X hits on this document

PDF document

Keywords: Caribbean, eutrophication, LTER (long-term ecological research), coastal settings, human ... - page 9 / 14





9 / 14

example, Hughes (1994) discussed the role of overfishing and reduced herbivory as the main factors inducing dramatic shifts from coral-dominated to algae-dominated systems in Jamaica. According to new surveys, all of the classic reef zonation patterns found in Jamaica in the 1950s have disappeared. Documented structural and functional changes in Jamaican reefs illustrate how overfishing, her- bivory, and large physical disturbances (e.g., hurricanes) in- teract to modify reef communities in the Caribbean region at the coastal seascape level. More recently, Lapointe (1997) argued that in addition to these causes, studies of macroalgal blooms need to evaluate how excess nutrient concentrations in water columns deteriorate reef areas. Documentation of nutrient enhancement in the Caribbean is scarce, despite the environmental stress caused by eutrophication (Aronson and Precht 2000). Because of the lack of long-term data on nutrient enrichment in coral reef habitats, controversies remain about the relative importance of eutrophication and herbivory in promoting macroalgae blooms in coral reefs (Hughes et al. 1999, Lapointe 1999, McCook 1999, Aronson and Precht 2000, Szmant 2002).

The shallow-water communities of Morrocoy National Park, Venezuela, provide a good example of the complexity involved in assessing the interaction of large-scale distur- bances (human and natural), overfishing, herbivory, and eutrophication on coral reef habitats in the wider Caribbean (table 2). The 320-km2 park consists of semienclosed, inter- connected embayments (Bone et al. 1998). Early in 1996, mass mortalities were recorded for coral reefs (60%–90%), gastropods, sipunculans, polychaetes, and sponges, following unusually low temperatures apparently caused by upwelling of cold water along the Venezuelan coast (Laboy-Nieves et al. 2001). Evaluation of coral recovery 3 years after the distur- bance shows that there are still large extensions of dead reefs and of reefs covered by algae and sand (Villamizar 2000).

The disturbance of reef communities associated with this upwelling event along the Venenzuelan coast occurred along with increased human impacts in the region. These included urban and industrial development around the park, sewage inputs from tourism facilities, unregulated recreational and commercial fishing, and coral bleaching and other diseases (Villamizar 2000). These impacts, along with the large inputs of sediments, nutrients, and organic matter transported by rivers during the rainy season, all slowed the recovery of coral reefs. Sea temperature was probably the trigger that initiated the massive mortalities (Laboy-Nieves et al. 2001), but lower temperatures increased the chance of mortality for already stressed coral reef habitats and associated commu- nities. Additional die-offs were observed at the end of 1996, when the highest precipitation in 28 years was registered. Low salinity and hypoxic conditions triggered mass mortal- ities of fishes, holothurians, sea urchins, and sea stars, along with a significant reduction in the area of sea grass (Laboy- Nieves et al. 2001). The high precipitation was apparently associated with the 1995–1996 ENSO, an event that further


confounded the identification of causes for the massive coral mortality in this park system.

Sea grasses are also affected by excessive nutrient inputs (Duarte 1995). Regional differences in nutrient availability determine the distribution of different sea grass species along salinity gradients (Fourqurean et al. 2001). High nutrient loading causes algal blooms, which increase turbidity and reduce the photosynthetic efficiency and growth of sea grasses. The excess nutrients also result in greater growth and pro- duction of epiphytes, to the extent of accelerating sea grass dieback. Phosphorus (P) has been identified as a limiting nutrient in karstic substrates, so an increase in this nutrient will certainly increase sea grass biomass in the short term (Jensen et al. 1998).

Some sea grass species have a narrow tolerance for salin- ity changes, which can trigger major shifts in species com- position (Lirman and Cropper 2003). Organisms are affected more by extreme salinities during major disturbances than by values observed under average environmental conditions. Salinity in association with nutrient enrichment can also become a stressor when freshwater inputs are drastically re- duced. For example, the mass mortality of sea grasses in Florida Bay has been attributed to long-term salinity stress caused by reduced freshwater low.Moreover,studies show that salinity stress can exacerbate susceptibility to other factors (e.g., pathogens, temperature), leading to extensive die-offs. Studies in Florida Bay have been critical in understanding how changes in salinity and P limitation affect species composi- tion and productivity patterns (Fourqurean et al. 1995). Florida Bay studies have also evaluated the physiological re- sponse of sea grasses to stress resulting from iron deficiency, low oxygen concentrations, and drastic reductions in salin- ity. Computer simulation models for sea grasses are being developed for Florida Bay; they incorporate different hier- archical levels, from physiological to spatially explicit seascape processes (Fong et al. 1997).

The high biological diversity found in the coral reefs of the Caribbean Sea is strongly influenced by the presence of man- grove forests and sea grasses. These three ecosystems form strongly coupled habitat complexes, which are not com- pletely understood, along the coastal seascape (Twilley et al. 1998, Koch and Madden 2001, McKee et al. 2002). There is a continuum across these ecosystems in which complex nutrient exchanges define the spatial and temporal distribution of mangroves, sea grasses, and coral reefs. Nutrient availability is highly controlled by nutrient recycling in mangrove forests and sea grasses (Duarte 1995, Rivera-Monroy and Twilley 1996, Feller et al. 2003a). The resulting high rates of primary production and organic material production sustain complex trophic food chains. Therefore, negative impacts in one ecosystem can cascade across the coastal seascape, affecting other areas. For example, mangrove deforestation in the coastal zone can significantly reduce the productivity of sea grasses and coral reefs by causing excessive sediment loads that increase turbidity.

September 2004 / Vol. 54 No. 9 BioScience 851

Document info
Document views18
Page views18
Page last viewedTue Oct 25 19:54:00 UTC 2016