The water flowing onto a coral reef acts as a transport medium for materials (organic matter, nutrients, sediments, propagules - sec Section 15.3) which are beneficial when delivered at appropriate concentrations and frequencies, but may be deleterious when delivered in excess.
Mechanisms for nutrient impacts on coral reefs Four mechanisms for nutrient impact on reefs are rccognised, although cause and effect have been difficult to establish unequivocally (Bell 1992). Should they reach a reef in solution, nutrients can affect reefs by (1) weakening coral skeletons (Hallock and Schlager 1986), and (2) fertilizing benthic algae on the reefs so that they smother living corals (Smith et al. 1981; Lapointe 1989). Nutrients may also act to the detriment of coral reefs because of the phytoplankton blooms initiated by elevated nutrient levels. These blooms may (3) reduce water transparency (Lapointe and O'Conncll 1989) and hence the vigor of coral growth, and (4) if they die on the reef en masse, starve reef animal communities of oxygen (Johannes 1975).
Symbiont diversity as a basis for adaptation? Within rather narrow limits, individual corals can "photoadapt" to gradual changes in available light by regulating pbotosynthetic pigments and zooxanthellae densities (Dustan 1979; Battey and Porter 1988). Diversity in photoadaptability is an important hedge against minor environmental deterioration, be it based in corals or zoothanthellae, or at the level of species or genotype (Buddemeier and Fautin 1993), A single species of coral can host several distinct taxa of zooxanthellae, with the dominant zooxanthelia being correlated with the amount of available light (Rowan and Knovvlton 1996). Different coral/ symbiont combinations may act to maximise calcification under particular light regimes, thus allowing the critical ecosystem outcome of accumulation of framework to be maintained over a wider depth range than would be possible if such a diversity of combinations did not exist.
Landscape ¡seascape diversity as buffers protecting coral reefs Coral reefs depend to a large extent on the existence of "properly functioning" adjacent habitats for their own well-being. This is an important aspect of biodiversity at the landscape/seascape scale. The concentrations and frequencies of water-borne materials carried to reefs are determined by events and processes occurring at a distance from the reef (e.g. in terrestrial catchments, rivers, estuaries, the open sea). For example, high flows of freshwater into reef waters can cause high rates of coral injury and mortality (Sakai and Nishihira 1991), whereas riparian and strand vegetation can restrain soil and freshwater runoff into coral reef waters (Kuhlmann 1988). Mangroves and seagrasses trap and utilize river-borne, nutrient-laden silts, thereby buffering coral reefs from potentially damaging excesses of nutrients or sediments (Ogdcn and Giadfelter 1983; Birkekmd ¡985).
In extreme cases of poor land use adjacent to reefs in poorly flushed embayments, land runoff of sediments and nutrients can lead to the transformation of coral reefs to piles of coral rubble dominated by microbes, worms and sponges (Smith et al. 1981). Even on some open coasts, where terrestrial discharges are carried along a reef-fringed shore, impacts may be major and widespread (Tomascik and Sander 1985. 1987a,b; Cortes 1990). On other open coasts, by contrast, direct impacts of even very nutrient-rich outfalls may be minor and localized (Grigg 1994), and those impacts, potentially mediated by the planktonic system, are adveeted away from reefs.
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