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We investigated how regional differences in environmental parameters influenced enrichment of amino acids in the sea surface microlayer relative to underlying bulk seawater. Concentrations and compositions of dissolved free (DFAA), dissolved combined (DCAA), and particulate (PAA) amino acids were measured in the sea surface microlayer and corresponding subsurface waters along a transect from coastal Massachusetts to open ocean waters of the Sargasso Sea. We also measured total bacteria concentrations, the percent bacteria with damaged membranes, and concentrations of virus-like particles. Microlayer samples taken by two different techniques-screen sampler (thickness 200-400 m) and rotating drum (thickness 30-60 m)-were compared. On average, concentrations of bacteria and amino acids in both subsurface and microlayer water were higher in water masses with higher fluorescence, lower temperature and salinity, and historically higher chlorophyll. The microlayer was enriched with amino acids and bacteria and, in most instances, virus-like particles. Consistent microlayer features, such as nonuniform preferential enrichment with DFAA and PAA and a uniform difference in dissolved amino acid composition between microlayer and subsurface water, were unrelated to environmental parameters measured. Wind, humidity, and light also had no apparent effects on amino acid concentration, composition, or enrichment in the microlayer in this study.

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