Concentrations and utilization rates of alanine, aspartate, lysine, glucose, glycollic, acetic and lactic acids have been determined on six occasions at least, during a full seasonal cycle in the water column of three stations (Scheldt estuary, coastal North Sea and English Channel) with greatly differing biological characteristics. No significant differences in substrate concentration could be detected between the three stations, although the rate of utilization of all substrates differed greatly, decreasing in the order estuarine〉coastal〉open sea environment. This apparent paradox is explained by aid of a simple model showing that the steady state concentration of a particular substrate is independent of its rate of production (and thus of consumption) and depends only on purely physiological characteristics of the bacteria. Using published data for the pertinent physiological parameters of marine bacteria, the model accounts for at least the order of magnitude of substrate concentrations observed, and the absence of important seasonal variations. © 1980 Academic Press Inc. (London) Ltd. ; SCOPUS: ar.j ; info:eu-repo/semantics/published