EPISODIC INPUTS OF ATMOSPHERIC NITROGEN TO THE SARGASSO SEA - CONTRIBUTIONS TO NEW PRODUCTION AND PHYTOPLANKTON BLOOMS

Atmospheric wet deposition rates of nitrate and ammonia on Bermuda col lected in the Atmosphere Ocean Chemistry Experiment (AEROCE) are compa red with the synoptic measurements of carbon and nitrogen cycling from the U.S. Joint Global Ocean Flux Study (JGOFS) Bermuda Atlantic Time Series Study (BATS) station, 75 km southeast of Bermuda. Measurable de position events were found on 23.8% of the days between October 1, 198 8 and June 30, 1991. However, only a few of these events significantly contributed to the standing stocks of nitrogen and phytoplankton or r ates of primary production. For 1.7% of the days each year, the atmosp heric nitrogen deposition would have equaled the sinking particulate n itrogen flux as estimated by sediment traps. For only 0.2% of the time , would adequate nitrogen be deposited to a 20 m mixed layer to change the surface concentrations of particulate organic nitrogen by 10%. Th e results are dramatically different if all of die deposition remains confined to the upper 1 m of die water column enabling intense, surfac e phytoplankton blooms to occur. The occurrence of these near-surface blooms may be an important signal in the interpretation of satellite o cean color imagery, particularly when the satellite data are used to i nfer whole water-column phytoplankton stocks or productivity. Numerica l simulations of the evolution of the near-surface mixed layer after a rainfall event indicate that low salinity surface waters would be mix ed to the upper 10 m or so within 2-4 hours, except for wind speeds le ss than approximately 5 m s-1. Thus, wet deposition induced surface al gae blooms should only be observed under relatively calm meteorologica l conditions. In summary, wet deposition of nitrogen is a relatively u nimportant process in affecting upper ocean nitrogen cycling for the S argasso Sea off Bermuda, although in oceans with lower productivity an d areas where deposition may increase with future industrial developme nt, episodic deposition events may eventually have some short-term imp acts on the local nitrogen cycle. To assess the total impact of atmosp heric deposition of nitrogen will require additional information on dr y deposition and the organic nitrogen content of rainwater.