Summury: Usually a water column consists of two big layers. The upper layer (10-50 m deep) is characterized by strong mixing, as a result, both the biomass and nutrients are nearly uniformly distributed there. Mixing in the lower layers is much smaller and all distributions there are highly nonuniform. In this article we show that in the presence of an upper mixed layer (UML) there can be two stable vertical distributions of phytoplankton biomass: either the bulk of the biomass is located within the upper layer or below it. In the first case the biomass shades sun light and prevents growth in deep layers, in the last case the deep biomass maximum (at around 70-100 m depth) stops nutrient flux from the bottom and makes the upper layer unfavorable. The result depends only on the initial conditions, thus preparing the system in a special way (putting a lot of nutrients or removing the biomass from the upper layers) we can switch between two different profiles of biomass. The UML also changes the outcome of competition between two species, favoring that one which has higher light requirements.