Box 82 How Are Rates Of Bioerosion Measured

While a piece of dead coral substratum can be cut open and the amount of calcium carbonate that has been removed calculated, and borers identified, this does not give you any idea of erosion rates. A better way to study bioerosion is to use experimental blocks of freshly killed coral that show no sign of boring and to lay them out on the reef (Fig. 8.5C) for fixed time periods. As the original dimensions and density of the blocks are known, losses and gains can be calculated and assigned to the various organisms. Obviously, replicates need to be collected for each time period in order to determine the variation within a site before considering variation between sites. Thin sectioning of the blocks allows the distribution and density of the various microborers to be determined. Part of the block can be dissolved in order to extract the borers that can then be counted and identified to species level. While net rates of grazing can be determined from changes to the dimensions of the block (See Fig. 8.5D), potential grazers in the region need to be identified, their densities calculated and the amount of calcium carbonate in their faecal pellets measured. In the case of scarids, the depth and dimensions of the feeding scars (Fig. 8.3A) can be measured and for echinoids, their population density can be measured and faecal pellets collected over a 24 hr period to estimate the amount of calcium carbonate they contain in order to calculate rates of grazing. Knowing rates of calcification in the area, a balance sheet of losses and gains for the area can be constructed. However, it must remembered that these rates may vary considerably within a reef, so numerous replicates must be used in order to gain reliable data.

Low Istes Snapper Is Lizard Is

Low Istes Snapper Is Lizard Is

Harrier Reef Ribbon No 3 Osprey Reef

Accretion Internal erosion Grazing

(coralline algae) by borefs

Figure 8.4 Variations in rates of grazing, accretion and internal erosion by borers across the Great Barrier Reef from the Daintree River out into the Coral Sea along a gradient of increasing water clarity determined from experimental blocks as illustrated in Figure 8.5C (after Hutchings et al. 2005). Loss (grazing and boring ) + gain (accretion) = net rates of bioerosion. (Drawing: K. Attwood.)

Harrier Reef Ribbon No 3 Osprey Reef

Accretion Internal erosion Grazing

(coralline algae) by borefs

Figure 8.4 Variations in rates of grazing, accretion and internal erosion by borers across the Great Barrier Reef from the Daintree River out into the Coral Sea along a gradient of increasing water clarity determined from experimental blocks as illustrated in Figure 8.5C (after Hutchings et al. 2005). Loss (grazing and boring ) + gain (accretion) = net rates of bioerosion. (Drawing: K. Attwood.)

and pharyngeal jaws. While all scarids have well developed scraping plates or a beak (Fig. 8.2D), the actual development of these plates and associated muscles determines what they can feed on. One type, the croppers, remove only algae and associated epiphytic material, whereas scrapers and excavators remove pieces of substratum together with the algae and leave distinctive feeding scars (Fig. 8.3A). The only difference between scrapers and excavators is the depth to which they can bite. These species are able to break down the calcium carbonate using their pharyngeal jaws. These latter two categories are feeding on the surface layers of dead coral substratum containing endolithic algae (Fig. 8.2A). The large double header Bolbometopon muricatum (Fig. 8.3C) feeds almost exclusively on live coral, often on the faster growing species such as Pocillopora and the tabulate acroporids. On the GBR, many species of scarids occur and their distribution and functional roles such as grazing, erosion, coral predation and sediment reworking vary across the reef. Inner shelf reefs support large numbers of scarids, although their biomass is low, and they exhibit high rates of grazing and sediment reworking. In contrast, the outer shelf reefs have much lower densities of scarids but there the biomass is much higher and they are responsible for higher rates of erosion by grazing and coral predation. Mid-shelf reefs have intermediate values. In areas of overfishing, loss of scarids and other herbivorous species of fish such as surgeonfishes, rabbitfishes and drummers can have significant impacts on the reef, as the algae are not being removed by grazing leading to intense competition for space between corals and benthic macroalgae. This can lead to a shift from a coral dominated environment to one dominated by macroalgae.

In the Caribbean and on some French Polynesian reefs echinoids are important grazers, especially Diadema setosum (Fig. 8.5A), D. savignyi and Echinometra mathaei (Fig. 8.2F, G). These species are relatively uncommon on the GBR. These echinoids graze on the algal covered reefal substratum using their Aristotle's lantern, a complex series of calcareous plates that scrape the surface (Figs 8.2G, 26.8C). The densities of these species are influenced by water quality and high levels of nutrients encouraging algal growth. A recent study in Papeete, Tahiti, showed that overfishing has reduced fish populations that graze on juveniles of echinoids and this has allowed high densities of Echinometra mathaei (Figs 8.2F, 8.6) to develop, which thrive on the excessive algal growth that is being driven by high levels of nutrients in the water column. These high levels of nutrients are being washed down from nearby rivers where untreated sewage is being discharged. Excessive algal growth restricts coral recruitment and over time if water quality is not improved, rates of grazing and associated boring will far exceed rates of calcification and this is already leading to substantial loss of reef framework on this reef (Fig. 8.6). This will have massive flow-on effects, including loss of protection from storm activity on nearby low lying areas, loss of coral reefs leading to reduced fish landings and the loss of tourism.

On the GBR, species of Echinostrephus (Fig. 8.5B) are present and can often be seen nestling in their home depression that they have eroded in the reef substratum. They feed on plankton and suspended matter and basically stay in these depressions.

The chiton Acanthopleura gemmata is common on the GBR in the intertidal zone (Fig. 8.2E). It forms deep depressions to which it returns after foraging on algae during low tides. It is has been suggested that feeding at low tide may decrease the predation risk from fishes and sharks that move over the area as the tide rises.

Figure 8.5 A, Diadema setosum a grazing echinoid linked to major erosion of western Indian Ocean reefs (photo: O. Hoegh-Guldberg); B, Echinostrephus sp., sitting in its home scar that it has eroded (photo: O. Hoegh-Guldberg); C, experimental study of bioerosion at Osprey Reef, Coral Sea, two replicate grids with newly laid coral blocks to be exposed for varying lengths of time (photo: J. Johnson). D, Diagrammatic representation of coral block illustrating how the various components of bioerosion (i.e. grazing, accretion and boring) are determined from a series of sections through each block. Knowing the density of the coral block, these measurements can then be scaled up to rates per square metre and then net rates of bioerosion calculated. a, original block; b, accretion; c, block remaining after grazing and boring (image: K. Attwood).

Figure 8.5 A, Diadema setosum a grazing echinoid linked to major erosion of western Indian Ocean reefs (photo: O. Hoegh-Guldberg); B, Echinostrephus sp., sitting in its home scar that it has eroded (photo: O. Hoegh-Guldberg); C, experimental study of bioerosion at Osprey Reef, Coral Sea, two replicate grids with newly laid coral blocks to be exposed for varying lengths of time (photo: J. Johnson). D, Diagrammatic representation of coral block illustrating how the various components of bioerosion (i.e. grazing, accretion and boring) are determined from a series of sections through each block. Knowing the density of the coral block, these measurements can then be scaled up to rates per square metre and then net rates of bioerosion calculated. a, original block; b, accretion; c, block remaining after grazing and boring (image: K. Attwood).

Figure 8.6 Experimental blocks after six months showing extensive grazing by Echinometra mathaei at Faaa, Tahiti. (Photo: M. Peyrot-Clausade.)

These chitons use their radulae to scrape the surface of the boulders, collecting the algae attached to the substratum. Examination of the extruded pellets of these chitons allows an estimate of the rate of sediment production to be calculated; in this restricted environment, rates are high.

A recent study of bioerosion across a cross shelf transect from the northern GBR out into the Coral Sea found that levels of sediment flowing down the Daintree River heavily influenced rates and agents of bioerosion. Experimental substrata at sites at the mouth of the river and Low Isles, after short periods of exposure are covered in a thick layer of silt that restricts development of the endolithic algae and hence levels of grazing are low. The silt settles out from the turbid water column. In contrast, at sites out in the Coral Sea where water is clear (Fig. 8.3D), substrata are heavily bored with endolithic algae, encouraging high rates of grazing by scarids. Boring communities vary between inshore and offshore sites with deposit feeding poly-chaete species dominant at inshore sites and filter and surface deposit feeders at offshore sites. Boring sponges are most abundant at inshore sites and boring bivalves at offshore sites. Net rates of erosion vary between sites and the relative importance of the components of erosion change markedly along the cross-shelf transect, supporting the data on the distribution, abundance and species composition of scarids across a similar transect (Fig. 8.4).

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