Processing watershed‐derived nitrogen in a well‐flushed New England estuary

Abstract

Isotopically labeled nitrate ( 15 NO 3 − ) 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 m 3 ) was achieved for the 3 weeks and allowed us to construct a nitrogen mass balance model for the upper estuary. Mean δ 15 NO 3 − in the estuary ranged from 300‰ to 600‰, and approximately 75%–80% of the 15 N was exported conservatively as 15 NO 3 − to the coastal ocean. Essentially all of the 20%–25% of the 15 N processed in the estuary occurred in the benthos and was evenly split between direct denitrification and autotrophic assimilation. The lack of water‐column 15 N uptake was attributed to low phytoplankton stocks and short water residence times (1.2–1.4 d). Uptake of watercolumn NO 3 − by benthic autotrophs (enriched in excess of 100‰) was a function of NO 3 − 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 15 NH 4 + . By the end of the study, 15 N storage in sediments and marsh macrophytes accounted for 50%–70% of the 15 N assimilated in the estuary. These compartments may sequester watershed‐derived nitrogen in the estuary for time scales of months to years.

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