Isotopic characterization of atmospheric nitrogen inputs as sources of enhanced primary production in coastal Atlantic Ocean waters

Abstract

Current estimates indicate that atmospheric nitrogen deposition is responsible for 26 to over 70% of “new” nitrogen (N) input to North Carolina estuaries and coastal waters. Concentrations of N in coastal rainfall events in a 2-yr period (August 1990 to 1992) ranged from 0.7 to 144 μM for NO 3 - and 0.5 to 164 μM for NH 4 + . The δ15N values of the NO 3 - and the NH 4 + were determined in 15 rain events. NH4 + values averaged-3.13‰ (range:-12.5 to+3.6), while NO 3 - plus dissolved organic N fractions had an average δ15N of+1.0‰ (range:-2.0 to+4.7). The uptake of this isotopically light N into particulate N, in parallel with primary productivity and biomass (as chlorophyll a) determinetions, was examined in microcosm and mesocosm bioassays. As phytoplankton productivity and biomass increased with added rainwater N, the δ15N of particulate N decreased. To investigate the effects of significant atmospheric N loading with stable isotope tracers, we measured the δ15N of the>1 μm fraction from surrounding coastal waters. Owing to the episodic nature of atmospheric deposition and the great variation in N loading with each event, a simple assessment of the atmospheric contribution was not possible. During a period in which rainfall inputs were significant and frequent (August 1992), δ15N values were several ‰ more negative than during periods of drought (Fall 1990). These experiments and observations emphasize the contribution of atmospheric nitrogen deposition to “new” production in coastal waters.