POTENTIAL EFFECT OF INCREASED GLOBAL CO_2 AVAILABILITY ON THE DEPTH DISTRIBUTION OF THE SEAGRASS POSIDONIA OCEANICA (L.) DELILE: A TENTATIVE ASSESSMENT USING A CARBON BALANCE MODEL

摘要

The hypothesis that seagrass depth limits could be increased as a consequence of the rising of CO_2 in the atmosphere and the sea has been examined for the Mediterranean seagrass Posidonia oceanica (L.) Delile. To do so, a carbon budget for this species near its depth limit was computed under natural (i.e., present day) and under experimentally increased [CO_2]_(aq) conditions. We obtained photosynthesis (as net oxygen release) vs. irradiance relationships at pH 8.2 (normal) and 7.5 for leaves collected at several months (March, May and July 1999) and of distinct ages. P_(max) (net oxygen release at saturating irradiance) increased significantly at low pH, while I_k (irradiance at which photosynthesis becomes light saturated) and dark respiration were not affected by changes in pH. Photosynthetic parameters were incorporated in an available carbon budget model. During the study period (March-July 1999), lowering of pH (increasing [CO_2]_(aq) caused an 80% increase in the net carbon balance. We tentatively conclude that an increase in [CO_2]_(aq) will significantly enhance the carbon budget of this seagrass, suggesting the possibility of colonisation beyond its present depth limit.