Episodic inputs of atmospheric nitrogen to the Sargasso Sea: Contributions to new production and phytoplankton blooms

Authors

  • Anthony F. Michaels,

  • David A. Siegel,

  • Rodney J. Johnson,

  • Anthony H. Knap,

  • James N. Galloway


Abstract

Atmospheric wet deposition rates of nitrate and ammonia on Bermuda collected in the Atmosphere Ocean Chemistry Experiment (AEROCE) are compared 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 deposition events were found on 23.8% of the days between October 1, 1988 and June 30, 1991. However, only a few of these events significantly contributed to the standing stocks of nitrogen and phytoplankton or rates of primary production. For 1.7% of the days each year, the atmospheric nitrogen deposition would have equaled the sinking particulate nitrogen 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%. The results are dramatically different if all of the deposition remains confined to the upper l m of the water column enabling intense, surface phytoplankton blooms to occur. The occurrence of these near-surface blooms may be an important signal in the interpretation of satellite ocean color imagery, particularly when the satellite data are used to infer whole water-column phytoplankton stocks or productivity. Numerical simulations of the evolution of the near-surface mixed layer after a rainfall event indicate that low salinity surface waters would be mixed to the upper 10 m or so within 2-4 hours, except for wind speeds less than approximately 5m s−1. Thus, wet deposition induced surface algae blooms should only be observed under relatively calm meteorological conditions. In summary, wet deposition of nitrogen is a relatively unimportant process in affecting upper ocean nitrogen cycling for the Sargasso Sea off Bermuda, although in oceans with lower productivity and areas where deposition may increase with future industrial development, episodic deposition events may eventually have some short-term impacts on the local nitrogen cycle. To assess the total impact of atmospheric deposition of nitrogen will require additional information on dry deposition and the organic nitrogen content of rainwater.

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