Optical Characterization and Spatial Distribution of Dissolved Organic Matter (DOM) in Seven Water Bodies of Mississippi, USA

Abstract:

Lakes and estuaries can serve as indicators of overall health of terrestrial and aquatic ecosystems. The characteristics of dissolved organic matter (DOM) in these water bodies provide insights into the biogeochemical processes undergoing at the source, during transport and in the water bodies. Land use and land cover plays not only a significant role in controlling the quantity of the exported DOM, but also influences the quality of DOM via various biogeochemical and biodegradation processes. We investigated the characteristics and spatial distribution of DOM in five major lakes - Sardis, Enid, Grenada, Okatibbee, and Ross Barnett Reservoir (RBR), an estuary, the Lower Pearl River (LPR) Estuary, and a coastal region, Grand Bay, in the state of Mississippi, USA. Water samples from the lakes and Grand Bay were collected during the summer of 2012-2014 while samples from LPR were collected during winter 2014 and spring 2015. We employed absorption and fluorescence spectroscopy including excitation emission matrix (EEM) combined with parallel factor analysis (PARAFAC) modeling techniques to determine optical properties of DOM and its characteristics in these study sites. A site-specific PARAFAC model was developed to evaluate DOM characteristics, which resulted in five diverse DOM compositions including two terrestrial humic-like, two microbial humic-like, and one protein-like DOM. The lakes and Grand Bay region showed high concentrations of microbial humic-like or protein-like DOM fluorescence signatures while the samples from LPR Estuary and the RBR showed relatively high concentration of terrestrial humic-like DOM. Moreover, we also observed strong correlations between microbial humic-like DOM (PARAFAC derived) and DOM indices such as biological/freshness and fluorescence indices (EEM based). DOM in the lakes indicated autochthonous characteristics predominantly probably because of photochemical degradation while the LPR Estuary and the RBR samples showed mainly allochthonous DOM of terrestrial origin. Preliminary analysis showed that forests, wetlands, agriculture, and urban areas are the major land use and land covers affecting DOM quality. We intend to investigate the influence of other land uses and land covers on DOM characteristics based on our nutrient measurements in all our study sites.