Oceanographic context of the First Aerosol Characterization Experiment (ACE 1): A physical, chemical, and biological overview

Abstract

The First Aerosol Characterization Experiment (ACE 1) intensive experiment, conducted between November 14 and December 15, 1995, covered an oceanographically complex region, including the subtropical (ST), Subtropical Convergence Zone (STCZ), Subantarctic (SA), and polar water masses. Oceanographic and atmospheric sampling to identify the chemical and biological communities that might affect biologically produced, aerosol precursors in these water masses was carried out from the RV Discoverer and FRV Southern Surveyor. Sea surface temperature was not a good indicator of water mass during ACE 1. The physical structure east of 147.5°E was more complex than west of this longitude. Nutrient concentrations (nitrate, silicate) in the mixed layer increased, but ammonia concentrations decreased from ST to polar waters. Ammonia concentrations in surface waters exceeded >5 μM, and ammonia fluxes peaked at 12 μM m−2 d−1 in ST water near Cape Grim between November 27 and 30. Seawater dimethyl sulfide (DMSSW) concentrations averaged 1.7±1.1 nM with maximum values near 6 nM in ST waters. The flux of dimethyl sulfide to the atmosphere averaged 4.7±5.0 μM m−2 d−1, with peak fluxes (>40 μM m−2 d−1) in SA water. DMSSW increased significantly only in SA waters during the experiment. Surface chlorophyll‐a concentrations measured from the FRV Southern Surveyor ranged between 0.3 and 0.6 mg m−3. Surface chlorophyll‐a concentrations along the Discoverer cruise track were similar, but 5 phytoplankton blooms (>1 mg m−3) were found. Integrated column chlorophylls ranged from 29.4 mg m−2 in SA water to 53.2 mg m−2 in ST water. Modeled primary production rates were ranged between 409 mg carbon m−2 d−1 in the polar front to 3180 mg carbon m−2 d−1 in ST water near Cape Grim. Mixed layer phytoplankton growth rates, estimated from net primary production, were 0.31 d−1 in the SAF to 1.07 d−1 in STCZ water east of Tasmania. Grazing dilution experiments showed microzooplankton grazing and phytoplankton growth rates were tightly coupled, with net growth rates ranging from ‐ 0.13 d−1 to 0.22 d−1. During the ACE 1 intensive period, the surface waters were in transition from the deeply mixed, winter conditions to the shallowly stratified, spring conditions. The spring phytoplankton bloom was just beginning in the northern water masses, and isolated blooms were present at several locations in SA surface waters. The low standing stocks of chlorophyll and tightly coupled grazing rates may have been responsible for the low concentrations of DMS and NH3 in the region, and the generally low fluxes of these compounds to the atmosphere during the ACE 1 intensive experiment.