||We isolated the effect phytoplankton cell size has on varying remote sensing reflectance spectra (R(rs)(lambda)) in the presence of optically active constituents by using optical and radiative transfer models linked in an offline diagnostic calculation to a global biogeochemical/ecosystem/circulation model with explicit phytoplankton size classes. Two case studies were carried out, each with several scenarios to isolate the effects of chlorophyll concentration, phytoplankton cell size, and size-varying phytoplankton absorption on R(rs)(lambda). The goal of the study was to determine the relative contribution of phytoplankton cell size and chlorophyll to overall R(rs)(lambda) and to understand where a standard band ratio algorithm (OC4) may under/overestimate chlorophyll due to R(rs)(lambda) being significantly affected by phytoplankton size. Phytoplankton cell size was found to contribute secondarily to R(rs)(lambda) variability and to amplify or dampen the seasonal cycle in R(rs)(lambda), driven by chlorophyll. Size and chlorophyll were found to change in phase at low to mid-latitudes, but were anti-correlated or poorly correlated at high latitudes. Phytoplankton size effects increased model calculated R(rs)(443) in the subtropical ocean during local spring through early fall months in both hemispheres and decreased R(rs)(443) in the Northern Hemisphere high latitude regions during local summer to fall months. This study attempts to tease apart when/where variability about the OC4 relationship may be associated with cell size variability. The OC4 algorithm may underestimate [Chl] when the fraction of microplankton is elevated, which occurs in the model simulations during local spring/summer months at high latitudes in both hemispheres. (C) 2011 Elsevier B.V. All rights reserved.