A biological-physical model was developed to study the population growth of Coullana canadensis (Willey), a meroplanktonic copepod, in the Saco River estuary, Maine. The biological processes were derived from extensive laboratory and field data, and the physical processes were based on a comprehensive estuarine hydrodynamic model. The study found that the physical environment significantly altered the seasonal pattern of population growth from that predicted from the biological model. Nauplii were lost from the estuary due to flushing in spring and early summer, and thus, the timing of the peak in copepod recruitment was delayed until late summer. A low river flux during late summer resulted in less mass exchange at the mouth of the estuary, that apparently was necessary for the viability of the population within the estuary.