The effect of phosphorus on nitrogen retention in lakes

A hypothesis has been presented that nitrogen retention in lakes has become less efficient at the same time as phosphorus concentrations have decreased due to high phosphorus reduction in wastewater treatment. This might have led to higher loading of nitrogen into the Baltic Sea where nitrogen is considered to be the minimum nutrient. The hypothesis was tested using the mass balance modelling approach. Oligotrophic, mesotrophic and moderately eutrophic lakes situated in Finland and Sweden were used in the study. The combined approach of the classical mass balance model of Vollenweider and the statistical sedimentation model of Bachmann was used. At first the model was tested for nitrogen without any impact of phosphorus and it was concluded that this modelling approach is reasonable to study the hypothesis. If phosphorus concentration or retention affects nitrogen retention a better fit for the model of nitrogen retention can be gained when a function of phosphorus input or concentration is included in the nitrogen model of Bachmann. The results indicate that inclusion of phosphorus does not improve the fit of the model and thus it can be concluded that phosphorus can only be a minor factor in regulating nitrogen retention, at least in lakes which are not hypereutrophic.