Isotopic tracers of nitrogen from atmospheric deposition to coastal waters

Abstract

Nitrogen deposition to global oceans from the atmosphere can be about two times that of natural riverine input [see Duce et al. (1991) for recent comprehensive review]. Even accounting for the increased loading of riverine nitrogen by anthropogenic inputs, total atmospheric deposition (AD) is essentially equal to that of riverine sources. Among growth-limiting nutrients, nitrogen plays a key role in regulating primary and secondary productivity both on regional and global scales (e.g., Ryther and Dunstan, 1971; Nixon et al., 1986). As such, proper identification and characterization of nitrogen sources and their impacts on nutrient flux and trophic utilization is of prime concern in understanding biogeochemical responses in coastal ecosystems (Eppley and Peterson, 1979; Paerl, 1985; Legendre and Gosselin, 1989). AD represents a quantitatively-important, but poorly assessed, nitrogen source in these ecosystems (Paerl, 1985; Fisher et al., 1988; Prado-Fiedler, 1990; Loye-Pilot et al., 1990). Current estimates indicate that from 20% to as much as 50% of annual nitrogen loading is attributable to AD alone in geographically-diverse coastal regions, including those along the U.S. Eastern Seaboard (Fisher et al., 1988 ). Even among large bodies of water such as the Baltic and Mediterranean Seas, AD constitutes a highly-significant N source (PradoFiedler, 1990; Loye-Pilot et al., 1990). In North Carolina's adjacent coastal Atlantic waters, AD is a major source (35-80%) of new nitrogen. Assumptions for this type of calculation include the following: ( 1 ) advective input of N from Gulf Stream is very low; ( 2 ) no upwelling is occurring; and (3) N2 fixation accounts for no more than 10% of new N inputs. Here, AD has been shown to significantly stimulate primary production by as much as 50-200% at natural input levels (Paerl, 1985; Paerl et al., 1990). The need to have a more detailed understanding of the impacts of AD of nitrogen in coastal waters is pressing, because previouslypristine segments of the coastal oceans are now exhibiting both incipient and advanced stages of eutrophication. Nitrogen from AD may be a unique nutrient source in the coastal zone for two reasons. Firstly, direct surface water deposition, as wet and dryfall, frequently occurs downstream of oligoand mesohaline estuarine nutrient stripping regions that assimilate a majority ofterrigenous N inputs (Boynton et al., 1982; Sharp et al., 1984). In shallow coastal areas, such as those present along the mid-Atlantic states, dilution of AD is minimized, which amplifies its importance as a new nitrogen source. Secondly, AD is highly dynamic in terms of the magnitudes of dissolved inorganic nitrogen (DIN) and dissolved organic nitro-