and diatoms, allowing green algae to increase in biomass during the experimental period. The presence of sesarmids was shown to affect cyanobacteria biomass more than either of the other two algal groups. Overall, MPB biomass was unaffected by fiddler crab density or sesarmid presence, suggesting that abiotic factors played a more important role in controlling biomass while biotic grazing and interference only affected microalgal group composition.