Irradiance Distributions Within the Seagrass Canopy

The two-flow approach described above provides a mechanistic density dependence to the determination of in-canopy light fields [Eqs. (11) and (14)], by linking absorption and reflection to leaf area [l(z)] in each layer, and, therefore, the total leaf area index (L) of the canopy. Self-shading within the canopy, however, is ultimately determined by the projected leaf area [lp(z)], which is a function of leaf orientation as well as shoot density. The vertical distribution of spectral downwelling irradiance [Ed(k, z)] predicted by the model for a moderately dense canopy (hc = 0.367 m, density = 458 shoots m-2, L = 1.85) of turtlegrass growing at 4 m depth in the Bahamas Bank showed very good agreement with measured spectra (Fig. 6A). The irradiance spectrum at 0 m (open circles) represented the boundary condition at the top of the canopy, 3.6 m below the surface of the water. Both predicted and observed Ed(k, z) decreased down through the canopy. Predicted Ed(k, z) was within 1.2% (RMS) of the measured spectrum at the midpoint of the canopy (0.2 m into the canopy), and within 2.5% of the measured spectrum near the bottom (0.3 m into the canopy, Zimmerman, 2003).

400 450 500 550 600 650 700

Wavelength (nm)

400 450 500 550 600 650 700

Wavelength (nm)

400 450 500 550 600 650 700 Wavelength (nm)

Fig. 6. Measured (lines) and modeled (symbols) downwelling irradiance spectra of the submarine light fields within a turtle-grass canopy, and an eelgrass canopy. From Zimmerman (2003). Copyright (2003) by the American Society of Limnology and Oceanography, Inc.

400 450 500 550 600 650 700 Wavelength (nm)

Fig. 6. Measured (lines) and modeled (symbols) downwelling irradiance spectra of the submarine light fields within a turtle-grass canopy, and an eelgrass canopy. From Zimmerman (2003). Copyright (2003) by the American Society of Limnology and Oceanography, Inc.

Simulated downwelling irradiance in a taller, denser and more heavily pigmented eelgrass canopy (hc = 1.0 m, shoot density = 110 shoots m-2, L = 2.72) submerged in the turbid waters of Elkhorn Slough, California, USA were within 15% of measured Ed(X, z) throughout the canopy (Fig. 6B). The RMS difference between modeled and measured spectra was 2.3% at 0.25 m, 1.5% at 0.5 m, 8.7% at 0.75 m and 14% at 0.85 m depth. The model predicted Ed(X, z) to peak more strongly in the green in the two bottom layers (0.75 and 0.85 m) of the canopy than was measured. Flattening of the measured spectra at the lower depths, however, may be attributed the effects of leaf epiphytes that were not accounted for in the model simulation (but see Drake et al., 2003) and to potential distortion of the measured spectra caused by low signal:noise in the red and blue portions at energy fluxes below 0.05 Wm-2 nm-1.

Was this article helpful?

0 0

Post a comment