Buoyant weight method

From the difference in density between calcium carbonate and seawater (known entities), it is possible to convert the weight of an object that has been measured in sea water into an absolute measure of calcium carbonate deposition. Assumptions made in applying this technique are that the only negatively buoyant part of the coral colony is its calcium carbonate, which is largely true for the tissues of corals. This technique has the advantage of producing an accurate measurement of the total calcium carbonate in an experimental coral as well as having the advantage that corals do not have to be killed during the measurement process. Many studies use the buoyant weight technique on corals that have been grown on tiles that can be brought in from the field and periodically weighed.

Figure 8.1 Wave action is a key process on coral reefs and underpins the physical erosion of reefs. A, section through the reef crest at Heron I. (L, living corals; C, consolidated framework of dead corals stuck together by calcareous red algae); B, reef crest at the Low Isles; C, reef crest at Heron I., showing the reef break where physical eroding forces are maximum; D, Heron I., spur and groove. (Photos: O. Hoegh-Guldberg.)

Figure 8.1 Wave action is a key process on coral reefs and underpins the physical erosion of reefs. A, section through the reef crest at Heron I. (L, living corals; C, consolidated framework of dead corals stuck together by calcareous red algae); B, reef crest at the Low Isles; C, reef crest at Heron I., showing the reef break where physical eroding forces are maximum; D, Heron I., spur and groove. (Photos: O. Hoegh-Guldberg.)

including polychaetes, molluscs, sponges, barnacles, sipunculans, and various micro-organisms such as bacteria and algae bore into coral substrata. Endolithic algae colonise the skeletons of corals (Fig. 8.2A) that colonise the surface layers of the substratum together with the turf algae (Fig. 8.2B). Algae is grazed by fish (Fig. 8.2C, D) as well as invertebrate grazers such as chitons (Fig. 8.2E), echinoids (Figs 8.2F, G; 26.8A-F), and gastropods (Fig. 8.2H). These organisms physically bite or scrape the substratum to collect the algae (Fig. 8.3A) and with it they take particles of the substratum that has become honeycombed by the action of the borers (Fig. 8.3B). This calcium carbonate matrix, together with the algae, passes through the gut of the grazers and is ground up, separating the algae from the calcium carbonate. Cellulose enzymes break down the plant cells, the nutrients are then absorbed, and then the calcium carbonate is defecated as a fine powder. Swimming behind schools of large schools of parrotfish (scarids, Fig. 8.3C) one often sees the water column becoming cloudy as this fine powder is ejected (Fig. 8.3D). Similarly the faecal pellets of grazing echi-noids consist largely of compacted finely ground calcium carbonate. The lagoonal sediments, especially those offshore, are largely composed of these products of bioerosion and physical erosion—along with mollusc

Figure 8.2 A, Endolithic algae inhabit the skeletons of corals, living amongst the crystals and over time weakening the skeleton. B, Dead coral substratum covered by turf algae. (Photo: O. Hoegh-Guldberg.) C, The parrot fish Scarus sp. With well developed jaws about to take a lump of dead coral substratum full of endolithic algae. (Photo: O. Hoegh-Guldberg.) D, Jaws of Bolbometopon muricatum on the outer barrier near Lizard I. (Photo: D. Bellwood.) E, Close-up of Acanthopleura gemmata from One Tree I., nestled onto its home scar. (Photo: B. Kelaher.) F, The grazing echinoid Echinometra mathaei, oral surface showing Aristotle's lantern partially protruding from the mouth that it uses to actually scrape off the surface of the coral. (Photo: A. Miskelly.) G, Diagram of Aristotle's lantern. (Illustration after Anderson, 1996.) H, Monodonta labio (Trochidae) feeding. (Photo: K. Gowlett-Holmes.)

Figure 8.2 A, Endolithic algae inhabit the skeletons of corals, living amongst the crystals and over time weakening the skeleton. B, Dead coral substratum covered by turf algae. (Photo: O. Hoegh-Guldberg.) C, The parrot fish Scarus sp. With well developed jaws about to take a lump of dead coral substratum full of endolithic algae. (Photo: O. Hoegh-Guldberg.) D, Jaws of Bolbometopon muricatum on the outer barrier near Lizard I. (Photo: D. Bellwood.) E, Close-up of Acanthopleura gemmata from One Tree I., nestled onto its home scar. (Photo: B. Kelaher.) F, The grazing echinoid Echinometra mathaei, oral surface showing Aristotle's lantern partially protruding from the mouth that it uses to actually scrape off the surface of the coral. (Photo: A. Miskelly.) G, Diagram of Aristotle's lantern. (Illustration after Anderson, 1996.) H, Monodonta labio (Trochidae) feeding. (Photo: K. Gowlett-Holmes.)

shells, carapaces of crustaceans, foraminifera tests and sponge spicules. Only sediments adjacent to the coast or large islands have a component of terrestrially derived sediments.

Storm activity will dislodge coral colonies both live and dead (Fig. 8.1B), which have often been weakened by borers attacking the base and branches of corals. These coral colonies and broken off branches of the stag-horn corals (Acropora) can be washed down to the bottom of the reef slope or thrown up onto sand cays. This band of coral rubble, which is often well developed on the windward side of cays, forms an important coral reef habitat for a wide range of organisms. Coral rubble is itself subjected to further bioerosion, although often it develops a protective coat of coralline algae that provides some protection from the colonisation of the substratum by endolithic algae. Such surfaces lacking endolithic algae are therefore not grazed by parrotfish

Figure 8.3 A, bite marks of a scarid (f ), and a boring barnacle embedded in Porites lutea (b) (photo: O. Hoegh-Guldberg); B, in situ dead coral habitat split open to reveal boring sipunculans and bivalves, burrow of boring bivalve (t) (photo: P. Hutchings); C, schools of Bolbometopon muricatum at Osprey Reef, Coral Sea (photo: P. Hutchings); D, defaecation by parrotfish, fine sediment produced by the grinding of the ingested coral fragments (photo: D. Bellwood).

Figure 8.3 A, bite marks of a scarid (f ), and a boring barnacle embedded in Porites lutea (b) (photo: O. Hoegh-Guldberg); B, in situ dead coral habitat split open to reveal boring sipunculans and bivalves, burrow of boring bivalve (t) (photo: P. Hutchings); C, schools of Bolbometopon muricatum at Osprey Reef, Coral Sea (photo: P. Hutchings); D, defaecation by parrotfish, fine sediment produced by the grinding of the ingested coral fragments (photo: D. Bellwood).

and echinoids. Dead coral substrata adjacent to river mouths, where large plumes of sediment loaded water flow out onto the reef during the wet season (see Chapter 11), tend to be covered in a thick layer of silt that again protects the substratum from endolithic algal colonisation. This has been documented on reefs adjacent to Low Isles and those at the mouth of the Daintree River in North Queensland. Live coral colonies have mechanisms to eject the sediment as it settles on the coral polyps, whereas it can accumulate on dead substratum.

While live coral colonies are typically unbored, once parts of the colony die the substratum is rapidly colonised by borers. As all borers have pelagic larvae it is difficult for them to settle on the living veneer of a coral colony without being eaten by the coral polyps. In cases where borers have settled in living colonies it is presumed that larvae have settled on damaged polyps allowing them to metamorphose and rapidly bore into the substratum before being eaten by a neighbouring polyp. Once coral substratum becomes available for colonisation by borers, a distinct succession occurs, with the early settlers being bacteria, fungi and endolithic algae that appear to condition the substratum and facilitate the next suite of colonisers, primarily polychaete worms, and later sponges, sipunculans and molluscs including bivalves and boring barnacles. Many of these early colonisers are short-lived and create burrows for a suite of non boring organisms or nestlers to colonise. In contrast, the sipunculans, sponges, bivalves and some of the larger polychaete borers are long-lived and once they have created their burrow they are entombed in the substratum and may then live for several years (Fig. 8.3A, B). These organisms bore by either physically eroding the substratum or chemically dissolving it, or by a combination of these methods.

Rates of boring decrease once the borers are established, and subsequent rates are just sufficient to allow those organisms to grow. This is particularly true for sponge colonies. Borers must obviously retain a link to the outside of the substratum in order to obtain their food, for respiration and for discharging their gametes. Once established in the substratum they are effectively entombed, often in flask-shaped burrows (Fig. 8.3B). They cannot leave their habitat, although some species of molluscs, primarily Conus spp., search out for particular boring species of polychaetes and sipunculans and insert their proboscis into the burrow and then proceed to suck out the worm.

Experimental studies have shown that recruitment of boring organisms is seasonal with maximum recruitment occurring during the summer months; however, some recruitment of borers occurs throughout the year. This means that within weeks of substratum becoming available it is already being colonised by borers. Recruitment also varies between years and this is presumably related to the availability of larvae, the supply of which will be influenced by weather patterns at the time of spawning. Net rates of bioerosion (losses due to grazing and boring plus gains from accretion from coralline algae and encrusting organisms, plus physical and chemical erosion) vary between sites on an individual reef as well as between reefs (Fig. 8.4), and differences occur between oceans. Factors such as water quality and sediment load influence not only the rates, but the agents responsible for grazing and boring.

On the Great Barrier Reef, various groups of scarids (parrotfish) are important grazers. Scarids can be divided into three distinct functional groups depending on the osteology and muscle development on the oral

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