Mass mortality of tropical marine communities in Morrocoy, Venezuela

Over a period of several days, in January 1996, various shallow-water marine communities and populations sustained massive mortalities at the Morrocoy National Park, Venezuela. Sixty to ninety-eight percent of the corals, depending on the location, were annihilated. Holothurians disappeared from several sites. Other groups, including sand dollars, gastropods, sipunculans, polychaetes and sponges, were decimated. The marine vegetation, mainly composed of the seagrass Thalassia testudinum and macroalgae, however, did not show any signs of degradation; neither did the mangrove prop root adherent community. Abnormal levels in several physicochemical parameters were registered in the area, including average water temperatures 10°C lower than usual, reduced concentrations of dissolved oxygen, salinities nearing 5 psu at the bottom-water interface, and lower than normal surficial current speeds due to water slacking. Satellite images show that during the period under study unusual masses of cold water were near the area. The coverage of the affected area was estimated to extend for at least 160 km². Although various physicochemical and oceanographical parameters were involved, it is likely that low temperatures, probably spurred by a nearby upwelling of cold-water masses, coupled with punctual drops of salinity at some sites, converged to trigger the complex processes that included planktonic blooms and mucilaginous aggregates and finally induced widespread mortalities.