PHOTOSYNTHESIS, RESPIRATION AND WHOLE PLANT CARBON BUDGET OF THE SEAGRASS THALASSIA-TESTUDINUM

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 P/I 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-alpha and P(max) 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-mu-g O2 g-1 min-1) than the belowground structures (0.9 to 4.6-mu-g O2 g-1 min-1), 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 C-14 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(sat) was 0 h at depths greater than 3 m, however.