Local heating rate within the oceanic mixed layer (ML) depends not only on the amount of solar radiation incident on the sea surface, but also on the vertical distribution of the irradiance in the water column. We have evaluated the effect of a phytoplankton bloom on mixed layer depth and temperature at a high latitude site near Iceland. The level 2½ version of the Mellor-Yamada (1982) turbulence scheme has been modified to include the vertical distribution of irradiance. This has allowed the investigation of the ML temperature and stability structure resulting from both physical and biological effects. An important part of the model is the parameterization of pigment-dependence which affects the spectral attenuation coefficient for downwelling irradiance as proposed by Morel (1988). Concurrent, high temporal resolution time series of physical and bio-optical data were used for the model. These data were acquired using a mooring deployed during the spring of 1989. We have estimated that the increase of phytoplankton abundance induced an increase of the sea surface temperature by about 0.2°C at the mooring site. This led to stronger near-surface thermal stratification and shallower mixed layers. The dependence of the upper layer thermal structure on biology is more important when vertical mixing is weaker and when phytoplankton concentrations are higher.