Several semi-analytic models exist for the inherent optical properties of sea water, at least for Case 1 waters. In these waters, models based on chlorophyll-a concentration seem to be fairly successful. For passive remote sensing, the critical properties are the backscattering coefficient and the zenith diffuse attenuation coefficient. The former describes the total scattering at angles > 0.5π steradians. The diffuse attenuation coefficient is not strictly an inherent optical property, because it depends on the sun angle. The zenith diffuse attenuation coefficient, defined as the attenuation of a diffuse source located at the zenith, depends only on the optical properties of the water. The observed remote sensing reflectance can be estimated from these two parameters and the solar zenith angle. Most of the investigations to date have assumed that the chlorophyll concentration does not vary with depth. This assumption is often quite good, because of the limited penetration of light into sea water. We will consider the case of intense thin plankton layers on a shallow pycnocline, where this assumption might not be valid. For active remote sensing, an additional parameter is important. This parameter is the volume scattering function at a scattering angle of π steradians, which is the sum of contributions from sea water and particles in the water. The sea water contribution is known. The particulate contribution can be modeled as the product of the scattering coefficient, which depends on chlorophyll concentration, and the phase function at π steradians, which does not.