Importance of atmospherically deposited nitrogen to the annual nitrogen budget of the Neuse River estuary, North Carolina

Abstract

Wet deposition of nitrogen, as NH 4 +, NO 3 −, and organic N, contributes up to 50% of the total externally supplied or ‘new’ N flux to the Neuse River Estuary (North Carolina). Excessive nitrogen (N) loading to N-sensitive waters such as the Neuse River Estuary has been linked to changes in microbial and algal community composition and function (harmful algal blooms), hypoxia/anoxia, and fish kills. In a 4-year study from July 1996 to July 2000, the weekly wet deposition of NH 4 +, NO 3 −, and dissolved organic N was calculated, based on concentration and precipitation measurements, at 11 sites on a northwest–southeast transect in the watershed. Data from this period indicate that the annual mean total wet atmospherically deposited (AD)-N flux was 11 kg ha −1 year −1. Deposition was fairly evenly distributed between nitrate, ammonium, and organics (32%, 32%, and 36%, respectively). Seasonally, the summer (June–August) months contained the highest weekly wet total N deposition; this trend was not driven by precipitation amount. Estimates of watershed N retention and in-stream riverine processing revealed that the AD-N flux contributed an estimated 20% (range of 15–51%) of the total ‘new’ N flux to the estuary, with direct deposition of N to the estuary surface accounting for 6% of the total ‘new’ N flux. This study did not measure the dry depositional flux, which may double the contribution of AD-N to the estuary. The AD-N is an important source of ‘new’ N to the Neuse River Estuary as well as other estuarine and coastal ecosystems downwind of major emission sources. As such, AD-N should be included in effective nutrient mitigation and management efforts for these N-sensitive waters.