UPWELLING AND TOPOGRAPHY - PROTECTION MECHANISMS FOR REEF BIOTA UNDER STRESS

The predicted rate of global warming and atmospheric CO2-increase are believed to exceed most reef's physiological limits and may lead to their extinction. Although the rate of the predicted changes appears extreme, their magnitude is within the limits reconstructed from earth history. Since reef biota have survived previous crisis refuge habitats must have existed. In general, the occurrence of coral reefs is restricted to shallow oligotrophic tropical/subtropical areas and the global coral reef distribution is negatively correlated with typical upwelling areas. Good indications for positive effects of small-scale upwelling are provided by studies of coral systems in the Bahamas and South Africa. In Cat Island (Bahamas), slightly cooler water of small-scale upwelling covered the terrace with the shelf-edge reefs in 15 - 25 m immediately behind the drop-off. Whereas the shallow water corals were in bad health condition, the health status of the shelf-edge reef was very good. In Eleuthera, where wider shelf areas are developed, cooler water currents did not reach the reefs and the reef health status was bad. Between 1992 and 1999, coral cover on South African reefs increased, indicating clearly that in 1998 no significant bleaching and/or coral mortality occurred. In a five year time series, a marked drop in temperature due to an upwelling event was observed every year at the onset of marine summer. Although this event was weaker in 1998 it was still strong enough to keep temperature below bleaching threshold. Bleaching is generally strongest in shallow reef zones and in typical reefs with a steep slope most of the coral habitat is within bleaching depth. In contrast, many areas with flat topography are characterized by coral carpets in medium depths (10 - 40 m) which are outside the zone with highest bleaching rates. This provides opportunity for big refuge areas. On the base of these observations and data we assume that two protection mechanisms exist: (1) Oceanographic protection: moderate upwelling of slightly cooler water may protect corals from bleaching by lowering the temperature and decreasing water clarity. (2) Topographic protection: intermediate deep areas of flat topography provide space for frameworks away from the "dangerous" immediate surface of a warming ocean.