Spatial and temporal variability of light attenuation in large rivers of the Amazon

The light field and its relationship with biogeochemical variables were investigated in the Solimes, Negro, Amazon, Madeira, Uatum, Trombetas, and Tapajs Rivers. In high suspended sediment rivers, total suspended matter is the primary control on light attenuation (r = 0.8), with colored dissolved organic matter (CDOM) being secondary (r = -0.6) due to scattering and absorption, respectively. Photosynthetically active radiation was the lowest (< 100.0 mu mol m(-2) s(-1) at the depth of half Z (1%)) and was limited to depths of less than 1.0 m and confined to red light. In low suspended sediment rivers, CDOM is the primary control on light attenuation (r = 0.9). The concentrations of chlorophyll a (Chla) and CDOM cause variations among these rivers. High CDOM rivers, Negro and Uatum, are depleted (< 0.5% of incoming irradiance) of blue and green light at the depth of half Z (1%). The light spectra of low CDOM and higher Chla waters, such as the Tapajs, Uatum, and Trombetas Rivers at rising water stage, are restricted to green and red wavelengths, and marked by high absorption at 620 and 670 nm, due to the presence of Cyanophyceae.