IMIS

Small-scale spatial patterns in benthic populations are mainly explicable by biological processes. This experiment was performed in order to see if it was possible to explain a macrobenthic population patterning at a scale of a few meters by an experimental hydrodynamic modification. Bamboo patches of two different densities were put on a mud flat in January 2002. Subsequently larval settlement (June) and adult macrofauna structure (August) were analyzed in a transect perpendicular to the ripple marks across the patch (upstream, inside and downstream). Sediment composition was also determined. Field observations and flume measurements permit to characterize the hydrodynamic modifications. The flume experiment showed that the bamboo patches induced an increase in turbulence. Moreover the field observations revealed erosion in the front part of the patch and sedimentation in the back. The analyses of the sediment composition showed a decrease in the percentage of mud in the high-density area and particularly inside the patches. Multivariate analysis of the fauna data showed differences in macrofauna assemblages that can be explained by one axis linked with the bamboo density. Analyses of variance of the macrobenthic density and biomass data showed a decrease of the suspension feeder density inside and behind the patches. Sub-surface deposit feeders decreased behind the patches relative to the other positions, but the high variability in the control did not allow to conclude whether this was due to an increase behind the patches, or to a decrease inside. No distinct pattern was seen for surface deposit feeders, and negative effects of the bamboo density were recorded for the predators and omnivores. Moreover, larval recruitment was also negatively affected by the bamboo patches. We suggest that the suspension feeders were directly affected by the hydrodynamic modification, whereas the other groups were more indirectly influenced.