Large-scale Ecological Impacts of Development on Tropical Islands Systems: Comparison of Developed and Undeveloped Islands in the Central Bahamas

The relationship between density of development and the health of nearshore marine habitats is explored through spatial and temporal comparisons of patch reef environments in the central Bahamas. Nearshore patch reefs are important fi sh habitats, and tend to have high, but variable, coral cover and benthic diversity in the Bahamian archipelago. Twelve patch reef stations were established off developed and undeveloped islands in the central Bahamas. Environmental parameters were measured over an 18-mo period to examine seasonal, tidal, and diurnal variability. Water quality measurements were not signifi cantly different between developed and undeveloped sites for temperature, salinity, dissolved oxygen, chlorophyll-a, total nitrogen and total phosphorus. Only turbidity measurements were signifi cantly different among sites, attributed to storm events. Ecological surveys recorded macroalgae species, stony coral species, coral cover, and coral vitality. Signifi cant differences in species composition between developed and undeveloped stations were seen, with a higher coral diversity, lower coral cover, and higher incidence of coral lesions on developed patch reefs. A 53yr comparison of nearshore environments from aerial imagery showed signifi cant loss of patch reefs and seagrass areas with increasing development density. Results stress the importance of comparison reefs in marine protected areas for evaluating impacts of coastal development on nearshore marine habitats. This paper is one in a series resulting from a workshop held at the Caribbean Marine Research Center (December 2001) to evaluate the importance of back reef systems for supporting biodiversity and productivity of marine ecosystems. This paper specifi cally addresses the potential impacts of coastal development on nearshore patch reef environments by spatial and temporal comparisons of heavily developed (“developed”) and less-developed (“undeveloped”) islands in the central Bahamas. The Bahamian archipelago is made up of shallow-water carbonate limestone environments with few surface water resources; a critical feature is the continuous solution of the limestone rock, and consequent permeation of seawater beneath all the islands (Sealey, 1994). The marine and terrestrial habitats adjacent to the shoreline interact ecologically in terms of nutrient fl ux through rainfall run-off, seepage of ground water, and detrital material moving on and off the islands. Most people in the Bahamian archipelago live within 2 km of the sea, and both countries that occupy the islands (The Bahamas and the Turks and Caicos Islands) share a common culture as well as economic interest in the coastal and marine resources. The typical pattern of development on these carbonate islands requires completely clearing the site of all vegetation, and leveling the rocky terrain, with fi ll if necessary. Threats to nearshore marine environments, particularly patch reefs, occur on several levels. The most severe threat is the physical elimination of reefs for the alteration and restructuring of the shoreline to accommodate marina, harbor, and residential development. A second threat to these reefs comes from the acute and chronic sedimentation generated when coastal vegetation is removed, and the soil/sand exposed to erosion by rain during construction. Often exotic plants are used in landscaping, and native coastal plants are BULLETIN OF MARINE SCIENCE, VOL. 75, NO. 2, 2004 296 removed permanently, leaving little or no vegetation buffer between buildings and the sea. Lastly, a threat exists, though poorly documented in the Bahamas specifi cally, from nutrient seepage from cesspit systems very close to the sea, or in contact with ground water. With the exception of downtown Nassau and some large resorts, the common means of wastewater treatment and disposal is the use of “soak-aways” or cesspit systems constructed onsite (Cant, 1997). All onsite disposal systems installed on limestone islands are shown to enrich nearshore water with inorganic nutrients and some amount of organic matter (U.S. EPA, 1983, 1991). Human activities on land inevitably increase nutrient inputs to coastal waters from deforestation, wastewater, fertilizer, and other sources (Bell, 1992). Human population growth in the Bahamian archipelago is one of the highest in the insular Caribbean, with a 10-yr intercensal growth rate of 19% for the Bahamas (Department of Statistics, 2002), and 41.7% for the Turks and Caicos Islands (UNEP, 2002). The population of the Bahamas is projected to increase from 303,000 in 2000 to 357,000 in 2010. Seventy percent of the population in the Bahamas live on New Providence Island. Although the population is small in comparison to the islands of the Greater Antilles, population size is similar to eastern Caribbean island nations. The small island nature of the archipelago places all development in close proximity to nearshore reef environments. The Bahamas is not unlike the Florida Keys (Hoffmeister, 1974), where the process of eutrophication has been studied intensively. Major pathways of nutrient input to waters of the Florida Keys include onsite sewage disposal systems (Lapointe et al., 1992), and submarine groundwater discharge (Lapointe and Matzie, 1992). A major difference between the Florida Keys and the central Bahamas is circulation. The Bahamian islands are not adjacent to the peninsula of south Florida, and have not been targeted for any signifi cant fi ll between islands for causeway construction (See Lott et al., 1996 for historical overview of island fi ll in the Florida Keys). Considerable concern exists over the loss of live coral and the decline of coral reefs caused by macroalgae proliferation in the tropics (e.g., Dustan and Halas, 1987; Hallock et al., 1993; overview by McCook et al., 2001). Coral reef ecosystems are very sensitive to environmental perturbations for several reasons: 1) key reef-building organisms such as corals have very narrow physiological tolerances, 2) interactions of key species (plant-herbivore, algae-coral) are easily disrupted by human-induced perturbations, and 3) the toxic effects of introduced materials are enhanced because of tropical water temperatures and higher metabolic rates (Pastorak and Bilyard, 1985). The stress of eutrophication or nutrient-enrichment on coral reefs likely contributes to this macroalgae proliferation, and appears to be related to the proximity of coastal development as a source of pollution, particularly sewage (Bilyard, 1985; Marszalek, 1987; Pastorak and Grigg and Dollar, 1990). Throughout developed areas of the tropics, nutrient input from human sources constitutes one of the greatest threats to coral reefs, often resulting in a chronic eutrophication process (Mee, 1988). Indeed, this threat may be even greater in the Bahamian archipelago because, despite having a large area of shallow-water banks, fi sheries production is in fact tied to habitats very close to islands along the platform margin (see Stoner et al., 1994; Colin, 1995; Sluka et al., 1996; Lipcius et al., 1997). How can nearshore habitats be monitored and evaluated to detect impacts from development? Clark and Green (1988) summarized several criteria to be used in pollution assessment, including the importance of selecting one or more sites as controls (relatively non-impacted), and the documentation of nuisance physical and biological variables from seasonal, tidal, or diurnal cycles. SULLIVAN SEALEY: DEVELOPMENT IMPACTS ON BAHAMIAN NEARSHORE REEFS 297 The present study evaluates criteria for the selection of control sites for nearshore patch reefs based on their proximity to coastal development. With coastal development, nearshore patch reefs can be physically destroyed (loss of habitat) or degraded by sediment and/or nutrient enrichment (habitat degradation or phase-shifts). Habitat degradation should be detected by changes in both environmental parameters (physical and chemical properties of the water column and sediments) and ecological parameters (benthic species composition and coverage). No islands in the archipelago can be considered “undeveloped” in the purest sense (Jackson et al., 2001) as historical changes in land cover and vegetation, as well as extinctions and extirpations, have accounted for unknown ecological changes over the past two millennia. For this paper, “undeveloped” is less-developed, and at the lowest level of contemporary human occupation and use in the central Bahamas. Both the undeveloped and developed patch reef stations are in areas of restricted fi shing; undeveloped patch reefs are within the Exuma Cays Land and Sea Park, a marine fi sheries reserve, and the developed patch reef stations are within the no-fi shing limits adjacent to settlements. This study examined three questions: 1) Are spatial comparisons made between patch reefs adjacent to developed and undeveloped islands valid, and are there appropriate “reference sites” within a larger ecological system (e.g., banks)? 2) What environmental and ecological parameters are most useful in characterizing the condition of reefs based on proximity to coastal development in the central Bahamas? and 3) What is the nature of temporal changes in reef environments as development density increases (e.g., Montagu Bay off New Providence ) given a detailed history of aerial photography and historical records? The answers are critical to developing both a better ecological understanding of nutrient fl ux in the coastal zone as well as management guidelines for development practices within the archipelago.