The erodibility of two microtidal mudflats in the Danish Wadden Sea area wa
s 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 threshol
d and decrease the erosion rate but the correlation between chlorophyll a a
nd erosion threshold was not strong. Biofilms formed in spring, summer and
autumn at one study site and the erodibility of the site was consequently g
enerally low during these seasons and higher during the winter period. Biof
ilms were absent where macro faunal populations were dominant and low erosi
on thresholds were found at these sites. The erosion rate was dependent on
the fecal pellet content of the bed material. A strong seasonal variation o
f the content of fecal pellets of the bed material was found and this cause
s a seasonal variation of the erosion rate. The temporal variation of the e
rodibility 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 warm
er seasons, probably due to the higher egestion-rate and higher production
of fecal pellets by H. ulvae under higher temperatures. The data demonstrat
es that sites dominated by H. ulvae are easily eroded, both because of the
snails grazing activities on benthic diatoms and the pelletization of the s
urface material. However, it is only by examination of the erosion rates in
addition to the erosion thresholds that the destabilizing effect of H. ulv
ae becomes clear. The study demonstrates that the prediction of the seasona
lity of the erodibility at intertidal mudflats is not straightforward but r
equires information on both benthic micro algae contents and macrofaunal co
mmunity structure and population densities. (C) 2001 Academic Press.