Influence of wave and sediment dynamics on cordgrass (<Emphasis Type="Italic">Spartina anglica</Emphasis>) growth and sediment accumulation on an exposed intertidal flat |
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| Authors: | Email author" target="_blank">Andrew?SwalesEmail author Iain?T?MacDonald Malcolm?O?Green |
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| Institution: | (1) Centre for Estuarine and Marine Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Yerseke, The Netherlands |
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| Abstract: | Isolated patches ofSpartina anglica (cordgrass) at two sites on a wave-exposed mid-intertidal flat of the 340 km2 Manukau Harbor (Auckland, New Zealand) have developed very differently since being planted in the mid-1970s. Although the
two sites are only 0.5 km apart and at the same intertidal elevation,Spartina patches at the easternmost site (site 1) have as much as an order of magnitude higher biomass and accumulated sediment volume
thanSpartina patches at site 2. A field experiment was conducted to characterize waves and associated sediment dynamics at each site,
which might explain whySpartina patches at the two sites have developed so differently over the past 25 yr or so. Suspended sediments were measured and wave
characteristics were inferred from subsurface pressure data measured for 5 wk at bothSpartina sites and at an intermediate location. Bed-orbital speeds and frictional wave-energy dissipation were consistently lower
at the easternmost site with the largerSpartina patches. The west-to-east reduction in wave energy is due to the spatial arrangement of theSpartina sites relative to the predominant wind fetches. The wave-energy gradient is maintained by tidal-cycle variations in fetch
and bed friction and results in a west-to-east reduction in sand suspension. Silt, which is largely resuspended under southwest
winds, is redeposited in the low wave-energy conditions in and around the larger site 1Spartina patches. Shell accumulation bySpartina patches at site 1 occurs infrequently, during southwest winds >10 m s−1 and water depths >0.7 m, when waves are least attenuated by bed friction. Large between-site differences in the growth of
and sediment accumulation by theSpartina patch are consistent with the observed wave-energy gradient. The resulting spatial patterns of silt, sand, and shell resuspension
and deposition directly influence the rate of sediment accumulation bySpartina patches and the composition of accumulated sediment on this wave-exposed intertidal flat. |
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