Discriminating the effects of bioturbation and herbivory by the intertidal burrowing crab Neohelice granulata on microphytobenthic assemblages of SW Atlantic mudflats

Abstract

In shallow soft-bottom intertidal systems, microphytobenthic assemblages (MPB) and macrofauna interact and play important roles in ecosystem functioning. Macrofauna can affect MPB through bioturbation or/and herbivory. Most bioturbators also exert herbivory pressure on MPB, hence the results of the interaction between benthic macrofauna and MPB is not easy to predict. In this study we performed a small scale field experiment in order to discriminate the effect of bioturbation and herbivory by the intertidal burrowing crab Neohelice granulata on MPB structure, and primary production in SW Atlantic mudflats. The implemented treatments allowed us to separate the effects of bioturbation from those of herbivory on MPB. Results showed that N. granulata, through bioturbation + herbivory did not affect MPB structure (i.e., biomass, abundance and composition); and crab bioturbation without herbivory also did not modify MPB structure. Nevertheless, crab bioturbation without herbivory almost doubled MPB primary production, compared to the values registered in the treatment without bioturbation and herbivory and the treatment with bioturbation + herbivory. Sediment properties were equally affected by bioturbation in the four treatments. Thus, our results shows that the only significant effect on MPB structure is due to crab bioturbation.