Canopy-level Photophysiology of the Seagrass Thalassia Testudinum in Florida Bay, Florida

Examination of a dense Thalassia testudinum meadow in Florida Bay showed that light variability can differ nearly an order of magnitude from the top to the bottom of the seagrass canopy and light penetration through the canopy is highly sensitive to the incident angle of the sun. Additionally, photosynthetically available radiation (PAR) may not be an appropriate indicator of light availability to seagrasses because this measure assumes that the leaves absorb all wavelengths equally. Becasue seagrass leaves absorb light more effectively in the red and blue regions of the spectrum and absorb green light only weakly, photosynthetically usable radiation (PUR) is a more accurate measure of light availability because it represents the differential absorption of light across the PAR spectrum. The red:far-red within a seagrass canopy increases, in sharp contrast to a terrestrial canopy. The lack of a red:far-red signal may be a key disadvantage limiting seagrasses ability to regulate canopy density. Examination of the high frequency light fluctuations within the canopy showed that irradiance varied by an order of magnitude within a fraction of a second due to sunflecks allowing saturating light to penetrate even to the bottom of the canopy. The top of a canopy also experienced rapid fluctuations in light due to the focusing of light beams by surface waves that often exceeded double the surface irradiance. Chlorophyll content and leaf thickness were significantly higher toward the base of adult leaves indicating a gradient in light absorbance ability through the canopy. Because of the basal growth structure of T. testudinum leaves, leaf biomass is strongly weighted toward the bottom of the canopy indicating the majority of leaf tissue iv experiences a shaded environment. Photosynthetic performance, as measured by chlorophyll fluorescence, also varied significantly along leaves indicating interleaf acclimation to light availability. The results of this dissertation suggest that the intercanopy variability in photosynthetic attributes plays a significant role in how a seagrass canopy adjusts to its light environment. By not representing this important factor, seagrass productivity models may significantly overestimate the gross photosynthesis in a seagrass meadow.

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