Seasonal aspects of photosynthesis in Posidonia oceanica: Influence of depth, temperature and light intensity

Photosynthetic rates of Posidonia oceanica (L.) Delile from 5-, 15- and 30-m depths were determined in the laboratory at three light intensities in September and November 1981 and May 1982. Highest photosynthetic rates were measured in plants from the 5-m depth. The seasonal maxima for all plants were reached in spring. With the exception of November, plants from the 15-m depth always had the lowest photosynthetic rates, and light saturation in these plants was achieved at lower irradiances than in those from 5 and 30 m. In general, the productivity pattern remained unchanged even after cultivation for one or two months in the laboratory under equal light and temperature conditions. During the beginning of the new growing season in autumn, higher cultivation temperatures increased photosynthetic rates. The same treatment in spring decreased photosynthetic rates due to earlier leaf senescence enhanced by higher water temperatures. The productivity pattern of Posidonia oceanica growing at different depths is well correlated to ambient light and temperature conditions and seems to be genetically fixed.