Seasonal variation in erodibility of two temperate, microtidal mudflats

The erodibility of two microtidal mudflats in the Danish Wadden Sea area was examined over a year at monthly intervals (EROMES erosion equipment). One site was dominated by macrofauna (mainly Hydrobia ulvae) whereas the other was only sparsely inhabited by macrofauna with the temporary formation of diatom biofilms. The erodibility of the mudflat surface was mediated by the presence of both biofilms and H. ulvae. Biofilms increase erosion threshold and decrease the erosion rate but the correlation between chlorophyll a and erosion threshold was not strong. Biofilms formed in spring, summer and autumn at one study site and the erodibility of the site was consequently generally low during these seasons and higher during the winter period. Biofilms were absent where macro faunal populations were dominant and low erosion thresholds were found at these sites. The erosion rate was dependent on the fecal pellet content of the bed material. A strong seasonal variation of the content of fecal pellets of the bed material was found and this causes a seasonal variation of the erosion rate. The temporal variation of the erodibility at the sites dominated by H. ulvae was actually opposite to the other sites as an increase in the erosion rate was observed during the warmer seasons, probably due to the higher egestion-rate and higher production of fecal pellets by H. ulvae under higher temperatures. The data demonstrates that sites dominated by H. ulvae are easily eroded, both because of the snails grazing activities on benthic diatoms and the pelletization of the surface material. However, it is only by examination of the erosion rates in addition to the erosion thresholds that the destabilizing effect of H. ulvae becomes clear. The study demonstrates that the prediction of the seasonality of the erodibility at intertidal mudflats is not straightforward but requires information on both benthic micro algae contents and macrofaunal community structure and population densities. (C) 2001 Academic Press.