Phytoplankton often faces the dilemma of living
in contrasting gradients of two essential resources: the light
that comes from above and nutrients that are often supplied
from below. In poorly mixed water columns, algae can be
heterogeneously distributed, with thin layers of biomass found
on the surface, in depth, or on the sediment surface. Here, we
show that these patterns can result from intraspecific competition
between light and nutrients. First, we present numerical solutions
of a reaction-diffusion-taxis model for phytoplankton, nutrients
and light. We argue that motile phytoplankton can form a thin
layer under poorly mixed conditions. The numerical solution of
this model indicates the depth at which phytoplankton is equally
limited by both resources. The layer becomes shallower with an
increase in the nutrient supply and deeper with an increase in
the light supply. These general patterns are in agreement with
field observations. Thus, this model can explain many patterns
of algal distribution found in poorly mixed aquatic ecosystems