THE BENTHIC MACROFAUNA OF THE BOT RIVER ESTUARY, SOUTH AFRICA, WITH A NOTE ON ITS MEIOFAUNA

SUMMARY

The species composition, abundance, biomass and distribution of the benthic macrofauna of the Bot River estuary are discussed. In general, the fauna is impoverished, comprising mainly amphipods, isopods, polychaetes and molluscs. Average total biomass figures are 8,29 g dry mass m-² for December 1982 and 4,32 g dry mass m-² for May 1983. The tanaid, Apseudes digitalis, was the most abundant species, while the bivalve, Arcuatula capensis, had the highest biomass. Low diversities and abundances are attributed to the lack of contact with the sea. The estuary can be divided roughly into four major zones, based on the distribution and abundance of the benthic macrofauna. Faunistically, the zones differ with respect to the characteristic and numerically dominant species. Highest total biomass figures (91,63 g dry mass m-²) are associated with the Ruppia beds, whereas the lowest figures were usually found in the deeper areas.

The meiofauna of the system is noticeably impoverished, in terms of both numbers and diversity. The main groups represented are the platyhelminths, nematodes and oligochaetes, with a few chironomids, polychaetes and juvenile forms of macrofaunal amphipods. Densities are low (0,02 to 1,82 cm-³ sediment) and the animals are relatively large for meiofaunal organisms. The abundance and diversity of these groups are closely related to the composition of the sediment, with the richest sediments being those with the lowest mud fractions, close to the sea.