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) d u n n g an annual period The validity of using laboratory leaf incubations to set hght requirements was tested by cornpanson to in situ whole-plant measurements of photosynthesis versus irradiance ( P v s I ) Cosine (2rt) and sphencal (4rt) sensors were used to measure photon flux density (PFD) in laboratory and field expenments, respectively In additlon differences in PFDs recorded with the different sensors were examlned by completing simultaneous in situ measurements T h ~s data was used to compare estimates of productlon based on numerical integration and the dally light saturation period (H,,,) Laboratory-based seasonal photosynthetic performance was dependent on temperature and was slte-specific Rates of photosynthesis varied from ca 45 to 345 1.1mol O 2 g dry wt-' h-', w h ~l e dark respiration values ranged from ca 5 to 105 pin01 0, g dry wt-' h-' Late s p n n g and suminer saturation irradiance (Ik) values derived from labolatory incubations (lklZ.) = 85 pm01 photon m-, S-') were about 3-told lower than in sltu whole-plant measurements (IL,,,, = 290 pm01 photon m-2 S-') Despite the large apparent differences in Ik12-, and IkI4,, values, integrated production calculated using the 2 estimations were similar because of lower ~n situ 271 cosine PFD values compared to the spherical 471 sensor (ca 5 0 % ) However integrated productlon estimates using laboratory P vs l data were still 12 to 22% lower than estimates based on whole-plant incubatlons Application of the H,,, 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 sltu PFD and entire-plant P vs I measurements for seagrasses, especially under conditions of low water transparency characteristic of estuarine and near shore environments