Population-based variation in resilience to hyposalinity stress in Halophila johnsonii
Plants of the threatened seagrass, Halophila johnsonii Eiseman, from a riverine and marine population were exposed to a series of salinity treatments within mesocosms. Survival and maximum photochemical efficiencies of PSII (Fv/Fm) were measured in response to varied frequency, duration, and amplitude of hyposalinity exposures. Both populations exhibited high survival after two cycles of short, pulsed hyposalinity treatments (100% and 89%, respectively). However, two cycles of long pulses of low salinity resulted in 100% mortality for marine H. johnsonii and >50% mortality for riverine plants. After two cycles of gradual salinity reduction to a salinity of 5, survival for marine and riverine plants was also low (22% and 33%, respectively). Fv/Fm values of riverine H. johnsonii were relatively high (0.65–0.70) after a single short pulse or gradual reduction to salinity of 10. Fv/Fm values of marine plants were lower in these two treatments and exhibited greater declines following a single, long pulse to a salinity of 10 or after a single gradual reduction to a salinity of 5. Fv/Fm values showed riverine plants were also more resistant to repeated pulses of moderate low salinity than marine plants. Our results indicate wide tolerances of H. johnsonii to short pulses of hyposalinity, but suggest that repeated or prolonged hyposalinity stress at near-tolerance salinity levels can have additive effects on photosynthetic health and survival. Differences in resilience of marine and riverine H. johnsonii populations are consistent with previous suggestions that estuarine ecophenes are more tolerant to hyposalinity than marine populations.
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Document Type: Research Article
Publication date: 01 July 2014
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