I investigated how photosynthetic performance of Thalassia testudinum changed along a naturally occurring salinity gradient in Florida Bay, and to laboratory controlled hyper and hypo-osmotic stress. I found significant differences between sites in Florida Bay for yield ratios (Y and Fv/Fm); however, this difference does not seem to be based on the salinity regime, since sites with the greatest salinity range were not significantly different from the site with the lowest salinity range. Laboratory results showed declines in the minimum and maximum fluorescence values after a gradual ramping-up of salinity and after long-term exposure to a sustained drop in salinity, but these declines were not seen with the Y and Fv/Fm ratios.
Caution should be used when drawing conclusions about physiological stress from results obtained by PAM fluorometry, as acclimation may play a large role in the fluorescence response, limiting the use of this technique.
