Ocean acidification in the Mediterranean Sea: Pelagic mesocosm experiments. A synthesis

Estuarine, Coastal and Shelf Science, 2017.

Référence :

Maugendre L., Guieu C., Gattuso J.-P. & Gazeau F. (2017). Ocean acidification in the Mediterranean Sea: pelagic mesocosm experiments. A synthesis. Estuarine, Coastal and Shelf Science. http://dx.doi.org/10.1016/j.ecss.2017.01.006

Extrait [en anglais] :

"[...] The Mediterranean Sea is a typical low-nutrient low-chlorophyll area which exhibits large changes in nutrient concentrations in the illuminated surface waters that depend strongly on the seasonal hydrological regime. Nutrients are severely depleted in the surface layer during summer oligotrophic conditions characterized by strong thermal stratification. During winter mixing events (January–February), nutrients are re-injected to the surface layer, providing favourable conditions for a bloom initiation. Superimposed to these well-known seasonal features is an important inter-annual variability (Marty et al., 2002 and de Fommervault et al., 2015). Short events driven by the atmosphere such as strong short wind events (i.e. Andersen and Prieur, 2000) and sporadic atmospheric inputs (i.e. Pulido-Villena et al., 2010) can lead to transient increase in nutrient concentrations impacting nutrient stocks and thus likely biota and biogeochemical fluxes. Indeed, while a wind event can inject nutrients from below by rapidly deepening the mixed layer depth (Andersen and Prieur, 2000), atmospheric inputs such as Saharan dust events, biomass burning or intense rain events can bring new nutrients to the surface of the water-column on short-time scales (The Mermex group, 2011, and references therein). In some cases, both nutrients from below and above can also be responsible of profound transient changes in nutrient dynamics and impact biota (Guieu et al., 2010). Nutrient availability is therefore a main control of ecosystem condition in the Mediterranean Sea. The perturbation experiments reported in the present special issue show no or low impact of ocean acidification on key biogeochemical processes, both in summer and winter whereas the natural assemblage was highly dependent on nutrient availability. Our summer in situ ocean acidification mesocosm experiment was representative of summer conditions in the Northwestern Mediterranean Sea. The results appear solid regarding the impact of ocean acidification on short time scale on the biogeochemistry of such oligotrophic system. Caution should be exercised to interpret the results of the winter experiment because the expected bloom conditions were not met, and important changes in nutrient availability were observed during the acidification step leading to conditions inside the mesocosms that were not representative of ambient conditions. Moreover, poor weather conditions at the beginning of the experiment (including variable and low light availability) prevented the stabilisation of blooming conditions in the bags. Yet, the time of the year to perform this experiment was carefully chosen according to the 18-year time series both at Point B and DYFAMED ( Fig. 4). Unfortunately, this experiment is a good illustration that biological activity in the Mediterranean Sea exhibits a large interannual variability and specific short-term events, such as blooms, are difficult to capture. [...] "

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