Catalogus

During the study period of 1997, 50–90% of the bed surface material was aggregated into fecal pellets. These pellets were mainly produced by the mud snail Hydrobia ulvae, which lives in densities of up to 40,000 indiv./m². During the study period of 1998, only ≈20% of the surface material consisted of fecal pellets due to lower numbers of H. ulvae, and the bed was covered with an algae mat in summer. Microscopic examination of the suspended material showed it to be temporarily consisting of high amounts (>20%) of fecal pellets during periods of erosion. These pellets have settling velocities that are orders of magnitude higher than their constituent particles, and the presence of pellets enhances the overall settling velocity of the suspended material. The settling velocity of individual, unbroken fecal pellets from H. ulvae and the worm Heteromastus filiformis were examined in the laboratory, and found to be 3–8 mm/s and 12–33 mm/s, respectively.

The lack of grain size discrimination in the feeding process of H. ulvae has the consequence that fast and slowly settling aggregates generally have similar primary grains. Thus, hydraulic sorting with respect to primary grains does not seem to occur.