the Caribbean. The influence of the Orinoco River plume on the ecology and biogeo- chemistry of the western Caribbean extends to areas close to Puerto Rico and the Do- minican Republic, again demonstrating the degree of connectivity in the Caribbean basin (Salisbury et al. 2001). These rivers repre- sent point sources for large fluxes of dis- solved and particulate constituents (figures 2, 3). These river constituents typically have dramatic effects on the primary productiv- ity of coastal waters. Turbid conditions lim- iting productivity, with a zone of high productivity at the distal end of turbid plumes, have been reported by several investigators (Demaster et al. 1996).
Coastal ecosystems of the wider Caribbean region Geomorphic features and geophysical ener- gies of coastal settings (Thom 1982), along with regional climate, have been identified as environmental signatures that constrain the structure and function of tropical coastal ecosystems (Thom 1982, Twilley 1995). These environmental signatures have been used to generalize patterns across coastal geomor- phological types, including reefs, lagoons, estuaries (drowned river valleys), and deltas (Downing et al. 1999). The vulnerability of different coastal ecosystems to human dis-
Figure 3. Spatially mapped correlation coefficients between the monthly clima- tology of Magdalena River (Caribbean coast of Colombia) discharge and the ocean color (monthly averaged water-leaving radiance at 555 nanometers (nm), from SeaWiFS [Sea-viewing Wide Field-of-view Sensor] data) between November 1997 and October 2002. The 555-nm band has been used as a proxy for light scattering by particles (sediment and detritus). The region of high positive correlation suggests that the Magdalena River is the source of a large sediment plume.
turbance can be associated with these environmental signa- tures. The combination of geophysical processes (e.g., river discharge, wind, and water residence time) and biogeo- chemical properties of a coastal setting provides a way to summarize how stressors limit the structure and function of coastal ecosystems. For example, changes in light availability and nutrients are two key factors that determine both the type and the rate of primary production among four geomopho- logical types of coastal settings (figure 4).
Our approach provides a conceptual framework to generalize the impacts of land-use changes on ecosystem health across different coastal settings in the Caribbean basin. Ecosystem health is the condition in which “an ecosystem is active and maintains organization and autonomy over time and is resilient to stress” (Costanza 1992). To characterize environmental signatures, we considered 10 categories, including regional climate, geomorphological type, and anthropogenic characteristics (table 2). These categories have been recognized as forcing functions that regulate the structure and function of coastal ecosystems (Thom 1982, Twilley 1995).Values for each category are represented in nom- inal and ordinal scales,since information is lacking across most coastal settings (e.g., nutrient loading rates, sea-level rise). The ordinal scales are ranges of values using established classifi- cation systems. Another characteristic of each site is the
initial effort to establish long-term ecological research. The selected sites were grouped along this continuum of envi- ronmental signatures based on geomorphological type (figure 4; Twilley 1995, Downing et al. 1999). The main assumption of this framework is that the susceptibility of tropical coastal settings to eutrophication will vary along this continuum of environmental signatures.
Availability of nutrient and light resources, along with water residence time, defines environmental signatures from marine to river-dominated coastal settings (figure 4). The dual resource gradient (light and nutrients), along with the variance in salinity, determines the diversity of ecosystems in the coastal setting. The coastal seascape consists of reef, sea grass, and wetland ecosystems distrib- uted across longitudinal and topographic gradients of any coastal setting. The longitudinal gradient in salinity occurs from tidal freshwater to euhaline environments. Within each of these salinity regimes, there are topographic gra- dients from submerged (subtidal) to emergent (intertidal and floodplain) zones. Emergent zones include fringe (edge of intertidal) to interior (supratidal) and floodplain re- gions. Reefs and sea grasses colonize the submerged zone, and wetlands (mangrove, marshes, and forested wetlands) colonize the fringe, interior, and floodplain regions of the emergent zone.
September 2004 / Vol. 54 No. 9 • BioScience 847