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

Isotopically labeled nitrate ((NO3-)-N-15) 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 delta(15)NO(3)(-) in the estuary ranged from 300parts per thousand to 600parts per thousand, and approximately 75%-80% of the N-15 was exported conservatively as (NO3-)-N-15 to the coastal ocean. Essentially all of the 20%-25% of the N-15 processed in the estuary occurred in the benthos and was evenly split between direct denitrification and autotrophic assimilation. The lack of water-column N-15 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 100parts per thousand) 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 (NH4+)-N-15. By the end of the study, N-15 storage in sediments and marsh macrophytes accounted for 50%-70% of the N-15 assimilated in the estuary. These compartments may sequester watershed-derived nitrogen in the estuary for time scales of months to years.