Biotic Changes in the Reef Complex of San Andres Island

Abstract

San Andres Island (SW Caribbean, Colombia), an uplifted oceanic atoll of Miocene origin, consists of a well-developed coral and carbonate bank-barrier reef complex. From a former cotton and coconut economy in 1953, it became a major trade and tourist center currently having a population of more than 50,000 inhabitants in a land area of just 25 km2. Human activities directly affecting the marine environment have included dredge and fill operations, shipwrecks and groundings, disposal of urban waste, thermal pollution, over-fishing, construction along the shoreline, and diving, boating and beach-going activities. From qualitative observations carried out from 1968 to 1979, and quali- and quantitative resurveys made in 1992 to 1996, major biotic changes became evident in the upper insular platform (0-25 m in depth). These were: a detectable decline in live coral cover (to an overall mean of 30% of hard substrata in 1992), extensive recent coral mortality (overall mean in 1992 of 52% of total live + dead coral cover), almost total disappearance of the long-spined urchin Diadema antillarum and the sea-fan Gorgonia ventalina, proliferation of algae (to about 60-70% cover of the overall reef hard substrata in 1992), and an almost total absence of commercial reef organisms, including both carnivores and herbivores. With exceptions due to localized solid and sewage disposal, coastal construction and ship groundings, coral death followed a spatial pattern of increased values in lagoonal, enclosed environments and lower values in high energy zones and in deep reef areas. The higher coral death in shallow lagoonal areas was interpreted as being directly caused by a greater susceptibility of coral species making up the shallow reef frameworks (i.e., Acropora, Porites) to hurricane and storm damage and to diseases. Indirectly, it may have been caused by the effect of the prevalent regime in lagoonal areas of higher illumination (allowing greater algal growth after the die-off of Diadema), low turbulence and limited water flushing (causing higher susceptibility to bleaching and diseases, and lower rates of natural and artificial lesion regeneration), and by a higher incidence of lethal or chronic effects of anthropogenic activities. San Andres constitutes a typical example of the widespread phenomenon of over-development and poor planning of small islands. Its growing recreational tourism, which is based mostly on attractive and "healthy" seascapes, will soon be seriously threatened.