Dominance of phosphorus over nitrogen as the limiter to phytoplankton growth rate

In mesotrophic lakes Washington and Sammamish growth rate of natural phytoplankton assemblages was shown to be a function of phosphate concentration, and to be relatively unaffected by nitrate. The situation was less clear in oligotrophic Findley lake. These findings indicate that experiments which show that nitrate plus phosphate produces more biomass than phosphate alone cannot be used to imply that a similar interactive effect occurs on growth rate. It may be fair to ignore nitrate when modelling phytoplankton dynamics in mesotrophic lakes.