Photophysiological responses of Halophila johnsonii to experimental hyposaline and hyper-CDOM conditions

The endemic seagrass Halophila johnsonii grows intertidally to 3 m deep, in both marine and riverine influenced habitats of eastern Florida. Salinity and chromophoric dissolved organic matter (CDOM) levels widely fluctuate across this broad habitat range, changing tidally and with variable influx of freshwater from watershed runoff, river discharge and stochastic storm events. CDOM exponentially absorbs light in the UV to blue wavelengths, affecting optical water quality. H. johnsonii produces 15 flavonoid compounds that maximally absorb in the UV range. These flavonoids are thought to function as UV-protectants (UVP) in high-light and UV-intense environments. This mesocosm study examined the photosynthetic capacity, quantum efficiency and pigment content of H. johnsonii under experimental treatments of three salinities (10, 20 and 30) with and without CDOM. Main treatment effects and possible interactive effects at both short- (1 day to 1 week) and longer-term (1 month) time scales were examined. There were no significant CDOM or CDOM x salinity effects over any of the experimental treatment durations. There was 100% mortality of plants at salinity 10 after 10 days regardless of water color. UVP content of leaves was not affected by CDOM in this study, but there was significant variation in UVP in response to salinity. Our results do not support the primary role of UVP in this species as a sunscreen, but indicate that different salinity environments contribute to changes in the levels of these flavonoids. The UVP response to salinity stress response was not mitigated by a decrease in UV-radiation (increased CDOM) as H. johnsonii continued to put energy into the production of the carbon-rich flavonoids regardless of potential UV-stress. The experimental results indicate that prolonged hypo-salinity conditions are an important environmental factor to manage in the limited geographic range of H. johnsonii.