Characteristic flow patterns generated by macrozoobenthic structures

A laboratory flume channel, equipped with an acoustic Doppler flow sensor and a bottom scanning laser, was used for detailed, non-intrusive flow measurements (at 2 cm s^− 1 and 10 cm s^− 1) around solitary biogenic structures, combined with high-resolution mapping of the structure shape and position. The structures were replicates of typical macrozoobenthic species commonly found in the Mecklenburg Bight and with a presumed influence on both, the near-bed current regime and sediment transport dynamics: a worm tube, a snail shell, a mussel, a sand mound, a pit, and a cross-stream track furrow. The flow was considerably altered locally by the different protruding structures (worm tube, snail, mussel and mound). They reduced the horizontal approach velocity by 72% to 79% in the wake zone at about 1–2 cm height, and the flow was deflected around the structures with vertical and lateral velocities of up to 10% and 20% of the free-stream velocity respectively in a region adjacent to the structures. The resulting flow separation (at flow Reynolds number of about 4000 and 20,000 respectively) divided an outer deflection region from an inner region with characteristic vortices and the wake region. All protruding structures showed this general pattern, but also produced individual characteristics. Conversely, the depressions (track and pit) only had a weak influence on the local boundary layer flow, combined with a considerable flow reduction within their cavities (between 29% and 53% of the free-stream velocity). A longitudinal vortex formed, below which a stagnant space was found. The average height affected by the structure-related mass flow rate deficit for the two velocities was 1.6 cm and 1.3 cm respectively (80% of height and 64%) for the protruding structures and 0.6 cm and 0.9 cm (90% and 127% of depth) for the depressions. Marine benthic soft-bottom macrozoobenthos species are expected to benefit from the flow modifications they induce, particularly in terms of food particle capture due to altered particle pathways and residence times, but also for the exchange of gases, solutes and spawn. The present results confirm previous studies on flow interaction effects of various biogenic structures, and they add a deeper level of detail for a better understanding of the fine-scale effects.     

Parameterisation of clastic sediments including benthic structures

The sediment transport processes in the south-western Baltic Sea are predicted by means of a numerical model in the project DYNAS. There are two sediment parameters that influence the results of modelling remarkably: critical shear stress velocity and bottom roughness. This paper presents the way how to parameterise these factors and extrapolate them into the investigation area. The critical shear stress velocity is parameterised basing on grain size data, combining approximations after Hjulström [Hjulström, F., 1935: Studies in the morphological activity of rivers as illustrated by the river Fyris. Geological Institution of University of Uppsala: Bulletin (25): 221–528.], Shields [Shields, A., 1936: Anwendung der Ähnlichkeits-Mechanik und der Turbulenzforschung auf die Geschiebebewegung. Mitteilungen der Preussischen Versuchsanstalt für Wasserbau und Schiffahrt (26): 26 pp.] and Bohling [Bohling, B., 2003: Untersuchungen zur Mobilität natürlicher und anthropogener Sedimente in der Mecklenburger Bucht. unpublished doctoral thesis, Mathematisch-Naturwissenschaftliche Fakultät, Ernst-Moritz-Arndt-Universität Greifswald/Germany, 156 pp.]. The roughness length, in the case of absence of macro zoo-benthos and their structures, is parameterised basing on grain size too employing Soulsby [Soulsby, R.L., 1997: Dynamics of Marine Sands: a Manual for Practical Applications. London, Thomas Telford Publications. 249 pp.], Nielsen [Nielsen, P., 1983: Analytical determination of nearshore wave height variation due to refraction shoaling and friction. Coastal Engineering 7, 233–251.] and Yalin [Yalin, M.S., 1977: Mechanics of Sediment Transport. Pergamon Press, New York. 298 pp.]. No equivalent simple parameterisations for biologically caused bed roughness exist. Here, findings of Friedrichs [Friedrichs, M., 2004: Flow-induced effects of macro zoo-benthic structures on the near-bed sediment transport. Dissertation, Universität Rostock, 80 S.] and estimations by the DYNAS biologists group were combined in order to derive roughness lengths from abundance measurements of four previously selected key species which represent the originators of the dominating benthic structures at the sea floor in the south-western Baltic Sea. Critical shear stress velocity and bed roughness are known at few sample sites only. They were extrapolated into the larger investigation area using a proxy-target concept. The mean near bottom milieu (bathymetry, median grain size, salinity, oxygen) which was derived using results from numerical modelling serves as the proxy. Since the milieu parameters are measured at the sampling sites for which the target parameters have been determined, a combined hierarchical and supervised classification was employed to transfer the local knowledge into the unknown investigation area.     

Environmental impact assessment of sediment dumping in the southern Baltic Sea using meiofaunal indicators

An experimental sediment dumping was carried out in the southern part of the Mecklenburg Bight in June 2001. Foraminiferans and ostracods from superficial sandy sediment were studied in a time series from before dumping until March 2004 in order to assess changes in associations and recolonization patterns of both groups. Additionally, an area sampling covering the dumping site and its surroundings from 15.5 to 20.7 m water depth made it possible to compare associations inside and outside the dumping area as well as the water depth dependent distribution of foraminiferans and ostracods. Salinity values vary within the high alpha-mesohaline and low polyhaline range. The dominating species are Ammotium cassis (Foraminifera) and Sarsicytheridea bradii (Ostracoda). The diversity is low (Fisher alpha index from 0.4 to 3.2 for foraminiferans and 1.0 to 2.5 for ostracods), but higher within the dumping site samples. These higher values are explainable by input of allochthonous tests and valves representing additional species. After the sediment dumping it took two and a half years to re-establish the total foraminiferan association and the total foraminifer/ostracod ratio within the dumping site. Total foraminiferan abundance increases remarkably with water depth (mean 83 tests in 100 ml) driven by higher nutrient availability and more suitable salinity and temperature values within the zone of the oscillating halocline. The distribution of shallow water species such as Cribroelphidium excavatum, Eucythere argus and Hirschmannia viridis, within the transient water layer A. cassis, Nodulina dentaliniformis, S. bradii and Palmoconcha laevata and below Eggerella scabra indicate the depth position of the halocline. Water depth and sediment dumping influence are the main driving factors for the distribution of foraminifer and ostracod associations within the study area. However, a significant sedimentological difference between samples inside and outside the dumping area is not recognizable.     

Biologically induced differences in erodibility and aggregation of subtidal and intertidal sediments: a possible cause for seasonal changes in sediment deposition

This study was carried out to describe the difference in erodibility and aggregation in a tidal basin including both subtidal and intertidal study sites and to use these results to explain the shifting erosion/deposition cycles at the sites. Erosion thresholds, erosion rates and settling velocities of the eroded material were measured at a mudflat transect and at sediment cores taken from a nearby tidal channel during surveys made in May 2000 and March 2002. Surface samples were analysed for grain-size, chl. a content, faecal pellet content, dry bulk density and organic content. Additionally, surface samples were taken at eight occasions in the period January 2002 to May 2003 from shallow tidal channels in the area. These samples were analysed for mud content and showed that major shifts in sediment distribution occurred in the period. The erodibility of the mudflat was generally high due to pelletization by the mudsnail Hydrobia ulvae but close to the salt marsh much lower erodibility was found, probably due to stabilisation by microphytobenthos. In contrast, the erodibility of the channel bed seemed to be very little influenced by biological activity and the relatively low erodibility found here was caused by physical characteristics of the sediment. The sediment eroded from the mudflat was generally strongly pelletized and showed high settling velocities whereas less aggregation and lower settling velocities were found for the channel bed sediments. Temporal variations of the mudflat stability and hydrodynamics resulted in temporal variations of deposition and erosion and the changing stability at the mudflat is likely to be one of the main reasons for a general transport of fine-grained sediment from the mudflat to the channel in the cold seasons and vice versa during the rest of the year.     

Effects of sediment resuspension on organic matter processing in coastal environments: A simulation model

A model, constructed using STELLA™, was used to simulate changes in standing stocks and flows of organic matter resulting from sediment resuspension in shallow coastal environments. Previous studies suggested that resuspension may determine the sites and rates of organic matter mineralization in shallow environments (Hopkinson, 1985, 1987). Those studies predicted that resuspended organic material could exert an enhanced demand on dissolved oxygen. Our model results support this hypothesis. Total system metabolism receives increasing contributions from the water column as settling rate decreases. Water column respiration also increases relative to benthic respiration as the frequency and intensity of resuspension events increases. This is driven by higher specific degradation rates in the water column than in the benthic environment. Furthermore, overall respiration (benthic + pelagic) increases in response to resuspension.