Direct wet and dry deposition of ammonia, nitric acid, ammonium and nitrate to the Tampa Bay Estuary, FL, USA

The average total (wet plus dry) nitrogen deposition to the Tampa Bay Estuary was 7.3 (±1.3) kg-N ha−1 yr−1 or 760 (±140) metric tons-N yr−1 for August 1996–July 1999, estimated as a direct deposition rate to the 104,000-ha water surface. This nitrogen flux estimate accounted for ammonia exchange at the air–sea interface. The uncertainty estimate was based on measurement error. Wet deposition was 56% of the total nitrogen deposition over this period, with an average 0.78 ratio of dry-to-wet deposition. Wet nitrogen deposition rates varied considerably, from near zero to 1.3 kg-N ha−1 month−1. About 40% of the total nitrogen flux occurred during the summer months of June, July and August when rainfall was the highest, except for 1997–1998 when the El Niño phenomenon brought unseasonal rainfall. Ammonia/ammonium contributed to 58%, and nitric acid/nitrate 42%, of the total nitrogen deposition over the 3-yr period. In one summer as waters of Tampa Bay warmed above 28°C and ammonium concentrations reached 0.03 mg l−1, the estimated net flux of ammonia was from the Bay waters to the atmosphere.