High-Frequency Concurrent Measurements in Watershed and Impaired Estuary Reveal Coupled DOC and Decoupled Nitrate Dynamics

Abstract

Rapid changes in land use, pollution inputs, and climate are altering the quantity, timing, and form of materials delivered from watersheds to estuaries. To better characterize these alterations simultaneous measurements of biogeochemical conditions in watersheds and estuaries over a range of times scales are needed. We examined the strength of watershed-estuarine biogeochemical coupling using in situ measurements of nitrate, terrestrial dissolved organic carbon (DOC), and chloride collected over a 7-month period in a nitrogen-impaired estuary in northeastern US. The watershed exerted strong control over concentrations of terrestrially derived DOC in the estuary, attributable to relative homogeneity of watershed sources from forested land combined with relatively conservative behavior in estuarine waters. Estuarine nitrate patterns were more complex, suggesting the influence of heterogeneous watershed distribution of non-point and point sources and high reactivity of nitrate in the estuary. Understanding estuarine biogeochemical patterns will be advanced through greater use of simultaneous sub-hourly measurements of inflows, salinity, and water quality in estuaries and their upstream watersheds.