Physiological Responses of \u3cem\u3eThalassia testudinum\u3c/em\u3e and \u3cem\u3eRuppia maritima\u3c/em\u3e to Experimental Salinity Levels

Thalassia testudinum, a stenohaline seagrass species, and Ruppia maritima, a euryhaline submerged aquatic vegetation species, were subjected to the same seven salinity levels (0 - 60) in a controlled environment. The response variables examined were the occurrence of leaf discoloration, plant growth rates, photosynthetic characteristics of blade segments (Pmax, respiration, alpha, and Ik), and osmolality changes within the plant tissues. These response variables were measured at exposure times of one, seven, and 28 days. Greater than 75% leaf discoloration occurred in Thalassia testudinum blades placed in 0, and 60 psu, while Ruppia maritima blades only became severely discolored in 60 psu. Plant growth rates were highest in 40 psu for T. testudinum and 20 psu for R. maritima. Pmax for both species was somewhat affected by salinity changes, but the plants did not appear to be photosynthetically compromised in their optimal ranges over time. Salinity effects on photosynthesis were less pronounced in R. maritima than in T. testudinum, which would be expected when comparing a euryhaline species to a stenohaline species. Both intercellular and intracellular osmolality showed a pattern of increase or decrease as the treatment salinities were altered from ambient levels (30 psu for T. testudinum and 20 psu for R. maritima). After one day of exposure to a new treatment salinity, the intercellular osmolality had changed significantly from ambient value, with a second shift, occurring mostly in the salinity extremes, for both seagrass species. This second shift is most likely due to the fact that at the extremes, the plants are being compromised. Changes in these physical and physiological responses indicate that significant increases and decreases in ambient salinity levels are initially stressful for both species. Both seagrass species had an optimal salinity as well as a range of salinities in which the long-term physiological stresses did not cause tissue death. Thalassia testudinum had the fewest stress responses in 40 psu, with an optimal range of 20 - 40 psu. Ruppia maritima had the fewest stress responses in 20 psu (growth salinity) with an optimal range of 0 - 40 psu. In this study, neither species was able to survive for 28 days in 60 psu (at which point the plants had been out of their respective optimal salinities for at least 42 days)