Trophic roles

The primary producers of coral reefs are extremely diverse. Like most shallow hard and sandy substrata throughout both tropical and temperate seas, they are inhabited by all the major algal groups (benthic micro- and macro-algae, coralline algae), and commonly by seagrasses. What sets coral reefs apart are the sj'mbiotic zooxanthellae, the single-celled, dinofiagellate algae of many species (Trench 1987; Rowan and Powers 1991) which live within the cells of many animal calcifiers (notably corals, foraminifera and mollusks) and are the powerhouses of coral reefs (see Section 15.2.3). Plank-tonic primary production (phytoplankton) is sometimes important in lagoons (Charpy-Roubard et at. 1988). but usually minor compared with overall benthic production on hard substrata and sands. The relative amounts of carbon going into the trophic as opposed to the bioconstruc-tional pathway depends on the apportionment of plant standing crop between calcifiers and all other algae.

Primary producer populations (density and biomass per hectare) vary greatly within and among reefs as a function of ambient nutrient regime, successional status, wave energy and grazing pressure (Littler and Littler 1985; Birkeland 1987, 1988). A diverse and abundant array of vertebrate and invertebrate grazers scrape, browse, crop and suck this plant production (Hatcher 1983), often inadvertently ingesting varying amounts of detritus, limestone and living material (e.g. coral tissue, epiphytic micro-invertebrates)

plant and animal biomass to detrital pathways. Arrow thicknesses and box sizes have no absolute quantitative meaning other than to signify that (a) is a "normal" reef with the "proper balance" between bioconstruction and protein production. Panels (b)-(f) exemplify the types of changes in ecosystem configuration which may signify degradation of the reef (i.e. relative sizes of arrows and boxes compared to (a)), as follows: (b) total fixed carbon depressed due to rapid attenuation of photo-synthetically active radiation caused by turbid water (shading): (c) excess carbon fixed as limestone rather than edible plants; (d) excess carbon fixed as edible plants rather than limestone; (e) excess edible plants "lost" as detritus rather than contributing to higher trophic levels; (f) excess of carbonate fixed as small skeletal elements in non-framework builders (shaded arrow) and/or cxcess framework transformed to sediments by bto-eroders. Causes of transitions among these various states are discussed in the text. Some of these configurations representing "degradation" at the scale of a whole reef represent "normal function" in specific rones within a reef (Section 15.3)

in the process. Coprophagy is common among certain reef fishes, and is believed to be of major importance in sustaining fish biomass in areas in which other food sources are intermittently limited (Robertson 1982). Where high daily plant production sustains high grazing rates, the standing crop of benthic algae is commonly very low, and export of plant material, either into the open sea or to deirital-based sites in sheltered sand accumulations such as lagoons, is minimal. By contrast, some high-latitude and disturbed reef systems support dense beds of annual macrophytic algae (Carpenter 1986; Crossland 1988).

Corals are food for many types of fish and invertebrates. A variety of fishes nip, crunch or scrape corals (Bellwood and Choat 1990; Bellwood 1994), leaving localized injuries which heal rapidly. Others kill entire colonies. In low abundances, coral predators such as crown-of-thorns starfish (Acanthaster planci), gastropods (Drupelia spp. and Coralliophila spp.) and bristleworms (Polychaeta, Amphinomidae), harvest coral soft tissue at rates that arc sustainable within local communities and promote diversity by opening substrata for colonization (Glynn 1982). There are also secondary predators on the adults and juveniles of the coral predators (e.g. fish, gastropods, shrimps for A. planci; fish for Drupelia and Coralliophila), although their efficacy in regulating local abundances of these corallivores has been difficult to demonstrate (Endean and Cameron 1990b; Ormond et al. 1990)

The marine trophic pyramid beginning with phytoplankton and benthic algae and culminating in the large predatory sharks and teleost fishes is multi-layered (Grigg et al. 1984) and each layer is diverse (Sale 1991). Benthic carnivores and mid-water carnivores (reflecting the sources of their prey) can comprise >60% of species (Sutton 1983), whereas the relative importance of herbivores and planktivores varies in different settings, presumably reflecting differences in the importance of benthic plants and plankton in reef trophodynamics (Williams 1982; Russ 1984a, b). Estimates of sustainable harvest of secondary production, mostly in the form of fishes, mollusks, echinoderms and crustacea, are up to 15 t wet weight ha"1 on reefs fished according to customary practices (Munro and Williams 1985).

15.2.3 Bioconstructional roles

Bioconsiructors Bioconstructors (Figure 15.4) are the sessile benthic organisms that produce skeletons of aragonite and calcite, minerals based on calcium carbonate CaCOj with traces of Mg and Sr (Chalker 1983; Smith 1983). They comprise two broad categories: framework builders and non-framework builders.

Framework builders, notably corals and encrusting coralline algae, accrete a framework of dense intergrowths of rigid skeletons and encrustations (Scoffin

1987). Primary framework-builders consist of massive and robustly branching or platy coral colonies (decimeters to metres across) which are analagous to the structural components of a building (Ginsburg and Lowenstam 1958), and various encrusting coralline algae, analogous to the cement, mortar, glues and plaster that hold the components of a building together. Secondary framework-builders are smaller (centimetres to decimetcrs) colonies of the same groups and other attached organisms such as bryozoans and bivalve mollusks. These add small-scale topographic complexity to the framework.

Framework and infilled sediments are also bound by submarine lithifica-tion, a process involving chemical transformation and micro-organisms (Macintyre and Marshall 1988). The development and strength of marine cements that bind the framework together is much greater on reefs with high water transparency and wave action than on those in turbid, sheltered waters (Marshall 1985). The life expectaiicy of a coral-derived framework depends on the strength of cementation, and also on species composition and diversity and the ambient wave regime (Done 1992b; Massel and Done 1993). In the absence of physical disturbance accretion can continue for centuries, but with episodic physical destruction, coral-dominated substrata are from time to time transformed to bare pavement, rubble and/or algal-covered framework, and a "recovery" period is initiated (see below).

A second important group of bioconstructors are the non-framework builders. These include foraminifera, erect coralline algae (notably the genus Halimeda) and most mollusks, which contribute loose shells and skeletal fragments to the extensive sedimentary deposits associated with coral reefs, and to the framework itself, as trapped sands, silts and gravels (Hopley 1982).

Modifiers Modifiers (sec Figure 15.4) include three functional groups which act at the levels of individuals and populations to affect benthic community performance and bioconstruction - calcification enhancers, bioeroders and sediment operators.

Calcification enhancers are the symbiotic, unicellular dinoflagellate algae, or "zooxanthellae", which occur by the thousands within cells of most corals and many other calcifying organisms on reefs (from microscopic foraminifera to giant clams). They are the reefs powerhouse, because the products of their metabolism "power" the critical reef process of calcification in the host. During sunny days, the host uses photosynthatc and 02 as quickly as they are generated by the zooxanthellae, increasing its calcification rates many times faster than it can achieve at night (Chalkcr 1983). However, in periods of environmental stress, host and symbiont may part company in a process called "bleaching" which reduces calcification rates to their night-time levels (Brown and Ogden 1992).

Bioeroders include a diverse array of fish, invertebrates (notably sponges, bivalve mollusks, sipunculans, echinoids and polychaete worms) and iilamen-

Atmosphere Sea

tous algae that bore into living or dead framework or etch, scrape or nip into its surfacc (Hutchings 1986). Grazers assume major importance as incidental bioeroders as they remove skeletal matter in the process of extracting the nutritional soft tissues. Echinoids commonly erode channels and depressions in the bases of living corals and the reef framework. This action simply adds to habitat complexity when echinoderm abundances are low (^ 1 m However, it can reduce coral framework to rubble and sand over vast areas when abundances are high (10-100 m~2), which can be a result of overfishing of the fish predators of urchins (McClanahan and Muthiga 1988).

Sediment operators are the animals which mobilise or immobilise sediments. Diverse mollusks, polychaete worms, holothurians and fish pass sand through their alimentary canals in order to assimilate diatomaceous algai films covering the sands. The fish may defecate considerable distances from where they ingested the sand. Tube worms bind sand grains and shells into their tubes. The coral framework itself can baffle sands against entrain-ment by passing currents.

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