Tidal sediment reworking was compared to biogenic reworking by benthic-feeding fishes and macrofaunal invertebrates on an intertidal sandflat in South Carolina. Flume-determined rates of ripple migration were combined with field measurements of current velocity to characterize physical reworking. Bioturbation by dasyatid rays was measured by counting newly formed feeding excavations at low tide. On both an areal and volumetric basis, ray reworking amounted to <1% of that reworked by ripple migration. Approximately 30% of the sediment surface was covered by feeding pits during the July maximum of ray activity, with only a small proportion of these newly formed per day. Sediment ingestion by the hemichordate Saccoglossus kowalevskii, was less important than ray bioturbation in comparison to tidal sediment reworking. Sediment displacement by free-burrowing haustoriid amphipods was estimated to be important relative to other biogenic sources, but still <1% of the disturbance produced by bedform migration. However, an abundant infaunal species, the polychaete Paraonis fulgens, manipulated sediment during feeding at rates approaching 33% of tidal reworking. In a community such as Debidue Flat, much of the conspicuous bioturbation by large animals is "noise" compared to the larger background of tidal reworking. In contrast, the more inconspicuous biogenic reworking by small infauna is substantial, but has yet to be incorporated into conceptual models of disturbance and soft-bottom community structure.
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