Short-term impacts of salinity pulses on ionic ratios of the seagrasses Thalassia testudinum and Halodule wrightii

•Total ions, Cl− and Na+ increased with higher salinity in leaves and rhizomes of both seagrass species.•K+ and Ca2+ generally decreased with higher salinity.•Cl− and Na+ were higher in rhizomes than in leaves, possibly reflecting effective ion exclusion mechanisms in leaves.•The ratios K+/Na+ and Ca2+/Na+ were higher in Halodule wrightii than in Thalassia testudinum leaves.

We examined the effects of short-term salinity pulses on ion accumulation in the seagrasses Thalassia testudinum and Halodule wrightii. Plant fragments were exposed for approximately 1 week to 10, 23 (ambient salinity), 30, 40, 50 and 70 psu. The concentrations of total ions, Cl− and Na+ increased with higher salinity in leaves and rhizomes of both seagrass species. In contrast, the concentrations of K+ and Ca2+ generally decreased with higher salinity, although the decrease was relatively small and only observed at extreme salinities. Our results indicate the concentrations of Cl− and Na+ were higher in rhizomes than in leaves, possibly reflecting effective ion exclusion mechanisms in leaves. Under ambient (control) salinity the ratios K+/Na+ and Ca2+/Na+ were 38% and 46% higher in H. wrightii than in T. testudinum leaves, which support the notion that H. wrightii is more tolerant of salinity increases than T. testudinum. In concert, our results show novel observations of ion osmolyte concentrations in these seagrass species that point to adaptive responses to salinity pulses. Despite these adaptive responses, pulses of extremely high salinity (>50 psu) lasting approximately 1 week are detrimental to these seagrass species.