Relative independence of dissolved organic carbon transport and processing in a large temperate river: The Hudson River as both pipe and reactor

Bacterial respiration (BR) of organic matter is an important flux in the carbon budgets of large rivers, yet the regulation of BR and the relationship of this respiration to various organic matter sources is poorly understood. Using detailed spatial transects, we evaluated transport and consumption of dissolved organic matter in the Hudson River estuary, and compared both with BR. Dissolved organic carbon (DOC) concentration, long-term DOC lability, and in situ BR were measured at 24 stations on each of five transects. DOC lability, measured in long-term bioassays, averaged 15 mg L21 d21 and was similar to the average net rate of decline of DOC from the upper to lower estuary. BR averaged 156 mg L21 d21 C, far exceeding the net downriver DOC decline and measured DOC lability. BR was well predicted by a model that included seston, chlorophyll, and DOC consumption. Rate coefficients derived from this model indicate that BR is primarily supported by carbon derived from seston and chlorophyll. Changes in DOC concentration along the Hudson flow path were well predicted from a combination of freshwater input, DOC concentration in the headwaters, and long-term DOC lability. Although most of the total respiration is due to free-living bacteria and thus mediated by DOC, ,20% of this respiration is actually supported by DOC loaded in the headwaters, and transported downstream. The Hudson River, therefore, acts as a pipe transporting dissolved terrestrial organic matter seaward while also functioning as a reactor where intense bacterial activity degrades organic matter associated primarily with particles or generated locally. Organic matter loading, transformation, and transport by large rivers and estuaries are important processes for determining regional carbon budgets, the fate of organic matter of terrestrial origin, resources to food webs, and inputs to receiving coastal ecosystems (Schlesinger and Melack 1981). The topic has been studied with multiple approaches including carbon mass balances, isotopic tracers, compound-specific methods, and microbial rate measurements (Cifuentes and Eldridge 1998; Raymond and Bauer 2001a; McCallister et al. 2006b). Collectively, these studies have determined the magnitude of carbon transport and processing in large rivers and estuaries and identified changes associated with bacterial processing. Uncertainty remains, however, about the sources and nature of carbon that is being processed, transported, and exported. This information is critical to establishing the temporal and spatial linkages between various sources within aquatic ecosystems, and between aquatic and surrounding terres-