  The horizontal irradiance at the sea surface is an informative light
pollution indicator to study Artificial Light at Night (ALAN) effects on marine
biodiversity (e.g.: zooplankton diel vertical migration). The Posch ratio (PR)
for the horizontal irradiance (that is, the ratio of the horizontal irradiance
to the zenith radiance) is a useful tool for estimating the irradiance from
easily available measurements of the zenith night sky brightness. The PR
definition has already been generalized for any pair of linear radiance
indicators in any pair of arbitrarily chosen photometric bands, and can also be
applied to estimate e.g. the average sky radiance or the radiance at some
elevation above the horizon as a function of the radiance in any other
direction of the sky. The PR for a single light source depends on the distance
from the source, its angular and spectral emission pattern, and the state of
the atmosphere. The PR for any set of sources is a linear combination of the
individual PRs that each one would produce separately, with weights that can be
easily derived from the relative contribution of each source to the zenith
radiance. Whereas in populated lands the ALAN PR varies relatively fast from
one location to another, due to the particular spatial distribution of lights,
in coastal waters and the high seas the light pollution PR is a smooth function
of the distance to the shoreline, due to the progressive lack of neighboring
sources and the absence of obstacles. In this work we present the fundamental
equations of the model and an example of application for the waters surrounding
the Iberian Peninsula, North Africa and the West Mediterranean islands.
