ISOTOPIC CHARACTERIZATION OF ATMOSPHERIC NITROGEN INPUTS AS SOURCES OF ENHANCED PRIMARY PRODUCTION IN COASTAL ATLANTIC-OCEAN WATERS

Current estimates indicate that atmospheric nitrogen deposition is res ponsible for 26 to over 70% of ''new'' nitrogen (N) input to North Car olina estuaries and coastal waters. Concentrations of N in coastal rai nfall events in a 2-yr period (August 1990 to 1992) ranged from 0.7 to 144 mu M for NO3- and 0.5 to 164 mu M for NH4+. The delta(15)N values of the NO3- and the NH4+ were determined in 15 rain events. NH4+ valu es averaged -3.13 parts per thousand (range: -12.5 to +3.6), while NO3 - plus dissolved organic N fractions had an average delta(15)N of + 1. 0 parts per thousand (range: -2.0 to +4.7). The uptake of this isotopi cally light N into particulate N, in parallel with primary productivit y and biomass (as chlorophyll alpha) determinations, was examined in m icrocosm and mesocosm bioassays. As phytoplankton productivity and bio mass increased with added rainwater N, the delta(15)N of particulate N decreased. To investigate the effects of significant atmospheric N lo ading with stable isotope tracers, we measured the delta(15)N of the > 1 mu m fraction from surrounding coastal waters. Owing to the episodi c nature of atmospheric deposition and the great variation in N loadin g with each event, a simple assessment of the atmospheric contribution was not possible. During a period in which rainfall inputs were signi ficant and frequent (August 1992), delta(15)N values were several part s per thousand more negative than during periods of drought (Fall 1990 ). These experiments and observations emphasize the contribution of at mospheric nitrogen deposition to ''new'' production in coastal waters.