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The effects of salinity and temperature on the photosynthetic and growth performance of the seagrasses Cymodocea nodosa and Ruppia cirrhosa were studied to understand their local seasonality and distribution. Cymodocea nodosa shoots were collected from Cape Vrasidas, and R. cirrhosa shoots from the coastal lagoon Fanari, all from the Eastern Macedonian and Thrace Region, Greece. Effective quantum yield (Delta F/Fm'), leaf chlorophyll-a content (mg g(-1) wet mass) and growth (% of maximum) were tested at different temperatures (10-40 degrees C) and salinities (5-60). The results showed that: (a) R. cirrhosa was more euryhaline (5-55/60) than C. nodosa (10-50), (b) the upper thermal tolerance of C. nodosa (34-35 degrees C) was higher than that of R. cirrhosa (32-34 degrees C), (c) C. nodosa could not tolerate 10 degrees C, whereas R. cirrhosa could, and (d) the growth optimum of C. nodosa was 30 degrees C and that of R. cirrhosa 20-30 degrees C. The thermal optima and tolerances of growth and photosynthesis confirm the seasonal patterns of R. cirrhosa but not of C. nodosa. However, the sensitivity of C. nodosa to low salinities and temperatures may explain its absence from shallow coastal lagoons. Ruppia cirrhosa could be vulnerable to future climate change.