Constancy and change in macrobenthic abundance, biodiversity and assemblage structure along the axis of a flood-tidal sand delta

Abstract

Flood-tidal deltas commonly occur along microtidal coasts in the mouths of those inlets kept open by tidal currents, but they have seldom been studied ecologically. Such a delta of fine sand occurs in the Knysna estuarine bay, South Africa, extending for some 2 km upstream. Although macrofaunal assemblage metrics (overall abundance, observed and estimated species density, geometric mean morphospecies abundance, evenness, and patchiness) varied little along the deltaic axis, faunal composition changed markedly though without species replacement except in the paraonid polychaetes. Unusually, the sand was dominated by these paraonids (especially Paradoneis lyra capensis) and, over its proximal half, also by the amphipod Urothoe pulchella, but worms steadily increased in importance upstream whilst crustaceans decreased. Noteworthily, a psammodrilid was present - the first record from Africa. The flood-tidal shore fauna contrasted in composition with that of the facing ebb-channel shore (dominated by Dipolydora) and areas upstream of the mouth.