Published 21 October 2009
Les émissions de CO2 entraînent une acidification croissante des océans. Avec un risque d’impacts en cascade sur de nombreux organismes marins.
Si les émissions de dioxyde de carbone monopolisent essentiellement l’actualité en raison de leur rôle dans le processus du réchauffement climatique, celles-ci sont également à l’origine d’un autre phénomène inquiétant : l’acidification des océans. Encore peu étudiée il y a une quinzaine d’années, cette problématique est aujourd’hui au centre des préoccupations d’un nombre croissant de scientifiques, avec à la clef une série de constats là encore inquiétants.
“La tonalité de ces études est assez grise en effet, mais nous manquons encore de recul. On découvre presque tous les mois de nouveaux processus affectés négativement ou positivement par ce phénomène d’acidification“, commente avec prudence l’océanographe Jean-Pierre Gattuso, coordinateur du projet européen Epoca (qui est le premier programme de recherche international sur le sujet. Lire ci-contre).
Continue reading ‘Les océans tournent à l’aigre (in French)’
Published 21 October 2009
A new pH modelling approach is presented that explicitly quantifies the influence of biogeochemical processes on proton cycling and pH in an aquatic ecosystem, and which accounts for time variable acid-base dissociation constants. As a case study, the method is applied to investigate proton cycling and long-term pH trends in the Scheldt estuary (SW Netherlands, N Belgium). This analysis identifies the dominant biogeochemical processes involved in proton cycling in this heterotrophic, turbid estuary. Furthermore, information on the factors controlling the longitudinal pH profile along the estuary as well as long-term pH changes are obtained. Proton production by nitrification is identified as the principal biological process governing the pH. Its acidifying effect is mainly counteracted by proton consumption due to CO2 degassing. Overall, CO2 degassing generates the largest proton turnover in the whole estuary on a yearly basis. The main driver of long-term changes in the mean estuarine pH over the period 2001 to 2004 is the decreasing freshwater flow, which influences the pH directly via a decreasing supply of dissolved inorganic carbon and alkalinity, and also indirectly, via decreasing ammonia loadings and lower nitrification rates.
Continue reading ‘pH modelling in aquatic systems with time-variable acid-base dissociation constants applied to the turbid, tidal Scheldt estuary’
Production (abundance and biomass) and net calcification rates of the coccolithophorid Pleurochrysis carterae under different partial pressures of CO2 (pCO2) were examined using short (15, 24 and 39 h), long (7 d) and dark (7 d) incubation experiments. Short incubations were conducted at ambient, 500 and 820 ppm pCO2 levels in natural seawater that was enriched with nutrients and inoculated with P. carterae. Long incubations were conducted at ambient and 1200 ppm pCO2 levels in natural seawater (0.2 µm filtered as well as unfiltered) that was enriched with nutrients and inoculated with P. carterae. Dark incubations were conducted at ambient and 1200 ppm pCO2 in unfiltered seawater that was inoculated with P. carterae. The abundance and biomass of coccolithophorids increased with pCO2 and time. The abundance and biomass of most noncalcifying phytoplankton also increased, and were hardly affected by CO2 inputs. Net calcification rates were negative in short incubations during the pre-bloom phase regardless of pCO2 levels, indicating dissolution of calcium carbonate. Further, the negative values of net calcification in short incubations became less negative with time. Net calcification rates were positive in long incubations during blooms regardless of pCO2 level, and the rate of calcification increased with pCO2. Our results show that P. carterae may adapt to increased (~1200 ppm) pCO2 level with time, and such increase has little effect on the ecology of noncalcifying groups and hence in ecosystem dynamics. In dark incubations, net calcification rates were negative, with the magnitude being dependent on pCO2 levels.
Continue reading ‘Effects of carbon dioxide on the coccolithophorid Pleurochrysis carterae in incubation experiments’