Ion concentrations in seagrass: A comparison of results from field and controlled-environment studies

Osmoregulation is essential for the survival of seagrasses in marine and hypersaline environments. The aim of this study was to examine ion concentrations of four seagrass species (Posidonia australis, P. sinuosa, Amphibolis antarctica and A. griffithii) after exposure to salinity changes. Plant fragments were placed in a series of aquaria at marine salinity (35) and, after one week of acclimation, exposed for 7 days to salinities between 20 and 70. Cl⁻, Na⁺ and total ion concentration increased with salinity in leaf tissue of the four seagrasses species. These results were compared with those of P. australis and A. antarctica samples collected at three locations at Shark Bay, Western Australia where higher salinities occurred, ranging from 46 to 51. Concentrations of K⁺ and Ca⁺² were higher in seagrass tissues from Shark Bay than in those in aquarium trials. Cl⁻, Na⁺ and total ions in P. australis and A. antarctica from Shark Bay were lowest at the highest salinity location. The K⁺/Na⁺ ratio in the aquarium trials (under ambient conditions) was in the following order: A. antarctica = A. griffithii > P. australis > P. sinuosa and Ca⁺²/Na⁺ ratio was: A. antarctica = A. griffithii > P. sinuosa > P. australis. This species order indicates a physiological capacity to tolerate variation in salinity. Furthermore, these ratios were higher in the locality with highest salinity in Shark Bay, indicating acclimation and adaptation of ion concentrations to the salinity regime in the environment.