Computing the carbonate chemistry of the coral calcifying medium and its response to ocean acidification.

Critical to determining vulnerability or resilience of reef corals to Ocean Acidification (OA) is a clearer understanding of the extent to which corals can control carbonate chemistry in their Extracellular Calcifying Medium (ECM) where the CaCO3 skeleton is produced. Here, we employ a mathematical framework to calculate ECM aragonite saturation state (Ωarag.(ECM)) and carbonate system ion concentration using measurements of calcification rate, seawater characteristics (temperature, salinity and pH) and ECM pH (pH(ECM)). Our calculations of ECM carbonate chemistry at current-day seawater pH, indicate that Ωarag.(ECM) ranges from ∼10 to 38 (mean 20.41), i.e. about 5 to 6-fold higher than seawater. Accordingly, Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) were calculated to be around 3 times higher in the ECM than in seawater. We also assessed the effects of acidification on ECM chemical properties of the coral Stylophora pistillata. At reduced seawater pH our calculations indicate that Ωarag.(ECM) remains almost constant. DIC(ECM) and TA(ECM) gradually increase as seawater pH declines, reaching values about 5 to 6-fold higher than in seawater, respectively for DIC and TA. We propose that these ECM characteristics buffer the effect of acidification and explain why certain corals continue to produce CaCO3 even when seawater chemistry is less favourable.

Spatial distribution of the copepod Centropages typicus in Ligurian Sea (NW Mediterranean). Role of surface currents estimated by Topex-Poseidon altimetry.

A particle-tracking model was used to simulate the dispersion and development of the planktonic copepod Centropages typicus during spring in Ligurian Sea. We show that mesoscale current structure, with a coastal jet and eddies, plays a key role in the transport and dispersion of C. typicus during its life cycle. Although, in the north, offshore Nice, cohorts can be advected southwestward out of Ligurian basin, more to the south others are retained in the central eddy and may give the start to the spring bloom of this species. However, input of individuals from the south through the Corsican Channel and along the west coast of Corsica may also be important in spring. This study shows that the ambit of C. typicus population is larger than the Ligurian Sea.

Phenological changes in the Northwestern Mediterranean copepods Centropages typicus and Temora stylifera linked to climate forcing.

Planktonic copepods play a major role in the fluxes of matter and energy in the marine ecosystem, provide a biological pump of carbon into the deep ocean, and play a role in determining fish recruitment. Owing to such ecological considerations, it is essential to understand the role that climate might play in the interannual variability of these organisms and the mechanisms by which it could modify the ecosystem functioning. In this study, a causal chain of meteorological, hydrological and ecological processes linked to the North Atlantic Oscillation (NAO) was identified in the Ligurian Sea, Northwestern Mediterranean. The forcing by the NAO drives most of the hydro-climatic variability during winter and early spring. Subsequently, interannual and decadal changes of the dominant copepods Centropages typicus and Temora stylifera were significantly correlated to the state of the hydro-climatic signal and tightly coupled to the NAO. Direct and indirect effects whose influence promoted phenological changes in the two copepod populations drove the species' responses to climatic forcing. Opposite responses of the analysed species were also highlighted by these results. While years characterized by the positive phase of the NAO leads to enhancement of the strength and the forward move of the C. typicus peak, they act negatively on the annual cycle of T. stylifera, the abundance of which drops twofold and the annual peak appears delayed in time. In contrast, low NAO years lead to high abundance of T. stylifera and a forward timing of its peak, and acts in turn negatively on the C. typicus annual cycle in both abundance (low) and timing (delayed). Owing to the synchronism between hydro-climatic conditions and the NAO, and the major role of these species in the pelagic ecosystem of the studied area, these results provide key elements for interpreting and forecasting decadal changes of planktonic populations in the Ligurian Sea.