Seasonal variation of temperature effects on photosynthesis and respiration of Cymodocea nodosa (UCRIA) ASCHERSON was investigated in a 1 m depth meadow in the Mar Menor lagoon (SE Spain). Light-saturated net photosynthesis (P-max) and dark respiration (R) rates were linearly correlated with water temperature between 10 and 30 degrees C, and 10 and 35 degrees C, respectively, during the year. Water temperature had a similar effect on R rates in August, November, and February, but increased P-max rates in August vs. those in February, thus promoting a shift of the maximum P-max/R ratio values from 15 degrees C in February to 30 degrees C in August. P-I curves had higher P-max values in May and August than in November or February, while I-c values remained relatively constant throughout the year. Whole-plant carbon balances suggested that C. nodosa might not be able to survive in the central basin of the lagoon and that the spatial distribution of this seagrass in the Mar Menor might be controlled by light availability.