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Tidal Inlets, Deltas and Flats
Effects of Seagrass Beds on Tidal Flat Sedimentation, Corner Inlet, Southeast Australia
Seagrass beds have been found to effectively baffle current flow and to increase sedimentation in shallow water environments, but few quantitative studies of this process have been carried out on tidal flats. Field monitoring of current velocity and suspended sediment concentrations (SSC) over the seagrass beds and adjacent sand flats (without seagrass cover) in intertidal environments of Corner Inlet, southeast Australia, indicates that the seagrass beds (Zostera muelleri, Posidonia australia) can reduce current velocity and suspended sediment concentrations near the bed by as much as 40 per cent and 25 per cent respectively. This effect is particularly evident near the sediment substrate, and it decreases toward the water surface. The general tendency of a shoreward decrease in wave and tidal current energy across intertidal flats is thus modified.
Because of the reduced bottom current velocity and the sediment trapping effect, seagrass has a significant influence on intertidal sedimentation. This is clearly shown by the distinct differences in sediment texture and sedimentary structures between sand flats with and without seagrass beds in the intertidal environments of Corner Inlet. Flats with seagrass generally accumulate a higher percentage of silt and clay than those without. Consequently, sediments accumulating on intertidal flats of Corner Inlet display finer and coarser sediment alternations across the intertidal zone, a characteristic apparently related to the presence or absence of seagrass beds, respectively.
The depositional effects of seagrass on intertidal sedimentation are also revealed by the internal sedimentary structures of tidal deposits. Seagrass beds tend to produce layers of highly bioturbated mud or sandy mud that overlap structureless, uniform sand or shelly sand deposited in areas free of seagrass. An upward fining stratigraphic sequence can be produced by the migration of seagrass. Thus, it is suggested that the temporal and spatial variations of seagrass density may leave a signature in the sedimentary record.
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