The photosynthetic capacity of the seagrass Posidonia oceanica: influence of nitrogen and light.

Photosynthetic capacity of the seagrass Posidonia oceanica was determined in two contrasting nutrient zones along a bathymetric gradient. Photosynthetic capacity was two times higher in the relatively eutrophic zone compared to the highly oligotrophic zone. The same trend was observed in the chlorophyll a+b and tissue nitrogen content. Photosynthetic capacity and chlorophyll a+b decreased with irradiance in the oligotrophic zone while increased in the eutrophic zone. Tissue nitrogen content followed the same trend in the oligotrophic zone and did not present any trend in the eutrophic zone. Overall, this study has shown the importance of large scale sampling when assessing metabolic parameters such as photosynthetic capacity where large differences have been observed associated to the nutrient regime. According to our results, in oligotrophic zones nitrogen seems to govern photosynthetic capacity and there is a coherent pattern between nitrogen content, pigments and photosynthetic capacities. In eutrophic zones, light is the dominant regulator and there is a coherent pattern between light and photosynthetic capacities.