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Marine Ecology Progress Series

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MEPS 539:127-137 (2015)  -  DOI:

Photochemical efficiency of Thalassia testudinum varies in response to repeated shading events and unpredictable weather

Melissa A. Genazzio, Michael J. Durako*

Department of Biology and Marine Biology, Center for Marine Science, University of North Carolina Wilmington, 5600 Marvin Moss Ln., Wilmington, NC 28409, USA
*Corresponding author:

ABSTRACT: A diurnal regression approach was recently proposed as an alternative to point measurements of effective photochemical efficiency of photosystem II (ΔF/Fm') 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 (Fv/Fm), estimates of Fv/Fm derived from ΔF/Fm' vs. photosynthetically active radiation (PAR) diurnal regression intercepts, and diurnal regression slopes. Estimated Fv/Fm values were more variable but generally only significantly different from measured Fv/Fm 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/Fm' 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

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Cite this article as: Genazzio MA, Durako MJ (2015) Photochemical efficiency of Thalassia testudinum varies in response to repeated shading events and unpredictable weather. Mar Ecol Prog Ser 539:127-137.

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