Spatial and Temporal Variability of Light Attenuation in the Amazonian Waters

Quantification of hyperspectral downwelling vertical attenuation coefficient (Kd) and irradiance (Ed) in the euphotic depth (Zeu) in 2005 (falling) and 2006 (rising waters) in Amazonian waters showed that Kd is higher in the blue (5-9 m), decreasing towards the green (4-7 m) and red (3-5 m) part of the spectra. Further, Kds are generally higher in white waters than in black and clear waters. A stepwise multiple regression analysis showed that when all the data is polled together, TSS and acdom are the main drivers of spectral Kds (R 2 ~ 0.65; p<0.005). However, when analyzing only black and clear waters Chla also explains the variability of Kds (R 2 ~ 0.96; p<0.005). The selective attenuation by the optical constituents result in maximum light reaching Zeu of 1.0 to 0.5 m (white waters) is dominantly at 685nm, Zeu of 1.8 to 3.0 m (black) is at 695nm, and Zeu of 4.0 to 3.0 m (clear) is at 590nm.