Influence of wave and sediment dynamics on cordgrass (Spartina anglica) growth and sediment accumulation on an exposed intertidal flat

Isolated patches of Spartina anglica (cordgrass) at two sites on a wave-exposed mid-intertidal flat of the 340 km(2) 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 than Spartina patches at site 2. A field experiment was conducted to characterize waves and associated sediment dynamics at each site, which might explain why Spartina 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 both Spartina sites and at an intermediate location. Bed-orbital speeds and frictional wave-energy dissipation were consistently lower at the easternmost site with the larger Spartina patches. The west-to-east reduction in wave energy is due to the spatial arrangement of the Spartina 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 1 Spartina patches. Shell accumulation by Spartina 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 the Spartina 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 by Spartina patches and the composition of accumulated sediment on this wave-exposed intertidal flat.