Direct atmospheric deposition of water-soluble nitrogen to the Gulf of Maine

Measurements were made at New Castle, New Hampshire, on the shore of the Gulf of Maine from 1994 to 1997 to assess direct atmospheric deposition of water-soluble nitrogen to the surface waters of the gulf. Daily dry deposition was highly variable and ranged from similar to 1 to 144 mu mol N m(-2) d(-1) (median 16 mu mol N m(-2) d(-1)). Wet deposition dominated dry deposition, contributing 80-90% of the total flux annually. Wet deposition was also highly variable and ranged from 3 to 4264 mu mol N m(-2) d(-1) (median 214 mu mol N m(-2) d(-1)). Fog water nitrogen deposition could contribute as much as large precipitation nitrogen deposition events, in excess of 500 mu mol N m(-2) d(-1). Dissolved organic nitrogen (DON) in precipitation constituted only a small fraction (3%) of the total precipitation nitrogen flux most of the year, except in spring where it comprised 14%, on average, of the total. The total atmospheric direct nitrogen (ADN) deposition numbers reported here do not include the contributions of fog and DON as they were not sampled regularly over the course of this study. The total ADN flux ranged from 1 to 4262 mu mol N m(-2) d(-1) (median 23 mu mol N m(-2) d(-2)), depositing 52 mmol N m(-2) yr(-1) to the surface waters of the Gulf of Maine, 3% of the total N input to those waters annually. However, this deposition was highly episodic with events over 500 mu mol N m(-2) d(-1) occurring in 8% of the days sampled but contributing 56% of the total measured flux and events in excess of 1000 mu mol N m(-2) d(-1) occurring in 2% of the samples and contributing 22% of the total measured flux. It is these large events that may influence biological productivity of the Gulf of Maine. The annual wet deposition of inorganic N measured at New Castle exceeded that reported by two National Atmospheric Deposition Program (NADP) sites by 42% on average of that reported from Cape God, Massachusetts, and by 69% of that at Mt. Dessert Island, Maine. Estimates of the episodic atmospheric nitrogen flux to the surface waters of the Gulf of Maine suggest large deposition events could be sufficient to support substantial chlorophyll a production, especially under calm conditions.