S. Z. Herzka á K. H. Dunton Light and carbon balance in the seagrass Thalassia testudinum : evaluation of current production models Received: 13 August 1997 / Accepted: 6 March 1998 Abstract The production dynamics and carbon balance of Thalassia testudinum in the lower Laguna Madre, Texas, USA, were examined during the 1995 summer period based on in situ photosynthesis vs irradiance P ±I  measurements and continuous measurements of underwater photon-¯ux density (PFD). The validity of applying the H sat model, used to calculate production for Zostera marina as the product of the maximum rate of photosynthesis P max  and daily hours of saturating ir- radiance H sat  was assessed for T. testudinum by com- parison with integrated production estimates derived through numerical integration. Gross integrated pro- duction values were combined with dark-respiration measurements of photosynthetic (PS) and non-photo- synthetic (NPS) tissues and areal biomass to generate daily whole-plant carbon balance. Production and whole-plant carbon balance are discussed in relation to surface and underwater PFD measurements, biomass and other physical and chemical parameters collected during a 1 yr period from January to December 1995. The H sat model signi®cantly underestimated production during all summer months, averaging 70% of integrated production over the entire study period. Gross inte- grated production ranged between 11.5 mg C g )1 leaf dry wt d )1 in June (during a period of unseasonably low PFDs caused by a drift-alga mat covering the seagrass bed) to 26.7 mg C g )1 leaf dry wt d )1 in July. Modeled net carbon gain was highest in July at 454 mg C m )2 d )1 (1.4 g dry wt m )2 d )1 ), sucient to account for mea- sured rates of leaf production in the study area and representative of T. testudinum populations of low pro- ductivity. During part of the summer period, however, the population was in negative carbon balance. The relatively low productivity of this population and the periods of negative carbon balance are attributed to low net photosynthesis:dark respiration P net : R d  ratios, sporadic low-light periods, the small fraction of PS tis- sue relative to whole-plant biomass (5 to 13%) and nutrient limitation. Production models are sensitive to both light availability and the proportion of PS tissue supporting NPS biomass as re¯ected in whole-plant P net : R d ratios. Introduction Light has long been recognized as the most important factor regulating the growth, distribution and health of seagrass populations (Buesa 1974; Backman and Bar- ilotti 1976; Dennison and Alberte 1982, 1985; Bulthuis 1983; Duarte 1991). Recent studies indicate that the light environment in seagrass beds is estuary-speci®c and variable on short time scales (Zimmerman et al. 1991; Dunton 1994; Dunton and Tomasko 1994). Therefore, continuous measurements of underwater photon-¯ux density (PFD; 400 to 700 nm) are the most indicative of light ®elds in seagrass meadows, and have become rou- tine in the monitoring of water quality (Batiuk et al. 1992; McPherson and Miller 1993; Dunton 1994). Continuous PFD measurements can be utilized in con- junction with whole-plant photosynthetic and respira- tory estimates to examine the eects of a variable light environment on the daily production and carbon bal- ance of seagrasses (Zimmerman et al. 1991, 1994). Based on a concept originally developed in the phy- toplankton literature, a computationally simple and frequently cited method of calculating production is based on the product of P max , the maximum rate of photosynthesis, and H sat , the number of hours of daily light saturation (Dennison and Alberte 1982, 1985). In addition, the daily H sat period required to maintain positive carbon balance can be utilized to examine growth depth-limits and light requirements. The H sat model has been primarily applied to the temperate spe- cies Zostera marina L. (Dennison and Alberte 1982, Marine Biology (1998) 132: 711±721 Ó Springer-Verlag 1998 Communicated by N.H. Marcus, Tallahassee S.Z. Herzka (&) á K.H. Dunton Marine Science Institute, The University of Texas at Austin, 750 Channelview Drive, Port Aransas, Texas 78373-5015, USA