Processing watershed-derived nitrogen in a well-flushed New England estuary
Tobias, Craig R., Matthew Cieri, Bruce J. Peterson, Linda A. Deegan, Joseph Vallino, and Jeffrey Hughes
Limnol. Oceanogr., 48(5), 2003, 1766–1778

Isotopically labeled nitrate (¹⁵NO₃^-) was added continuously to the Rowley estuary, Massachusetts, for 22 d to assess the transport, uptake, and cycling of terrestrially derived nitrogen during a period of high river discharge and low phytoplankton activity. Isotopic enrichment of the 3.5-km tidal prism (150,000 m³) was achieved for the 3 weeks and allowed us to construct a nitrogen mass balance model for the upper estuary. Mean ¹⁵NO₃^- in the estuary ranged from 300‰ to 600‰, and approximately 75%–80% of the ¹⁵N was exported conservatively as ¹⁵NO₃^- to the coastal ocean. Essentially all of the 20%–25% of the ¹⁵N processed in the estuary occurred in the benthos and was evenly split between direct denitrification and autotrophic assimilation. The lack of water-column ¹⁵N uptake was attributed to low phytoplankton stocks and short water residence times (1.2–1.4 d). Uptake of watercolumn NO₃^- by benthic autotrophs (enriched in excess of 100‰) was a function of NO₃^- concentration and satisfied up to 15% and 25% of the total nitrogen demand for benthic microalgae and macroalgae, respectively. Approximately 10% of tracer assimilated by benthic autotrophs was mineralized and released back to the water column as ¹⁵NH₄⁺. By the end of the study, ¹⁵N storage in sediments and marsh macrophytes accounted for 50%–70% of the ¹⁵N assimilated in the estuary. These compartments may sequester watershed-derived nitrogen in the estuary for time scales of months to years.