Phytoplankton distributions in Long Island and Block Island Sounds measured during a 1978 fall equinox cruise are interpreted in terms of tidal mixing variations and water column stratification. A stratification depth-scaled-by-light diagram is used to quantify the preferred physical environments of the two major morphological groups (diatoms and microflagellates). The success of the method in clearly distinguishing these physical regimes suggests its value as a useful biological growth index in estuarine systems.