PHOTOSYNTHETIC RESPONSE TO LIGHT AND TEMPERATURE OF THE SEAGRASS CYMODOCEA-NODOSA AND THE PREDICTION OF ITS SEASONALITY

Photosynthesis-irradiance (P-I) curves and the response of both light-saturated photosynthesis and dark respiration to temperature manipulations have been studied under laboratory conditions for the seagrass Cymodocea nodosa (Ucria) Aschers. from a shallow bed in the Ebro delta (northeast Spain). The experiments were performed seasonally in order to describe the annual variation of the characteristic parameters of the P-I curves (P(max), I(sat), I(comp)). The parameter showing the clearest seasonal pattern was P(max), with a summer maximum of around 5 mg O2 per g dry weight h-1 and a winter minimum of around 2 mg O2 per g dry weight h-1. Water temperature appears to have a significant effect on rates of net photosynthesis and dark respiration. Both rates showed a positive response to increasing temperatures, although respiration was more sensitive than net photosynthesis. Using experimental laboratory data, an attempt was made to establish a carbon budget for the Cymodocea bed. The calculated primary production (800 g dry weight m-2 year-1) differed from the primary production obtained from field measurements (500 g dry weight m-2 year-1). This discrepancy occurred only between May and October. It is concluded that although light and temperature are the major factors controlling seasonality of Cymodocea growth, during summer months growth is limited by other factors, mainly self-shading and nutrient limitation.