The 'big 3' tropical coastal ecosystems; coral reefs, seagrass and mangroves; are intricately linked, both spatially and ecologically, existing in a dynamic equilibrium. These systems are considered the major ones due to their high rates of primary productivity, associated biodiversity and areal coverage. A fundamental understanding is needed of functional processes in these ecosystems so that we can understand, and hopefully predict, what the effect of human impacts and development will be. Existence of each system is not only dependent on health within but also relies on the health of each adjacent system.

Mangroves and seagrass beds are ecologically essential components in protecting adjacent land from wave and storm erosion, while preventing terrestrial sediment and nutrients from affecting nearby reefs by acting as filters of freshwater runoff. In turn, coral reefs act as a buffer, attenuating wave energy thus minimising damage and erosion to the coastal zone creating lagoons and protected waters that favour the growth of seagrasses and mangroves. The principal and perhaps most important interaction between tropical coastal ecosystems is that of buffering the effect of the sea on the one hand and the land on the other.

Schematic diagram of the tropical coastal zone (Ogden 1988)

Coral reefs, seagrass and mangroves are among the most productive ecosystems on the planet. Although each system is closely associated, the productivity of each is reliant on very different environmental factors:

Mangroves and seagrass beds depend on nutrients from external sources (heterotrophic). Mangrove forests are generally found in sheltered tidal zones exposed to varying degrees of salinity such as coastal flood plains and along the shores of lagoons and estuaries. Those that are found in areas of low runoff demonstrate significantly less productivity than areas with abundant nutrient supplies. Similarly, areas of high seagrass productivity are found where there is a balance between an enhanced nutrient supply and high water clarity. Seagrass systems, like mangroves are predominantly detritus based, with energy and nutrients flowing from abundant dead plant material, processed by macro and micro-decomposers to higher trophic levels.

Unlike mangroves and seagrass beds, coral reefs rely less on external sources cycling nutrients internally (autotrophic). This exchange takes place predominantly between coral organisms themselves and symbiotic zooxanthellae which live within their cells. Development of coral organisms is dependent on clear water in order to allow transfer of organic molecules from zooxanthellae to coral via photosynthesis. Therefore sediments and pollution from terrestrial sources which block out light and smother feeding surfaces are highly detrimental to their productivity. If allowed, sedimentation and nutrification can result in wide-scale destruction of reef communities. Clearing of mangroves and seagrass beds resulting in excess sediment and nutrients from entering the system is one of the greatest threats to reefs in the Caribbean. All three ecosystems are threatened by the human migrations towards coastal areas, deforestation, aquaculture, nutrient loading, pollution, and overexploitation of key species.