The total mangrove area for the wider Caribbean region is estimated at 25,882 km2 (Spalding et al. 1997), representing about 50% of the total mangrove area in the Neotropics. The physiognomy of mangrove forests in the Caribbean re- gion is diverse, from scrub forests (less than 1.5 m tall) in the interior of islands to well-developed forests (more than 30 m tall) in river-dominated coastal seascapes. These gradients pro- vide insights into the multiple stressors that can affect man- grove forest growth. Mangrove forests growing under nutrient-limited conditions respond quickly to fertilization. Recent studies in karstic environments in the Caribbean (Twin Cays, Belize; table 2) have found that nitrogen (N) and P are not uniformly distributed within mangrove ecosys- tems (Feller et al. 2003b). At Twin Cays, both N and P en- richment significantly increased productivity along a tidal gradient; trees were generally P limited in the interior zones of the forest but N limited in the fringe. This response is as- sociated with low P availability in carbonate-rich sediments. Nutrient-enrichment field experiments, like the one per- formed in Belize, are limited in the Caribbean, although it ap- pears that mangrove forests in Bocas del Toro, Panama, are also prone to P limitation (Lovelock et al. 2004). Chen and Twilley (1999) showed how patterns of forest development in Shark River, Florida, differ along a fertility gradient defined by total P concentration. This fertility gradient is apparently caused by greater inputs of mineral material (enriched with P) at the mouth of the estuary, where mangrove trees grow significantly larger (more than 10 m tall) than at the head of the estuary (less than 5 m tall). The fact that the main P source at this site originates at the mouth of the estuary un- derscores the importance of nutrient sources for mangrove development.
Rehabilitation of ecosystems in the wider Caribbean region A rehabilitation project in the Ciénaga Grande de Santa Marta, Colombia, and a restoration project in the Florida coastal Everglades are the largest such projects in the coastal Neotropics (table 2). The degradation of water quality, alteration of hydrological regime, and loss of species and habitat in both coastal regions are the main conditions that prompted the planning, development, and financial support of these projects. The Ciénaga Grande region provides a dra- matic example of negative anthropogenic effects; the loss of an extensive mangrove area (about 350 km2) and the reduc- tion or disappearance of important commercial fish stocks have increased the region’s poverty and social unrest. Hydrological alteration in the Ciénaga Grande was the result of the construction of roads and levees in the 1950s to improve transportation and ranch farming. In the Everglades, an ex- tensive canal system was developed in the 1940s to optimize water management for agriculture practices, urban devel- opment, and flood protection (Davis et al. 1994). Negative environmental impacts in the Everglades include massive sea grass die-offs, coral reef mortality, low water quality, noxious algae blooms, habitat fragmentation, and major changes in
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vegetation distribution and diversity that have altered the density and spatial dispersion of wild and commercial animal species (Davis et al. 1994). Freshwater rediversion is the main restoration measure in both the Ciénaga Grande and the Everglades, but the geomorphological differences between the two sites (figure 4) make the outcome of both projects com- paratively uncertain. Because there have been no previous long-term projects of this type, it is unclear what the actual ecological response, and the effectiveness of restoration measures, will be.
The overall objective of the Ciénaga Grande rehabilitation project was to reduce soil salinity by reconnecting the flood- plain to the Magdalena River through five dredged channels in areas of historical distributaries (Botero and Salzwedel 1999). All five planned channels were dredged by 1998, and mangrove recovery was apparent after the practical salinity level dropped from more than 100 to 40–50, a range normally observed in mangrove forests (Rivera-Monroy et al. 2004). In the Everglades, restoration aims to redivert fresh water to al- low a more natural overland flow (sheet flow) through the freshwater marshes of Taylor and Shark River sloughs to the estuaries and coastal zone. The full Everglades restoration plan will be completed over the next 10 to 30 years; some hydro- logic structures have already been removed, and freshwater flow has been restored in some regions.
Research questions to test the environmental signature hypothesis We propose a series of questions describing the vulnerabil- ity of coastal ecosystems to human and natural disturbance across the coral reef–sea-grass–wetland coastal seascape, based on findings at each of our research sites (figure 5). The vulnerability of this coastal seascape depends on the en- vironmental signatures of tropical coastal settings. These questions follow our main hypothesis that the susceptibility of tropical coastal systems to disturbance will vary the envi- ronmental signature of coastal settings. The questions are grouped by ecosystem to facilitate their location in our con- ceptual framework, but ecosystems should be viewed as closely interconnected to each other in a seascape. Figure 5 in- dicates the location of the ecosystems and illustrates their hy- drologic connectivity. For example, sea grasses are absent on deltaic systems because of high sediment loads (figure 4). As mentioned above, the study sites listed in table 2 represent dis- crete points across the range of coastal settings, allowing an intersite comparison of factors that control nutrient cycling and primary productivity. Thus, multiple questions could be tested simultaneously at multiple sites, which represent a combination of environmental factors. Rather than trying to establish an exhaustive list of questions, we focused on the re- search problems that integrate different ecological processes at different temporal scales along an environmental signature, from uplands to oligotrophic coastal waters.
The general research questions that encompass the envi- ronmental signature concept are as follows: