A novel μCT analysis reveals different responses of bioerosion and secondary accretion to environmental variability

Corals build reefs through accretion of calcium carbonate (CaCO3) skeletons, but net reef growth also depends on bioerosion by grazers and borers and on secondary calcification by crustose coralline algae and other calcifying invertebrates. However, traditional field methods for quantifying secondary accretion and bioerosion confound both processes, do not measure them on the same time-scale, or are restricted to 2D methods. In a prior study, we compared multiple environmental drivers of net erosion using pre- and post-deployment micro-computed tomography scans (μCT; calculated as the % change in volume of experimental CaCO3 blocks) and found a shift from net accretion to net erosion with increasing ocean acidity. Here, we present a novel μCT method and detail a procedure that aligns and digitally subtracts pre- and post-deployment μCT scans and measures the simultaneous response of secondary accretion and bioerosion on blocks exposed to the same environmental variation over the same time-scale. We tested our method on a dataset from a prior study and show that it can be used to uncover information previously unattainable using traditional methods. We demonstrated that secondary accretion and bioerosion are driven by different environmental parameters, bioerosion is more sensitive to ocean acidity than secondary accretion, and net erosion is driven more by changes in bioerosion than secondary accretion

Internal bioerosion of Acropora formosa in Reunion (Indian Ocean): microborer and macroborer activities

Bioerosion by grazing and boring organisms is one of the major destructive forces operating on reef. The aim of this study was to estimate the intensity of internal bioerosion by both microflora and fauna of Acropora, a branching scleractinian, on the reef flat at La Saline (Reunion, Indian Ocean). Internal bioerosion was estimated at two sites, varying in degrees of eutrophication. At each site, dead Acropora subjected to heavy grazing or covered by algal turf were examined. Acropora formosa is subjected to high bioerosion due to its high porosity and its branching form, which facilitates colonisation by boring organisms. Three endolithic microflora species, Plectonema terebrans, Mastigocoleus testarum and Ostreobium queckettii colonised the branches. The mean density of polychaetes was high (72 individuals cm(-3)), and all were regarded as meiofauna (diameter < 0.5 mm). The main agents of bioerosion of Acropora formosa were boring microflora whose baring activities were four times greater (0.29 g cm(-3) of CaCO3) than that recorded by the boring fauna (0.07 g cm(-3) of CaCO3). Under high levels of eutrophication, the microflora were responsible for high bioerosion (30.8% of the surface of the substrate). In contrast, the composition of the fauna changed under these conditions but not the rate of bioerosion (4.4% maximum of the volume of dead Acropora). At the undisturbed site, the substrate covered by algae (within damselfish territory and therefore not subjected to grazing) had low levels of bioerosion caused by microflora (18.9% of the surface) and high bioerosion by fauna (9.5% maximum of the volume of dead Acropora) and in addition the composition of boring fauna was different to that found at the disturbed site with sipunculans being the dominant agent of bioerosion (6 individuals cm(-3) maximum).La bioérosion par les organismes brouteurs et perforants est un processus majeur dans la destruction des récifs coralliens. Le but de cette étude est dˈestimer lˈintensité de la bioérosion interne dˈAcropora formosa par la microflore et la microfaune sur le récif de La Saline. La bioérosion interne est étudiée sur deux sites en fonction du degré dˈeutrophisation et du type de recouvrement du substrat (brouté ou recouvert dˈun gazon algal). La bioérosion dˈA. formosa est très élevée en raison de sa très forte porosité et de sa forme branchue, qui augmente la surface disponible à la colonisation par les perforants. Plectonema terebrans, Mastigocoleus testarum et Ostreobium queckettii sont les trois espèces principales de la microflore endolithique qui colonisent ces branches. Les polychètes, dont la densité moyenne est élevée (72 individus cm–3), appartiennent à la meiofaune (diamètre < 0,5 mm). La bioérosion résulte principalement de lˈactivité de la microflore (0,29 g cm–3 de CaCO3) qui est 4 fois supérieure à celle de la meiofaune (0,07 g cm–3 de CaCO3). En milieu eutrophisé, 30,8% de la surface du squelette est perforée par la microflore et 4,4% du volume érodé par la meiofaune. En milieu témoin, le plus faible pourcentage de bioérosion (18,9%) par la microflore est relevé dans les substrats couverts de feutrage algal alors que la bioérosion par la faune atteint 9,5% du volume du squelette due principalement à lˈactivité des siponcles (6 individus cm–3)

Influence of land runoff on rates and agents of bioerosion of coral substrates.

Annually large volumes of fresh water laden with sediment are washed down the Daintree River in North Queensland into the Great Barrier Reef lagoon. To investigate the effects of land runoff on bioerosion, samples of recently killed colonies of Porites were laid at 6 sites on a cross shelf transect from Snapper Island at the entrance to the river to Osprey Reef, ∼328 km from the river mouth out in the Coral Sea. Rates and agents of bioerosion were determined over 4 years and inshore sites exhibited significantly lower rates of total bioerosion than the other sites. Offshore sites experienced high rates of bioerosion primarily due to grazing and internal bioerosion by macroborers such as sponges and bivalves was also important at some of these sites. Inshore sites were covered in heavy layers of silt which inhibited colonization and growth of microborers, primarily algae. This resulted in lower levels of grazing than at offshore sites. However the activity of macroborers (primarily sponges and bivalves) was often high at these sites. The macroboring communities differed between sites and over time and it is hypothesised that these site differences were due to different levels of terrestrial runoff. These results are compared with those from French Polynesia where contaminated terrestrial runoff greatly influenced rates and agents of bioerosion. However other factors such as overfishing may also play an important role in some locations

The effects of eutrophication-related alterations to coral reef communities on agents and rates of bioerosion (Reunion Island, Indian Ocean)

International audienceThis study investigated the variation of bioerosional processes in relation to disturbances of reefal communities due to eutrophication. La Saline fringing reef (Reunion Island) is subjected to nutrient inputs from the adjacent land. Bioerosion by grazers, microborers, and macroborers was measured using experimental substrata exposed for 1 year in three sites characterized by different levels of nutrient input and benthic community response. The relationship between bioerosion and epilithic algal cover of hard substrata and the interactions between the various agents of bioerosion were analyzed with parametric statistics. Significant variations in bioerosion were found among sites, ranging from 1.63 to 3.52 kg CaCO3 m–2 year–1 for grazing rates, from 6.73 to 32.25 g m–2 year–1 for macroboring rates, and from 43.78 to 67.56 g m–2 year–1 for microboring rates. One of the major factors controlling these variations appeared to be changes in the epilithic algal cover on substrata in response to changes in reefal water chemistry. In low nutrient areas, where dead corals were colonized mainly by algal turfs, erosion by microorganisms was low (43.78 g m–2 year–1) due to intense grazing (3.52 kg m–2 year–1). In reef zones receiving high nutrient inputs, the development of encrusting calcareous algae and macroalgae was associated with the lowest grazing (1.63 kg m–2 year–1) and macroboring (6.73 g m–2 year–1) rates recorded among sites. In contrast, high microboring rates (57.54 and 67.56 g m–2 year–1) were found in enriched areas in association with high macroalgal cover