Seasonal variation of P-I relationship and the effect of temperature on P(max) and dark respiration rates of Caulerpa prolifera (Forsskal) Lamouroux were investigated in a 1-m depth meadow in the Mar Menor lagoon (south-eastern Mediterranean Spanish coast). P(max) and R rates increased linearly with water temperature between 10 and 30-degrees-C and 10 and 35-degrees-C, respectively, all through the year. The effect of temperature on R was greater in February than in May, August and November, while the effect of temperature on P(max) did not change during the year. C. prolifera maintained a positive net photosynthesis at 10-degrees-C only in February, and the effect of temperature on P-I relationship, expressed as higher P(max) and alpha at higher temperatures, increased from February to August. These results suggest that C. prolifera adjusts its metabolism to the annual course of water temperature. Daily carbon balances indicated that C. prolifera maintains a photosynthetic surplus in the lagoon throughout the year.