Dissolved free amino acid cycling in an estuarine outflow plume

Microbial cycling of dissolved organic C and N is a major pathway in marine ecosystems, and dissolved free amino acids (DFAAs) are significant components of this cycling. Measurements from 4 cmises (February, June, August 1985, and April 1986) in the outflow plume of Chesapeake Bay (USA) show that DFAAs cycle rapidly, with tumover times averaging 0.5 to 1 h in spring and summer and ca 3 h in winter, as measured by brief (10 min) tritiated tracer uptake and respiration experiments under extremely clean conditions. DFAA uptake and release rates, calculated from tumover and concentration measurements of four of the most abundant amino acids (glu, ser, gly, ala) as the ship followed a surface drogue, were closely coupled and varied over the day, often tending to be highest near noon and lowest at night, suggesting a connection to primary production. Rates also tended to dechne as the relatively biologically-rich plume water mixed with less active coastal seawater. Winter rates were much lower than those in spring and summer. Despite concentrations in the low nM range, the release and uptake rates of these 4 amino acids represented a significant fraction (4 to 18 %) of the primary production rate, and incorporation of DFAA-C was 15 to 64 % of estimated bacterial secondary production. N uptake from these 4 DFAAs represented about 4 4 to 131 % of the calculated bacterial N demand, suggesting that the bacteria in this system probably regenerate inorgamc N. One set of measurements in the Hudson plume in summer showed even faster DFAA cycling, with turnover times averaging < 7 min.