Seasonal photosynthetic patterns of the seagrass Thalassia testudinum in the western Gulf of Mexico

Abstract

Seasonal photosynthetic performance was compared between 2 distinct populations of Thalassia testudinum in Texas (USA) during an annual period. The validity of using laboratory leaf incubations to set light requirements was tested by comparison to in situ whole-plant measurements of photosynthesis versus irradiance (Pvs I). Cosine (2π) and spherical (4π) sensors were used to measure photon flux density (PFD) in laboratory and field experiments, respectively. In addition, differences in PFDs recorded with the different sensors were examined by completing simultaneous in situ measurements. This data was used to compare estimates of production based on numerical integration and the daily light saturation period (H(sat)). Laboratory-based seasonal photosynthetic performance was dependent on temperature and was site-specific. Rates of photosynthesis varied from ca 45 to 345 μmol O2 g dry wt-1 h-1 while dark respiration values ranged from ca 5 to 105 μmol O2 g dry wt-1 h-1. Late spring and summer saturation irradiance (I(k)) values derived from laboratory incubations (I(k)(2π)) = 85 μmol photon m-2 s-1) were about 3-fold lower than in situ whole-plant measurements (I(k)(4π)) = 290 μmol photon m-2 s-1). Despite the large apparent differences in I(k(2π)) and I(k(4π)) values, integrated production calculated using the 2 estimations were similar because of lower in situ 2π cosine PFD values compared to the spherical 4π sensor (ca 50%). However, integrated production estimates using laboratory P vs I data were still 12 to 22%, lower than estimates based on whole-plant incubations. Application of the H(sat) model to the data showed that the predictive capacity varied as a function of water transparency, source of P vs I data and sensor utilized; values were 14 to 100% lower than numerically integrated production. The results underscore the value of in situ PFD and entire-plant P vs I measurements for seagrasses, especially under conditions of low water transparency characteristic of estuarine and near shore environments.