Effects of climate and land-surface processes on terrestrial dissolved organic carbon export to major U.S. coastal rivers

This study aims to understand the influences of climate change and land surface processes on the variation of in-stream DOC concentrations in coastal rivers crossing different climate zones. Monthly observations spanning multiple years in seven major rivers in four different climate zones within the U.S. were analyzed for correlations between dissolved organic carbon (DOC) concentration and surface air temperature, precipitation, land cover and discharge. The major watersheds were the Altamaha River (GA), the Apilachicola River (FL), the Columbia River (OR), the Delaware River (NJ), the Sacramento and San Joaquin Rivers (CA) and the Susquehanna River (MD). One minor watershed, the Neponset River (MA) was also analyzed. Results indicate that temperature is the most important variable for DOC export when the variation of annual mean temperatures is large (e.g., >5 ◦C) with sufficient precipitation levels. Land-surface characteristics and discharges are better correlated to DOC concentrations when the variations of annual mean temperatures are small (e.g. <2 ◦C). However, results from the small watershed (Neponset) showed that land surface processes can vary annual DOC concentrations about ±1.65 mg/L from mean value. This study is the first to examine DOC relationships in watersheds in multiple climate zones, and it was determined that weak correlations between temperature and DOC found in previous studies may be attributable to the fact that those studies examined small watersheds contained within a single climate zone. DOC flux per square meter was calculated based on incremental temperature increases. The results indicate that an increase of 1 ◦C would result in a 0.476 mg/L increase of in-stream DOC in large watersheds. Climate warming would have a greater impact on riverine DOC yields in cooler climate zones (up to 26% per ◦C) than on those in warmer climate zones (up to 6% per ◦C).