Climate change futures: health, ecological and economic dimensions

Physiological stresses on reef organisms are additive.Although reef changes are triggered by elevated temperature,several other factors compound the negativeeffects upon the ecosystems. Pollution from industrial,agricultural, petrochemical and domestic sources, sedimentationfrom shoreline erosion, daytime penetrationof intense ultraviolet radiation, and hyposalinity fromfreshwater runoff during times of extreme rainfallimpose added environmental stresses that impederecovery of the compromised reef.Caribbean reefs are now subject to annual episodesof bleaching and persistent expression of diseases (seefigure 2.31). The imminent collapse of Jamaica’s reefsand their overgrowth of macroalgae was alreadyapparent by 1991 (Goreau 1992) as a result ofbleaching stress (Goreau and Hayes 2004), massmortality of sea urchins (Lessios 1984), overharvestingof reef fishes (Jackson et al. 2001), and excess nutrient-loadingwith nitrates and phosphates released fromthe land sources (Hughes et al. 2003).The collapse of reefs means the loss of net benefits thatvary in economic value depending upon the degree ofcoastal development. However, in Southeast Asia thetotal loss is estimated to represent an annual value ofUS $20,000-151,000 per square kilometer of reef(Burke 2002). This estimate is based upon an assessmentof benefits derived from fisheries, coastal protection,tourism and biodiversity.CORAL DISEASESIn addition, all species of keystone reef-building coralshave suffered mounting rates of mortality from anincreasing assortment of poorly defined emerging bacterialdiseases. Within the last three decades, coralcolonies have become hosts to various microbes andhave demonstrated limited capacity to either resistmicrobial colonization and invasion or defend againsttissue infection and premature death (Harvell et al.1999; Sutherland et al. 2004).Bacteria normally live in dynamic equilibrium within thesurface mucous layer secreted by corals. This relationshipbetween bacteria and coral mucus is one of mutualbenefit, since tropical waters are nutrient poor andincapable of sustaining bacterial metabolism. Coralmucus serves as a source of nutrition for these bacteria,and the bacteria, in turn, provide protection tocorals from predators. However, once bacteria colo-nize, invade and infect the coral tissue, that relationshipof mutualism shifts to parasitism. The corals, lackingother effective defense mechanisms against infection,are rapidly overgrown by macroalgae, as wellas bacteria and other microbes.The most prevalent signs of infection are discoloration,detachment from the skeleton, and loss of tissue integrity.Discolorations may be due to bacterial growth (forexample, black band), loss of algae (for example,bleaching, coral pox), or local accumulation of pigment(for example, yellow band, dark spot).Detachment of tissue from the skeleton represents dissolutionof anchoring filaments and/or solubilization ofthe aragonitic (a calcium compound) skeleton itself (forexample, coral plague). Tissue necrosis is slowly progressiveand usually includes a patterned loss ofepithelial integrity (for example, white band).HEALTH AND ECOLOGICAL IMPACTSDecline in the health and integrity of coral reefs hasmultiple implications for sea life and for the coastalzone bordering low-lying tropical lands. Declines andloss will decrease their function as nurseries for shellfishand finfish (approximately 40% of stocks worldwide),and affect the lives and livelihoods for communitiesthat depend on fishing for consumption and commercialization.The disruption of reefs as shoreline barriersallows salt water intrusion and salinization of groundwater.This can lead to hypertension in humans anddecreased soil fertility for agricultural production (furtheraffecting health and nutrition). Reefs are alsobecoming reservoirs for microbial pathogens that cancontaminate the food chain, and are associated withhuman health risks from direct contact with microbeladencoastal waters (Epstein et al. 1993; Hayes andGoreau 1998).79 | NATURAL AND MANAGED SYSTEMSCASE STUDIES