Carbon pathways reef function and reef degradation

The essential functional characteristics that distinguish coral reefs from other ecosystems are illustrated in Figure 15.3a. (A representation of the enormous network complexity, not represented here, is provided by Johnson et al. 1995.) Photosynthesis by diverse plant forms (from unicellular dinoflagellates to fleshy macro-algae and coralline crusts) fixes carbon into compounds which are directed into pathways that are primarily bioconstructional (arrows 1 and 2) or trophic (arrow 3) in output. Total photosynthetic output per unit area depends on total solar energy and its reduction in intensity and changes in spectral composition as it passes down through the water column. The former is a function of geographic location, and the latter of water transparency, which is affected by human activities on and among the reefs, proximity to sources of terrestrial runoff, and land use. The total carbon fixed also depends on other water quality characteristics (such as nutrient concentrations. pH, dissolved C02, 02, HCD3, temperature and salinity).

The partitioning of fixed carbon between the bioconstructional and the trophic pathways (Figure 15.3a) depends on the composition and relative abundance of benthic biota. The bioconstructional pathways refer to the accumulation of limestone building blocks, the cements which bind them together into reef framework, the sediments derived from both the physical and biological erosion of blocks and framework, and the sand-sized skeletal elements of a variety of plants and animals. The trophic pathway refers to the food web, including plant-herbivore-predator links accumulating harvestaWc protein resources, and "losses" (arrow 5) to a microbial -processed detrital compartment.

Consumer and decomposer elements of the trophic pathway are supplemented to a greater or lesser degree by imported organic matter (not shown in Figure 15.3) comprising detritus, phytoplankton, zooplankton and vertebrates. Wilkinson (1986) and Birkeland (1987) identified a "nutritional spectrum" of coral reefs and reef benthic communities, ranging from those which are predominantly autotrophic (as in Figure 15.3), to those which are highly reliant on imported organic matter (see also Section 15.3).

The trophic characteristic that humans value most highly in any reef is a maximum sustainable yield of protein (e.g. as fishes, crustaceans, mollusks and cchinoderms). The bioconstructional characteristic most valued by humans, and essential for the long-term structural integrity of a reef, is the net accumulation of framework.

Reef scientists, users and managers often describe coral reefs as "degraded" when they fail to match the presumed (but rarely documented), sustained protein yields, structural integrity or aesthetic qualities of earlier times. Assuming, for the sake of illustration, that the arrow thicknesses and box sizes in Figure 15.3a represent this preferred state, the degradation may take several forms:

• when total fixed carbon is depressed, leading to low yields of protein and/ or limestone (Figure 15.3b);

• when an imbalance between trophic and bioconstructional pathways leads to low yields of either protein (Figure 15.3c) or limestone (Figure 15.3d);

• when the apportionment of carbon within the trophic pathway favors detritus over food webs sustaining protein production (Figure I5.3e);

• when the apportionment of limestone within the bioconstructional pathway leads to prolonged transformation of material from framework into rubble, sands and silts (Figure 15.30*

Within this conceptual framework, reef ecosystem function is driven by

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