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

The photosynthesis versus irradiance (P/I) response of the seagrass ThaJassia 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 ? fa of the variance in the data. The estimates of the P/I model parameters IX and Pmax produced ~ the 4 d,ifferent 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 Itg O2 g-lmin-1 than the belowground structures (0.9 to 4.61tg O2 g-lmin-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 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. Hsat was 0 h at depths greater than 3 m, however. INTRODUCTION must be understood. Previous studies have shown that in general seagrasses show a light-limited phase of photosynthesis at low light intensities, where there is a linear increase in photosynthetic rate as light increases, while at higher light intensities there is a light-saturated response in which photosynthetic rate remains constant as light increases (see Drew 1979, Dennison 1987 for reviews). In this behavior, seagrasses are similar to phytoplankton and terrestrial C3 plants. In addition to an understanding of the environmental variables which govern the PII relationship, the mathematical formulation of the PII curve must be understood if successful mechanistic models of seagrass growth as a function of light are to be produced. Most of the work done on the mathematical formulation of the relationship of photosynthesis to irradiance in aquatic and marine systems has been done with laboratory cultures or natural assemblages of phytoplankton. Several models (Table 1) have been proposed to describe the light-saturated response of photosynthesis to irradiance illustrated in Fig. 1. The 2 parameters common to all of these models are IX and In the last 50 yr, there has been a precipitous decline in the areal extent of seagrass beds in many parts of the world. Often these declines have been linked to reduced water quality and increased turbidity, which decrease the amount of light available to benthic plants (Zieman 1915, Orth & Moore 1983, Cambridge & McCoomb 1984, Geisen et al. 1990, Larkum & West 1990). Reduced light levels may restrict seagrasses to shallower areas, since the depth limits of some seagrasses are considered to be controlled by the penetration of light in the water column (Wetzel & Penhale 1983, Dennison & Alberte 1985, Dennison 1981). Vicente & Rivera (1982) reported that the depth limit of Thalassia testudinum Banks ex Konig in Puerto Rico is closely correlated to the mean Secchi depth over the seagrass beds, illustrating the importance of light intensity to depth limits. In order to predict the response of seagrasses to changes in environmental light regimes, the photosynthesis versus irradiance (P/I) response of seagrasses @ Inter-Research/Printed in Germany 0171-8630/91/0069/0161/$ 03.00 162 Mar. Ec;ol. Prog. gel. 69: 161-170, 1991 Table 1. Two-parameter gross photosynthesis (Pg) versus irradiance (I) models. All models expressed in terms of IX and