The rates of photosynthesis of the seagrassSyringodium isoetifolium(Aschers.) Dandy were measured by a gas‐release technique with whole shoots incubated in the field. Greater than 95% of the variation of the photosynthetic data could be accommodated by the Photosythesis‐Irradiance (PI) model presented here. As expected, the shape of the Pi‐curves differed between shoots. The different shapes resulted from the different photosynthetic maxima and the efficiency with which shoots used light but not the amount of light required by the shoot to reach saturating light intensities. Irrespective of the units (whole shoot, leaf chlorophyll content, or leaf surface area) used to generate the PI‐curves, there was no significant(P >0.05) difference between saturating light intensities (Ik), the mean value was 180 ± 33 μmol m‐2s‐1. The quantity and quality of light available to these plants can modify leaf surface and chlorophyll content of seagrass shoots. Presenting the rates of photosynthesis as a function of chlorophyll or leaf surface area diminishes the ability to observe differences between plants. Hence, the health of seagrass plants may be more obvious and quantifiable if seagrass productivity data are expressed as a function of whole seagrass shoots rather than on the basis of chlorophyll content or leaf surface. During the 20‐day study, the photoperiod ranged from 6.2 to 10.8 h and gross productivity estimates ranged from 10 to 18 g C day‐1m‐2. These estimates were more than 10 times greater than thein situnet leaf daily productivity of 0.9 g C day‐1m‐2that was measured with a traditional leaf marking technique. This result implies that most of the production was directed below ground. The health of seagrasses is related to the productivity of the whole plant not just the tissue that appears above ground. In comparative studies that are used to determine the habitat requirements of seagrasses and the impacts of pollutants on seagrass survival whole plant productivity measurements would appear to provide a better indicator of seagrass health.