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Abstract
UNDERWATER SPECTRAL ENERGY DISTRIBUTION AND SEAGRASS
DEPTH LIMITS ALONG AN OPTICAL WATER QUALITY GRADIENT
Charles L. Gallegos1, W. J. Kenworthy2,
P.D. Biber3 and B.S. Wolfe4
1Smithsonian Environmental Research Center
2National Oceanic and Atmospheric Administration
3University of Southern Mississippi
4University of Virginia
Bio
We measured in situ inherent optical properties and seagrass distributions
in widely differing optical water types, including turbid green
waters of the Indian River Lagoon (IRL, Florida, USA), a mix of
turbid and clear waters in Panama, and very clear waters in Belize.
We used Hydrolight to model in situ spectral energy distributions,
and measured leaf absorbance spectra (Thalassia testudinum)
to distinguish between photosynthetically available (PAR) and photosynthetically
usable (PUR) radiation. Attenuation coefficients for PAR and PUR
were nearly indistinguishable in Belize and Panama, and differed
only slightly in the IRL. Grass grew to depths of penetration of
31% of PAR in the IRL, 14% in Panama, and ca. 10% in Belize. Corresponding
percentages for PUR were 28%, 12%, and 10% for IRL, Panama, and
Belize, respectively. These regional differences in light requirements
were striking, and less than half of the difference could be attributed
to latitudinal variations in incident light. We conclude that factors,
other than spectral energy distribution, that covary with water
clarity control site-specific light requirements of seagrasses.
Possibilities include epiphytes, and sediment quality.
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