Possible effects of ocean acidification on coral reef biogeochemistry: topics for research

International audienceABSTRACT: This paper is a short review of recent literature on how ocean acidification may influence coral reef organisms and coral reef communities. We argue that it is unclear as to how, and to what extent, ocean acidification will influence calcium carbonate calcification and dissolution, and affect changes in community structure of present-day coral reefs. It is critical to evaluate the extent to which the metabolism of present-day reefs is influenced by mineral saturation states, and to determine a threshold saturation state at which coral communities cease to function as reefs

Sources of particulate organic matter at the ecosystem scale: a stable isotope and trace element study in a tropical coral reef

ABSTRACT: Spatial and seasonal variability of sources of particulate organic matter (POM) were evaluated in a coral reef ecosystem. Reef water POM was sampled monthly along a south-north gradient. The passage of tropical cyclone Gamede, which crossed Reunion Island in February 2007, provided an ideal opportunity to examine the impact of high river discharge. Stable isotope composition (δ13C, δ15N) was determined in oceanic, riverine and reef water POM, sedimentary organic matter (SOM), benthic primary producers and detritus. Trace elements (Ti, Fe, Cr, Ni, Cu, Zn) measured in SOM were used as additional terrestrial tracers. Stable isotope analysis of reef water showed that POM was affected by anthropogenic non-point nutrient sources, water circulation patterns and residence time. During the cold and dry season, the southern end of the reef received organic matter input derived from ocean primary production, while the middle and northern reef water POM originated primarily from SOM, reflecting an important bentho-pelagic coupling process. During the hot and wet season, reef water was enriched in benthic detritus due to higher autochthonous production. We found important spatial variability in POM sources, highlighting the importance of small-scale (hundreds of meters) studies when investigating ecosystem functioning. Although coastal tidal currents transported some river discharge material to the south of the reef, riverine POM input was limited, even after cyclone passage. We hypothesized that the major effect of the cyclone was the export of SOM and benthic detritus out of the system and that persistent disturbances such as groundwater discharges can be of greater importance than a cyclone in long-term degradation of ecosystems

Sources de l'enrichissement en sels nutritifs de l'ecosysteme recifal a la Reunion: impact des eaux souterraines.

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Community metabolism on reef flats at La Reunion Island (Indian Ocean): natural versus anthropogenic disturbance

International audienceThis paper documents community metabolism on four reef flats at La Reunion Island: (1) two undisturbed reef flats, (2) a reef flat damaged by the combined effects of nutrient loading and undergrazing (Planch’Alizés), and (3) a reef flat destroyed in 1989 by a hurricane (La Varangue). The undisturbed sites display similar excess productions (E), slightly positive in summer (56-59 μM O2 m-2 d-1, gross primary production to respiration ratio (Pg/R) = 1.11-1.13), and nil in winter. Whatever the season, E is positive at La Varangue (177-325 μM O2 m-2 d-1; Pg/R = 1.18-1.22); data suggest a tendency towards net heterotrophy at Planch’Alizés (E between -125 and -71 μM O2 m-2 d-1; Pg /R = 0.87-1); E is higher at La Varangue, and the Pg /R ratio lower at Planch’Alizés, than at the other sites. In winter, Planch’Alizés displays a lower daily net calcification than the other sites (18 and 110-182 μM CaCO3 m-2 d-1 respectively); in summer, a significant CaCO3 dissolution occurs at night at this site (respectively -16 and 3 to 5 μM CaCO3 m-2 h-1). Therefore, calcification and/or net productivity allow the characterization of the disturbed areas. At Saint-Leu, they reflect the shift in community structure, which occurred after the hurricane. The dominance of massive corals may affect the carbonate budget at Planch’Alizés. However, the negative E values and the night-time CaCO3 dissolution observed in summer at this site presumably result from an input of exogenous organic matter, and, therefore, mainly reflect anthropogenic disturbance

Seawater carbonate chemistry and protein and chlorophyll a content per nubbin of Acropora muricata during observations of La Saline fringing reef, La Reunion Island, western Indian Ocean, 2011

The effect of decreasing aragonite saturation state (Omega Arag) of seawater (elevated pCO2) on calcification rates of Acropora muricata was studied using nubbins prepared from parent colonies located at two sites of La Saline reef (La Réunion Island, western Indian Ocean): a back-reef site (BR) affected by nutrient-enriched groundwater discharge (mainly nitrate), and a reef flat site (RF) with low terrigenous inputs. Protein and chlorophyll a content of the nubbins, as well as zooxanthellae abundance, were lower at RF than BR. Nubbins were incubated at ~27°C over 2 h under sunlight, in filtered seawater manipulated to get differing initial pCO2 (1,440-340 µatm), Omega Arag (1.4-4.0), and dissolved inorganic carbon (DIC) concentrations (2,100-1,850 µmol/kg). Increasing DIC concentrations at constant total alkalinity (AT) resulted in a decrease in Omega Arag and an increase in pCO2. AT at the beginning of the incubations was kept at a natural level of 2,193 ± 6 µmol/kg (mean ± SD). Net photosynthesis (NP) and calcification were calculated from changes in pH and AT during the incubations. Calcification decrease in response to doubling pCO2 relative to preindustrial level was 22% for RF nubbins. When normalized to surface area of the nubbins, (1) NP and calcification were higher at BR than RF, (2) NP increased in high pCO2 treatments at BR compared to low pCO2 treatments, and (3) calcification was not related to Omega Arag at BR. When normalized to NP, calcification was linearly related to Omega Arag at both sites, and the slopes of the relationships were not significantly different. The increase in NP at BR in the high pCO2 treatments may have increased calcification and thus masked the negative effect of low Omega Arag on calcification. Removing the effect of NP variations at BR showed that calcification declined in a similar manner with decreased Omega Arag (increased pCO2) whatever the nutrient loading

Experimental shift in benthic community structure

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Spatial variability in submarine groundwater discharge (SDG) occurrence and benthic and fish communities patterns on St-Gilles/La Saline reef: a tentative interpretation through an hydrogeological model (Reunion Island)

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The carbonate budget of a fringing reef in La Reunion Island (Indian Ocean): sea urchin and fish bioerosion and net calcification

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Carbonate dissolution by reef microbial borers: a biogeological process producing alkalinity under different pCO₂ conditions

International audienceRising atmospheric CO2 is acidifying the world's oceans, affecting both calcification and dissolution processes in coral reefs. Among processes, carbonate dissolution by bioeroding microflora has been overlooked, and especially its impact on seawater alkalinity. To date, this biogeological process has only been studied using microscopy or buoyant weight techniques. To better understand its possible effect on seawater alkalinity, and thus on reef carbonate budget, an experiment was conducted under various seawater chemistry conditions (2 ≤  Ωarag ≤ )3.5 corresponding to 440 ≤  pCO2 (µatm) ≤ 940) at 25 degrees C under night and daylight (200 µmol photons m-2s-1) with natural microboring communities colonizing dead coral blocks (New Caledonia). Both the alkalinity anomaly technique and microscopy methods were used to study the activity of those communities dominated by the chlorophyte Ostreobium sp. Results show that (1) the amount of alkalinity released in seawater by such communities is significant and varies between 12.8 +/- 0.7 at ΩArag ~ 2 and 5.6 +/- 0.4mmol CaCO3 m-2 day-1 at ΩArag ~ 3-3.5 considering a 12:12 photoperiod; (2) although dissolution is higher at night (~ 80 vs. 20% during daylight), the process can occur under significant photosynthetic activity; and (3) the process is greatly stimulated when an acidity threshold is reached (pCO2920 µatm vs. current conditions at constant light intensity). We show that carbonate dissolution by microborers is a major biogeochemical process that could dissolve a large part of the carbonates deposited by calcifying organisms under ocean acidification