Estimating direct and episodic atmospheric nitrogen deposition to a coastal waterbody

Direct deposition of atmospheric nitrogen to shallow coastal embayments is usually estimated, since insufficient field measurements are available. Using Waquoit Bay (Cape Cod, MA. USA) as a case study, and a recent review of literature, we determined reasonable bounds on wet and dry inputs of inorganic and organic N. Since precipitation and wind vary daily, we explored the potential of episodic events to stimulate phytoplankton blooms. Many coastal waterbodies like Waquoit Bay are small relative to their watersheds. Nevertheless, direct deposition of NH3, NO3−, and HNO3 is significant in the loading budget. For Waquoit Bay, direct deposition was calculated to be 7-15.5 kg total N ha−1 yr−1, representing 70-150% of the atmospheric N reported to reach the bay via the watershed, and 20-45% of the total N reaching the bay from all land-based sources. Episodic events were estimated to deliver up to 65 mg N m−2 day−1, representing a phytoplankton stock of 12.3 mg Chl m−2, an amount unlikely to stimulate dense blooms in shallow coastal waters in the northeastern United States.