Sources and transformations of chromophoric dissolved organic matter in the Neponset River Watershed

Abstract

The major source of chromophoric dissolved organic matter (CDOM) in coastal waters is often terrestrial dissolved organic matter originating in coastal watersheds and delivered to the ocean by rivers or streams. While studies of carbon cycling in large rivers dominate the literature, small rivers deliver the bulk of the freshwater to coastal waters (∼60%). However, the variability of organic matter fluxes in small rivers requires high spatial and temporal resolution measurements to adequately represent the processes controlling distributions and fluxes. In this study, CDOM was measured monthly for 2 years at 15 sampling sites throughout a small, coastal, urban watershed (∼300 km2), the Neponset River Watershed, and CDOM variability was found to depend on land use type, discharge/precipitation, and growing degree days (temperature). Forest, wetland, residential, industrial, and golf course CDOM end-members were measured and differentiated using CDOM fluorescence, CDOM absorbance, absorbance slopes, total organic carbon and total nitrogen, and photo- and biological lability. Simple statistical models of CDOM end-member behavior could explain 40-70% of the observed seasonal variability. Using these simple relationships, trends in total CDOM loading behavior can be predicted under various scenarios of land use change and global warming. Overall, detailed measurements of CDOM throughout the Neponset River Watershed yield new insights into the watershed processes that affect terrestrial CDOM delivery to coastal oceans. Copyright 2009 by the American Geophysical Union.