The role of fecal pellets in sediment settling at an intertidal mudflat, the Danish Wadden Sea
Andersen, T.J. (2001). The role of fecal pellets in sediment settling at an intertidal mudflat, the Danish Wadden Sea, in: McAnally, W.H. et al. (2003). Coastal and estuarine fine sediment processes. Proceedings in Marine Science, 3: pp. 387-401. https://dx.doi.org/10.1016/S1568-2692(00)80133-3 In: McAnally, W.H.; Mehta, A.J. (Ed.) (2001). Coastal and estuarine fine sediment processes. Proceedings in Marine Science, 3. Elsevier: Amsterdam. ISBN 0-444-50463-X. xxix, 507 pp., more In: Proceedings in Marine Science. Elsevier: New York. ISSN 1568-2692; e-ISSN 2352-2860, more |  |
| Abstract | Settling tube samples were collected at a very fine-grained intertidal mud flat in the Danish Wadden Sea and the suspended sediment examined under a microscope. The temporal variability in in situ settling velocity is discussed mainly in relation to the biological activity at the site. In situ settling of suspended sediment is shown to be strongly dependent on suspended sediment concentration as shown before in numerous publications; however, a strong temporal variability has been found as well. It is argued that this temporal variation is mainly due to differences in the aggregation of the bed material at the site, as no differences in hydro-dynamics, primary grain size distributions or organic content which could explain the variation have been observed.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. |
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