Mireille Peyrot-Clausade

Bioerosion rates on coral reefs: interactions between macroborers, microborers and grazers (Moorea, French Polynesia)

Abstract A two-year experimental study of bioerosion at Moorea Island, French Polynesia, clearly demonstrated the importance of microborers in the initial stages of the establishment of infaunal boring communities. Rates of erosion by micro- and macroborers and by grazers were estimated from measurements of carbonate removal from experimental substrates, using Image Analysis. The studied substrates have been exposed for 2, 6, 12 and 24 months. After 2 months of exposure, the only borers present in the substrates were cyanobacteria and one chlorophyte (Phaeophila sp.) and their bioerosion rate was estimated at 0.6 kg CaCO3 m−2 yr−1. In the course of the 2 years of exposure, recruitment of macroborers occurred and their estimated rates of erosion increased during this period from 2.15 to 90g CaCO3 m−2 yr−1. Carbonate removal by grazers was the dominant agent of erosion, responsible for 89% of the total bioerosion: 2.6 kg CaCO3 m−2 yr−1, as recorded in substrates exposed for 2 years. The measurable rates of bioerosion by microborers apparently decreased with the time of exposure from 0.6 to 0.2 kg m−2 yr−1, but these values are underestimations which need to be corrected by including the intensity of microboring in substrate layers removed by grazing. Bioerosion is dependent on numerous environmental factors such as depth, light availability, and nutrient supply. A good knowledge of bioerosional processes in modern environments could highlight bioerosion significance in the fossil record.

Bioerosion of experimental substrates on high islands and atoll lagoons (French Polynesia) during 5 years of exposure

Rates of loss of CaCO3 from bioerosion (grazing and boring) and gains from accretion were determined from experimental coral substrates exposed for 5 years, which were laid in the lagoons of high islands and atolls in French Polynesia. Significant differences in net rates of bioerosion occurred between sites with the highest rates of loss being recorded at one of the lagoonal sites. The processes contributing to these losses and gains varied between sites. The results were compared with those obtained at the same sites after 6 and 24 months exposure. Rates of bioerosion increased with increasing exposure as did rates of internal erosion as densities of borers increased and boring sponges began to colonise the experimental substrates. High rates of bioerosion were recorded at both eutrophic and pristine sites suggesting that environmental conditions only influence some of the factors controlling rates of bioerosion. Worldwide, the amount of dead coral substrate available for colonisation by boring organisms is increasing as reefs are being subjected to anthropogenic impacts. We discuss how this may change the balance between reef growth and reef destruction by borers and grazers. In cases where the impact may be short lived as in the case of bleaching, we suggest that the maintenance of water quality is especially important in trying to minimise the rates of increase of bioerosion and increases in density of grazers.

The distribution and abundance of boring species of polychaetes and sipunculans in coral substrates in French Polynesia

Abstract The distribution and abundance of the dominant initial macroborers of dead coral substrate, sipunculans and polychaetes were investigated over time at seven sites within French Polynesia. Sites were located in the lagoon of high islands and atolls, and varied from highly eutrophic to oligotrophic. Significant differences occurred between sites and patterns of recruitment varied over time and between sites. With increasing exposure, the densities of polychaetes increased but not the number of species present, whereas both the densities and number of species of sipunculans increased. The atoll sites tended to be dominated by suspension feeding polychaetes and the high island sites by deposit feeding polychaetes. Sipunculans tended to dominate the high island sites in comparison to the atoll sites and they all fed by scraping algae and detritus from the substrate. We suggest that this distribution of feeding types is related to water quality and to land run off. In the atolls, the lagoonal waters are oligotrophic and little land run off occurs, whereas at the high island sites, high rates of land run off occur during the wet season with high levels of suspended material in the water column. These variations in densities of boring species, affect rates of bioerosion and have the potential to alter the equilibrium between reef growth and reef destruction. We suggest that it is critical for reef managers to try to maintain water quality and limit land-based terrestrial run off and associated nutrients into coastal waters. This is especially important if the reefs have been affected by bleaching events or Crown of Thorns plagues, resulting in extensive death of coral colonies and with it, the potential for a massive increase in the rate of bioerosion. The long-term maintenance of the reef structure is critical if coral recruitment and recovery of the reef are to occur.

Ingestion and transformation of algal turf by Echinometra mathaei on Tiahura fringing reef (French Polynesia)

The sea urchin Echinometra mathaei is the most abundant herbivore on many tropical reefs. We studied the ingestion and digestion diel rhythms, transformation of algal turf and bioerosion attributable to this species on the Tiahura fringing reef in French Polynesia. Ingestion rates showed a circadian rhythm with most feeding taking place during the night. Absorption of food occurred throughout the day with urchins digesting food outside of the feeding period. A total of 73% of the faecal pellets consisted of CaCO(3) eroded from the reef, 20% consisted of organic matter and 7% the refractory organic matter. Of the organic matter, lipids, carbohydrates and chlorophyll were digested and absorbed and proteins were expelled in the faecal pellets. An average individual bioerosion of 0.32 g day(-1) was estimated for E. mathaei from approximately a 35-mm test diameter on the Tiahura fringing reef. We further estimated that E. mathaei release 70.5 g m(2) y(-1) of carbohydrates, 43.8 g m(2) y(-1) of lipid, 23.3 g m(2) y(-1) of protein and 2.0 g m(2) y(-1) of total chlorophyll pigments.

Influence of some coral reef communities on the calcium carbonate budget of Tiahura reef (Moorea, French Polynesia)

The calcium carbonate budget of coral reefs is the result of the interaction of the processes of calcification and biological degradation, and is reflected in the chemical properties of the seawater overlying the reefs. A series of experiments at Moorea Island (French Polynesia) in 1988 monitored the diurnal and nocturnal variations in the chemical properties of seawater under field and laboratory conditions. Our results revealed that in the study area (Tiahura barrier reef flat), the calcium carbonate budget varied over space and time as a function of location in the water current. Two in-situ sites were investigated; one was situated 100 m from the algal crest of the barrier reef, the other 300 m further downstream. As a result of cumulative upstream events, the daily net calcification was ten times higher at the downstream (5.22 gm-2 d-1) than at the upstream (0.45 gm-2 d-1) site. The carbonate uptake by in situ Porites lobata in enclosures (8 kgm-2 yr-1) was ten times higher than the uptake by the whole community in the surrounding water (0.8 kgm-2 yr-1) and five times higher than that recorded for P. lobata in laboratory experiments (1.4 kgm-2 yr-1), where illumination levels were 10% of in situ levels. In laboratory experiments, the planktonic fraction of the seawater had no perceptible influence on the calcium carbonate budget. In the absence of bioeroders, living coral totally depleted the carbonate content of the seawater (3.7 gm-2d-1). Bioerosive organisms played an important role in restoring this calcium carbonate; e.g. sea urchins grazing on algal turf covering dead coral ingested CaCO3 and released this as a carbonate powder (1.26 gm-2d-1); a form of carbonate which is extremely accessible to chemical dissolution.

Transformation of algal turf by echinoids and scarid fishes on French Polynesian coral reefs

The respective roles of regular echinoids and scarid fishes in the transformation of turf algae, the main food resource for reef herbivores, were investigated on French Polynesian coral reefs. The role of one species of parrotfish (Scarus sordidus) was compared with that of four species of echinoids. The degree and ways of degradation of the algal matter were determined by the organic matter percentage, the composition of the sugar fraction, and the concentration and composition of chlorophylltype pigments as assayed by HPLC analysis. Chemical analyses were performed on anterior and posterior intestines for scarids, intestinal contents and faeces for echinoids, and on fresh algal turf as a control of initial food quality. A decrease in mean percentage of organic matter in gut content was observed from intestine (9.7%) to faeces (7%) in sea urchins, but not in parrotfishes. The total sugar fraction decreased from fresh algal turf (32% of total organic matter) to echinoid (28%) to scarid (18%) gut contents. The ratio of insoluble to soluble sugars (I/S ratios) was higher in echinoids (2.6) than in scarid gut contents (1.0). A decrease in the total pigment concentration was measured from fresh algal turf to echinoid and it was found to be even lower in scarid gut contents. Chromatograms showed that the composition of chlorophyll-type pigments in scarid intestines was very similar to fresh algal turf, with a dominance of native forms, mainly chlorophyll a and b. On the contrary, degraded pigment forms dominated in echinoids. The main degraded products were pheophorbides in sea urchins, and chlorophyllides in parrotfishes. These results provided evidence for differentiation in digestive processes occurring in the two types of grazers. Echinoids released higher degraded algal material than did scarids. Thus, these two types of grazers play different roles in the recycling of organic matter on coral reefs.