SHORT-TERM EFFECTS OF THE 1988 ORANGE RIVER FLOODS ON THE INTERTIDAL ROCKY-SHORE COMMUNITIES OF THE OPEN COAST

SUMMARY

Extreme flooding of the Orange River during 1988 led to abnormal dilution of the coastal waters and mass mortalities of intertidal and shallow-water organisms. Species occupying the shallower portions of the subtidal and lower portions of the intertidal zones suffered greatest mortalities. In deeper water mortality was negligible, probably because the fresh water formed a stable layer over the more dense seawater. In the intertidal zone, mortalities tended to be correlated with the physiological tolerances of different species.

Mortalities were greatest near the mouth of the river, and within 10 km virtually all patellid limpets, mussels, octopus, chitons, urchins, red bait (Pyura stolonifera), barnacles and Cape reef worms (Gunnarea capensis) were eliminated, as were almost all rock lobsters and kelps in shallow waters. Further south the magnitude of the mortality declined, but even as far south as Brazil (near Kleinzee, ca. 140 km south of the mouth), increased mortalities and other detrimental effects were detected.

Following the floods, an entirely different intertidal community developed in the areas most affected. In the absence or near-absence of patellid limpets, opportunistic green algae (Ulva sp. and Enteromorpha sp.) proliferated in the mid- and low-shore zones, although they had been largely absent from these zones previously. In the high-shore zone Porphyra capensis, normally patchy, developed a dense cover and occupied virtually 100% of the substratum. With the formation of these algal beds, several species of cryptic fauna colonized the mid- and high-shore regions, elevating species richness. Initially, high recruitment of Siphonaria spp. took place under the algal canopy. After two months, however, the algal mats accumulated sufficient sediment to smother many of the rock-dwelling species, including Siphonaria spp., whose numbers became inversely correlated with algal cover and silt load. Littorina africana knysnaensis, a high-shore specialist, migrated higher on the shore as the algal beds developed, possibly because the macroalgae prevented growth of their microalgal food. Encrusting coralline algae diminished or disappeared, apparently becoming smothered by foliar algae. Thus the shore was transformed from one dominated by patellid limpets to one dominated by opportunistic foliar algae. Several questions remain to be answered by further monitoring and experimentation, the most interesting being whether the recruitment of colonists is influenced by their mobility or the accessibility of adult spawning stocks.