Effects of elevated temperature on seasonal in situ leaf productivity of Thalassia testudinum Banks ex König and Syringodium filiforme Kützing

Net leaf productivities of Thalassia testudinum Banks ex König and Syringodium filiforme Kützing, measured in situ over a 1-year period in a sub-tropical estuary that receives thermal addition from a local power plant, were significantly temperature dependent. Primary production in both species followed seasonal temperature variation, with increase in leaf dry weight as a proportion of total leaf biomass ranging from 0.12% to 2.54% day^?1 for T. testudinum and 0.33% to 3.80% day^?1 for S. filiforme. From September to May, S. filiforme exhibited significantly higher productivity at the thermally-impacted (experimental) station than at the control station. However, from June to August, S. filiforme productivity at the experimental station was significantly lower than at the control station. T. testudinum productivity showed similar trends, but interstation differences were not statistically different. Maximal growth occurred between 23 and 29°C for S. filiforme and 23 and 31°C for T. testudinum. Thus, the thermal addition generally enhanced the seagrass productivity of both species in the autumn, winter, and spring, but reduced that of S. filiforme in the summer, as the optimal temperature range of this species was exceeded. The fact that T. testudinum exhibited a less marked response to the thermal addition and had a greater optimal temperature range than S. filiforme supports its status as the more eurythermal of the 2 species.