Dissolved free amino acid cycling in an estuarine outflow plume

Abstract

Measurements from 4 cruises (February, June, August 1985, and April 1986) in the outflow plume of Chesapeake Bay (USA) show that DFAAs cycle rapidly, with turnover 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 turnover 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. 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 44 to 131% of the calculated bacterial N demand.