BREACHING THE MOUTH OF THE BOT RIVER ESTUARY, SOUTH AFRICA: IMPACT ON ITS BENTHIC MACROFAUNAL COMMUNITIES

SUMMARY

The changes occurring in the benthic macrofaunal communities of the Bot River estuary, after an artificial opening of its mouth on 29 June 1983, are presented and the possible short- and long-term effects of this disturbance on their ecology are described. Surveys of the benthos conducted in December 1982 and May 1983, before the mouth-breaching, are compared with similar surveys afterwards: in October 1983, a week prior to reclosure of the estuary, and three months later, in January 1984. The total number of species decreased from 23 in May to 15 in October; however, only those species which contributed less than 5 % to the total biomass in May were lost. The largest reductions in species abundance and biomass were related to the collapse of the macrophyte, Ruppia maritima, which is intolerant of high salinities. The species which were reduced most were Arcuatula capensis, Melita zeylanica and Cyathura estuaria. These species were adversely affected by the loss of their attachment substrate (Arcuatula capensis), and possibly by increased predation pressure due to the reduction in macrophytic cover. In the areas not colonised by Ruppia, the dominant species exhibited an increase in densities and biomass. This phenomenon may be attributed to tidally induced currents which existed while the mouth was open, and which transported nutrients from the decaying macrophytes in the shallows to communities in the deeper waters. This assumption is strengthened by the fact that densities in the deeper areas returned to their usual low levels once closure of the mouth had eliminated these currents.

In the longer term, breaching of the mouth seems to influence species composition and diversity. After an extended period of separation from the sea, the benthic macrofauna appears to develop towards that of a coastal lake. The unpredictable, catastrophic disturbance caused by opening the mouth, however, interrupts this development by eliminating the ‘lagoonal’ species and thereby also maintaining very low species diversities.