The bulk concentration of phytoplankton chlorophyll and carbon may be represented in proxy by summations derived from flow cytometric spectra of cellular fluorescence and cellular forward light scatter. Changes in the bulk properties reflect changes in the adaptive properties of cells (cellular content of chlorophyll and carbon) as well as the weighting factor which is the cell abundance. In coastal waters off Morocco, depth profiles of total fluorescence and total forward light scatter matched those of bulk chlorophyll, suggesting a relative invariance of the carbon: chlorophyll ratio with depth. In the oceanic central North Atlantic, depth profiles of bulk chlorophyll were matched by those of total fluorescence but not by those of total forward light scatter. Phytoplankton in the subsurface chlorophyll maximum of oceanic waters are thus inferred to have a substantially lower carbon: chlorophyll ratio than phytoplankton near the surface. In the oceanic North Atlantic, maxima in ultraphytoplankton abundance and light scatter are located at depths shallower than the maximum in chlorophyll.