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A Dynamic Model for Wave-Induced Light Fluctuations in a Kelp Forest

Stephen R. Wing, James J. Leichter and Mark W. Denny
Limnology and Oceanography
Vol. 38, No. 2 (Mar., 1993), pp. 396-407
Stable URL: http://www.jstor.org/stable/2837818
Page Count: 12
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A Dynamic Model for Wave-Induced Light Fluctuations in a Kelp Forest
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Abstract

We formulated a dynamic model with linear wave theory to predict the effects of wave height, wave length, and water depth on modulation of the local surface cover in a Macrocystis pyrifera canopy. This model was incorporated into a general model for attenuation of photosynthetically active radiation (PAR) in a M. pyrifera forest with particular emphasis on light reaching understory algae. The model predicts that the period of fluctuationa of PAR on the bottom matches that of the dominant wave period and that the amplitude of irradiance peaks is increased by positive local surface areal stain during the trough of a wave. Field measurements of instantaneous irradicance and surface elevation and simultaneous video recording of the surface canopy allowed investigation of these predictions. Cross-correlation analysis on irradiance and surface elevation measurements showed a significant negative correlation between surface elevation and PAR reaching the understory. This correlation cannot be attributed to the decrease in lightpath length alone, but can explained by the changes in M. pyrifera surface cover. Measured changes in instantaneous fractional canopy cover exceeded theoretically predicted values by a factor of as much as 3. Consequently, the intensities of light flecks exceeded predictions. A combination of mechanisms, as well as assumptions of the model, may explain the deviation between the magnitudes of predicted and measured fluctuations in canopy cover and light. The dominant period of ocean swells is typically in the range of 5-20 s, and light flashes with these periods have been shown to effect significant gains in light utilization efficiency by certain algae.

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