Ecosystem Processes At Scales Of Whole Reefs And Zones

15.3.1 Whole reefs

Forty years ago, Odum and Odum (1955) proposed that the high accumulation of biomass on coral atolls compared with the surrounding oceans depended on two factors: (a) effective use of a non-limiting supply of solar energy, and (b) tight recycling of potentially limiting nutrients. Solar energy is fixed by the zooxanthellae within coral cells, by micro-aigae covering virtually every non-living surface, and by abundant macro-algae, turfs and corallines. By contrast, planktonic production is close to zero. Recycling is accomplished at two levels: as exchange in metabolic products between host and zooxanthellae within the coral cells and as the transfer of plant and animal production within complex and many layered food-webs on. the reef.

Subsequent work, which has embraced a greater variety of reef types, notably those on continental shelves, has modified some aspects of this view of reef function (Pomeroy el al. 1974; Hatcher 1988; D'Elia and Wiebe 1990; Rougherie and Wauthy 1993). Rougherie and Wauthy (1993) proposed that atolls receive a major input of nutrient-rich water from the deep ocean. Geothermal heating reduces the density of deep, nutrient-rich water, causing it to percolate to the surface of the atoll, where the nutrients are incorporated into microbial systems, plankton and higher food chains. This hypothesis provides a mechanism for sequestering a major exogenous supply of nutrients.

Other work suggests that most reefs occupy waters that are in no sense "deserts" (D'Elia and Wiebe 1990). Although P and N in reef waters normally occur at concentrations not much above limits of detection (Furnas el al. 1990), the supply is continuous (in dissolved forms, suspended detritus, phytoplankton and zooplankton). Moreover, N is fixed from dissolved N: by a number of groups, notably cyanobacteria (Order Cyanophyta), and much N and P are remineralized within detrital systems, primarily by invertebrates and microbes within lagoonal sands (Hansen el al. 1987). The main limiting nutrient in oligotrophic coral reef waters may be Fe (needed for chlorophyll Entsch et al. 1983), suggesting that tight recycling may be very important for this element at least.

15.3.2 Zones

The reefs effectiveness in "harvesting" nutrients and organic matter from the passing flow is assumed to be related to biodiversity at the levels of feeding guilds and reef zones. "Zones" are belts of reef, usually a few metres to tens of metres wide and having characteristic combinations of substratum, benthic assemblage and fish assemblage. They occur down the sides of reefs (reflecting gradients in wave action, light and sediment stress) and horizontally, across their tops (reflecting differences in exposure to waves, currents, water quality and air (Figure 15.2d; Cicister 1977; Done 1983). These "reefscape"-scalc expressions of biodiversity are both reflected in, and a product of, local differences in bioconstruction, transport and cycling of materials and in community metabolism.

If a guild of planktivorous fishes inhabits the reefs seaward slopes, it transfers nutrients and organic matter from plankton to the front of the reef (Glynn 1973, 1989; Hamner et a!. 1988). Benthic planktivores and filter feeders (e.g. corals, gorgonians, crinoids, antipatharians) similarly affect transfers from the water column to the reef (Sebens and Johnson 1991). The quantitative importance of transfer through these upstream "walls of mouths" should depend on the composition and abundance of the zooplankton, the planktivorous fish and the benthic planktivores (Johannes et al. 1972). Subsequent zones across the reef flat receive "used" water, whose nutrient, organic and dissolved gas concentrations arc determined by the amounts added and subtracted by the communities of all upstream zones (Crossland and Barnes 1983). There is thus an alternation of production and consumption within different zones ("sources" and "sinks" in Figure 15.2d) that is a key characteristic of coral reefs; downstream zones consume some of what those upstream produce (Crossland and Barnes 1983). The absolute and relative magnitudes of production and consumption of the zones determine the performance and "health" of the whole reef (see below).

However, the composition of zones varies among oceans, regions and environmental gradients (Done 1982; Wilkinson and Cheshire 1989). In clear, oligotrophic waters in the tropics, zones within a reef are most easily distinguished by differences in the distribution and abundance of coral species, while on sub-tropical coral reefs (Crossland 1988), and in more turbid tropica! reefs (Birkeland 1989a), fleshy macro-algae often dominate the reef crest and shallow slopes. In highly eutrophic areas, hard substrates in nutrient-enriched waters may be totally dominated by benthic filter feeders such as sponges, oysters and tube worms, sometimes to the exclusion of corals (Birkeland 1988). Major anthropogenic and natural impacts on biodiversity (see below) commonly cause phase shifts from coral to macro-algal dominance on the scales of zone and reef (Done 1992a; Hughes 1994), and changes in the spatial mosaic within zones (viz. fragmentation, richness, grain and pattern) owing to patchy mortality, invasions and physical redistribution of biogenic sediments (Scoffin 1993).

Similar within- and among-region differences are also seen in other groups. Western Atlantic coral reefs have a relatively small number of fish species with less specialized feeding requirements than those on Indo-Paeific reefs (Bellwood 1994), where species richness is about six times as great (Thresher 1991). Within the Great Barrier Reef, herbivores are more diverse and abundant on windward slopes of more offshore reefs, and planktivores are more diverse and abundant on mid-shelf reefs (Williams 1982; Williams and Hatcher 1983; Russ 1984a,b). Feather stars (Echinodermata; Crinoidea), that are a diverse and important component of the "wall of mouths" on western Pacific reefs, are entirely absent from central and eastern Pacific reefs (Birkeland 1989). Here the transfer of nutrients and organics is effected without them, the role presumably being taken up entirely by other benthic filter feeders and planktivorous fishes.

15.3.3 Ecosystem processes and "reef health"

Kinsey (1988) categorised the substrata of coral reefs into three all-inclusive types: "continuous framework", "algal pavement" and "sand/rubble" (see Figure 15.2d). Metabolically, these substrata have been shown to perform within narrow ranges, and to exhibit up to a 20-fold difference in their rates of calcification and photosynthesis (Table 15.1). These patterns are widespread across a wide range of reefs around the world (Kinsey 1985). In coral framework areas, and on "healthy" reefs at the "whole reef* scale, production and respiration are approximately balanced (i.e. P/R~l). By contrast, algal pavements produce more than is consumed (P/R> 1.0) and the biota of sand/rubble areas consume imported detritus (P/R< 1).

Likewise, the "proper" configuration for the apportionment of carbon among pathways and compartments at the scale of a whole coral reef (Figure 15.3a) reflects contrasting configurations at the scale of its individual zones (i.e. Figure 15.3b represents a deep zone where low light levels limit both plant and limestone production; Figure 15.3c represents a zone of vigorous framework accumulation; Figure 15.3d represents a zone where there is a high transfer of plant matter into animal protein; Figure 15.3e represents a zone of unpalatable macrophytes; Figure 15.3f represents a zone which is a

Table 15.1 "Standards of metabolic performance" for three main types of benthic substratum. Source: Kinsey (1991)

Biogentic substratum1

Photosynthesis (gC in" day"1)

PiR

Calcification (kg CaCO) m"2 y"1)

"Continuous coral"

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