Cohesive sediment transport in coastal lagoons with special emphasis on the biological influence on the erosion and deposition of fine-grained material

PhD-thesis
Thorbjoern J Andersen, Institute of Geography, University of Copenhagen.

The thesis deals with transport of fine-grained, sediment in the marine environment with special focus on the biological mediation of both erosion and settling of cohesive sediment at two temperate, micro-tidal mudflats. Additional data on the transport of cohesive sediment at one of the field sites is discussed as well as data on long-term deposition on some European mudflats.

A portable field-version of the EROMES erosion equipment has been designed and built and the instrument has been used for determination of the spatial and temporal variation in erodibility of the sediments on the two mudflats at Kongsmark and Kjelst in the Danish Wadden Sea area. The instrument has also been used for calculation of the settling velocity of the material which were eroded from the mudflats during erosion experiments.

The erodibility of the fine-grained intertidal deposits were generally affected substantially by both microphytobenthos and macrozoobenthos. It was found that one study site was dominated by the presence of the gastropod Hydrobia ulvae whereas the other site was dominated by temporary formation of biofilms. The fecal pellets produced by H. ulvae greatly increased the erosion rate whereas the erosion threshold was little affected by the content of fecal pellets. Biofilms increased the erosion threshold substantially when present. A strong seasonal variation was observed, especially of the erosion rate at the site dominated by H. ulvae and this is mainly ascribed to the temperature dependent ingestion by the snail.

The material was eroded as aggregates, not as single particles, and the equivalent settling diameter (ESD) of the eroded aggregates were generally in the range 20 – 80 micro-m whereas the primary particles had an ESD below 0.5 micro-m - about two orders of magnitude smaller. The settling of the material when resuspended from the bed was strongly influenced by the fecal pellet content of the material with higher settling velocities at larger fecal pellet contents and the ESD was not significantly correlated to any of the other investigated sediment properties.

The study showed that the erosion, transport and deposition of cohesive sediments at these intertidal sites was strongly mediated by biological factors. Both microphytobenthos and macrozoobenthos caused potential changes in the transport of sediment and strong seasonal variations were observed. Although increased erodibility were observed for stations dominated by macrozoobenthos, the net-effect of the different biological interactions on the transport of fine-grained sediment is probably a increased net-deposition in the area due to the stabilization and aggregation of the material.