Heat stress of two tropical seagrass species during low tides - impact on underwater net photosynthesis, dark respiration and diel in situ internal aeration

Seagrasses grow submerged in aerated seawater but often in low O-2 sediments. Elevated temperatures and low O-2 are stress factors. Internal aeration was measured in two tropical seagrasses, Thalassia hemprichii and Enhalus acoroides, growing with extreme tides and diel temperature amplitudes. Temperature effects on net photosynthesis (P-N) and dark respiration (R-D) of leaves were evaluated. Daytime low tide was characterized by high pO(2) (54 kPa), pH (8.8) and temperature (38 degrees C) in shallow pools. As P-N was maximum at 33 degrees C (9.1 and 7.2 mu mol O-2 m(-2) s(-1) in T. hemprichii and E. acoroides, respectively), the high temperatures and reduced CO2 would have diminished P-N, whereas R-D increased (Q(10) of 2.0-2.7) above that at 33 degrees C (0.45 and 0.33 mu mol O-2 m(- 2) s(-1), respectively). During night-time low tides, O-2 declined resulting in shoot base anoxia in both species, but incoming water containing c. 20 kPa O-2 relieved the anoxia. Shoots exposed to 40 degrees C for 4 h showed recovery of P-N and R-D, whereas 45 degrees C resulted in leaf damage. These seagrasses are 'living near the edge', tolerant of current diel O-2 and temperature extremes, but if temperatures rise both species may be threatened in this habitat.