Variation in bare-sand and seagrass macrobenthos along a South African micro-estuary-like delta channel.

Faunal variation in subtidal bare sand and Nanozostera capensis seagrass along a small (c. 6.5 ha) flood-tidal delta channel cum saltmarsh creek cum micro-estuary was investigated in the warm-temperate Knysna Estuarine Bay, South Africa. As in the mouth of the delta as a whole, the freshwater-deprived channel's proximal end was amphipod dominated, but near its head polychaetes dominated instead, this marked faunal change occurring abruptly over a short distance (<160 m), between one station and the next Urothoe reducing from a mean 2570 m-2 to 12 m-2, and Simplisetia and Capitella increasing from a mean 99 m-2 to 4135 m-2. However, there was no significant change along the channel in any bare-sand assemblage metric (e.g. species density, alpha or gamma diversity, assemblage abundance, evenness, taxonomic distinctness, or patchiness). Seagrass occurred in only limited patches near the head, but insofar as evidence is available its fauna also displayed a equivalent change to that in the bare sand in the same section of the channel. Despite its increasing confinement by saltmarsh and very small size, this subsidiary channel therefore displayed the same relatively constant biodiversity and abundance but marked change in faunal composition known to characterise the bay's main 66 ha flood-tidal delta system, although its fauna was significantly different. The marked faunal change along the channel likely reflects short spells of greatly reduced salinity near the discharge point of the small Bigai river into its small low-tide water volume.