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


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.

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We thank the editors and two anonymous reviewers for their insightful comments. This research was funded in part by National Science Foundation grants EPS 1101245 and OCE 1637630 (Plum Island Ecosystem LTER), NH Sea Grant (NOAA NA10OAR4170082), NH Agricultural Experiment Station, and UNH ESRC’s Iola Hubbard Endowment for Climate Change. This is AES Scientific Contribution Number 2675 supported by the USDA NIFA Hatch Project 0225006. Raw data underlying this work is provided with the supporting information. Code related to the use of frequency-dependent coherence is available upon request.

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Correspondence to Gopal K. Mulukutla.

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Communicated by Dennis Swaney

Key Points

• Simultaneous water quality measurements in watershed and N-impaired estuary show strong watershed control for estuarine DOC but complex coupling for nitrate.

• DOC exhibited near-conservative behavior in the estuary.

• For nitrate, spatial distribution of watershed sources and interactions with internal estuarine processes produce complex response.

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Mulukutla, G.K., Wollheim, W.M., Salisbury, J.E. et al. High-Frequency Concurrent Measurements in Watershed and Impaired Estuary Reveal Coupled DOC and Decoupled Nitrate Dynamics. Estuaries and Coasts 45, 445–461 (2022).

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  • Water quality
  • Watershed-estuary biogeochemical coupling
  • Eutrophication