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The photosynthesis versus irradiance (P/I) response of the seagrass Thalassia testudinum from Florida Bay, USA, was measured using the oxygen evolution of intact short shoots enclosed in sealed chambers, and found to have a light-saturated P/I behavior. All 4 of the commonly used mathematical formulations of the PA curve were of equal utility in describing the data. When fit to the data using a least-squares fitting procedure, they produced nearly identical lines explaining 90 % of the variance in the data. The estimates of the P/I model parameters ol and P,,, produced By the 4 different formulations varied widely, however, so parameter values generated using one model cannot be used in other models. Green photosynthetic leaves account for only 15.0 % of the total biomass of Florida Bay T. testudinum. The remaining 85 % is apportioned into belowground short shoots, rhizomes and roots. Leaves had higher respiration rates (7.4 yg O2 g-'min-l) than the belowground structures (0.9 to 4.6 yg O2 g-'min-l), and accounted for 42.6 % of total plant respiration. The P/I curve and respiration data were used to build a daily carbon budget for Florida Bay T. testudinum. Estimated carbon fixation rates agreed closely with previously collected field 14C uptake measurements. Under average summer light conditions, the budget was positive as deep as 4 m, suggesting that plants can survive at this depth. H,,, was 0 h at depths greater than 3 m, however.

Photosynthesis, respiration and whole plant carbon budget of the seagrass Thalassia testudinum