Landscape Control on the Spatial and Temporal Variability of Chromophoric Dissolved Organic Matter and Dissolved Organic Carbon in Large African Rivers

Abstract

The characteristics of colored dissolved organic mat- ter (CDOM) as well as the concentrations and stable isotope composition (d 13 C) of dissolved organic car- bon (DOC) were characterized in several large rivers of Africa including the Congo, Niger, Zambezi, and Ogoouebasins. We compared the spatial and tem- poral patterns of dissolved organic matter (DOM) quantity and quality along with various environ- mental gradients, including hydrology, river size, catchment vegetation, and connectivity to land. The optical proxies used include the absorption coeffi- cient at 350 nm, the specific ultra-violet absorbance, and the spectral slope ratio (SR = 275-295-nm slope divided by 350-400-nm slope). Our results show that land cover plays a primary role in controlling both DOC concentration and optical properties of DOM in tropical freshwaters. A higher cover of dense forest in the catchment leads to a higher quantity of highly aromatic DOM in the river network, whereas an increasing savannah cover results in lower DOC concentrations and less absorptive DOM. In addition to land cover, the watershed morphology (expressed by the average slope) exerts a strong control on DOC and CDOM in tropical rivers. Our results also show that the percentage of C3 and C4 vegetation cover is not an accurate predictor for DOM and CDOM quality in rivers due to the importance of the spatial distribution of land cover within the drainage net- work. The comparison of our results with previously published CDOM data in temperate and high-lati- tude rivers highlights that DOM in tropical freshwa- ters is generally more aromatic, and shows a higher capacity for absorbing sunlight irradiance.