The Delaware estuary was sampled at 10–14 stations along the main salinity axis from February to August 1985. Dissolved free and combined amino acid concentrations ranged from 50 nM to 1.4 µM and 0.1 to 8.0 µM, respectively. Combined amino acid concentrations were generally higher than dissolved free amino acids. Highest amino acid concentrations were measured in the lower estuary in early spring. Molecular weight analysis of the dissolved combined amino acids revealed that a high percentage of the pool contained peptides of <1,000 daltons.

Uptake of ¹⁴C‐labeled amino acids and an algal protein were used to estimate the relative importance of these potential bacterial substrates. Laboratory experiments verified that peptide and protein uptake was nonspecific and that labeled algal protein was a good model substrate. Combined and free amino acid uptake rates ranged spatially and temporally from 15 to 600 nM h⁻¹. Combined amino acid uptake averaged 51% of the bacterial amino nitrogen demand.