I examined the production dynamics, reproductive ecology and demography of the largest Thalassia testudinum meadow on the Texas coast, which occurs in Lower Laguna Madre. This study represents the first measurements from Lower Laguna and the first application of several new methods to examine seagrass health and production. Annual whole plant production was estimated between 600-1000 gdw m-2 y-1. On an annual basis, rhizome production accounted for 25 to 35% of total plant production and 80-90% of total biomass. Determination of total plant productivity must take into account seasonal patterns of production and the large fraction of production in the below-ground tissues. Sexual reproductive effort was quantified, with up to 15% of total above-ground biomass allocated to reproduction. Concurrent depression of plant growth suggests that a substantial energetic cost is associated with reproduction. T. testudinum seedlings undergo dramatic ontogenetic changes in photosynthetic parameters, biomass allocation patterns and soluble carbohydrate reserves. These changes probably control seedling survival and the establishment of new genets within the seagrass population. Biomass allocation and photosynthetic patterns became more representative of adult plants as the seedlings matured. Gross Pmax increased from 80 to 220 µmol O2 gdw-1 hr-1, while whole plant respiration decreased from 170 to 60 µmol O2 gdw-1 hr-1. Daily carbon balance, assessed using the Hsat model, indicates that T. testudinum seedlings become photosynthetically self-sufficient between 2 and 6 months after seed release. Preliminary calculations imply a potential seed production equivalent to 90 seedlings m-2 of bare area, which is consistent with field observations. I also examined the age structure and demography of two Thalassia populations. Long-term leaf marking suggests that T. testudinum violates a critical assumption of the plastochron interval technique, which is used to determine shoot age. As a result of site-specific, seasonal and interannual variability, annual leaf formation rates only provide preliminary estimates of shoot age. Static life-table calculations were used, to determine age-specific rates of short shoot survival, mortality and life-expectancy. Maximum shoot life expectancy was about 2.5-4 years, equivalent to a turn-over rate of 0.25-0.4 y-1.