MEPS 539:127-137 (2015)  -  DOI: https://doi.org/10.3354/meps11498

ABSTRACT: A diurnal regression approach was recently proposed as an alternative to point measurements of effective photochemical efficiency of photosystem II (ΔF/F_m') for assessing seagrass physiological condition at the landscape scale. We conducted a mesocosm study, which assessed the responsiveness of the diurnal regression approach and timescales of photoacclimation of Thalassia testudinum to repeated week-long cycles of reduced irradiance (Shade treatment), followed by full sunlight. Shade-treatment shoots exhibited photoacclimation responses 1 to 3 d after shades were added or removed in maximum photochemical efficiencies (F_v/F_m), estimates of F_v/F_m derived from ΔF/F_m' vs. photosynthetically active radiation (PAR) diurnal regression intercepts, and diurnal regression slopes. Estimated F_v/F_m values were more variable but generally only significantly different from measured F_v/F_m during stressful light or hyposalinity conditions, indicating a higher sensitivity of photosystem II to environmental stress under light pressure. Compared to unshaded controls (Sun treatment), the Shade-treatment plants exhibited 26% shorter leaves, a 10% reduction in leaf width, a 43% decrease in leaf area and a 50% decrease in dry weight after 47 d. Shade treatments also showed an increase in leaf chlorophyll a and b content with a decrease in chlorophyll a:b ratio. These patterns suggest that Shade-treatment plants were light-limited. Significant day-to-day variation in measured and regression-derived estimates of photochemical efficiency in response to overcast and hyposaline conditions reflected rapid changes in chlorophyll fluorescence in response to short-term weather changes. This day-to-day variation frequently equaled or exceeded treatment responses, suggesting that the use of diurnal regressions of ΔF/F_m' vs. PAR may not be an effective method for assessing seagrass physiological health at landscape scales.

KEY WORDS: Thalassia testudinum · Photochemical efficiency · Light reduction · Photoacclimation · Seagrass monitoring