The Dependence of Chlorophyll a Concentration on Total Phosphorus in Water Bodies with Increasing Water Color

For Karelian lakes, it is shown that, as the water color increases, an increase in the total phosphorus content in water leads to a slower increase in the concentration of chlorophyll a compared to the generally accepted Dillon–Riegler dependence. This is explained by the fact that a significant amount of phosphorus is associated with iron in complexes with humic substances, which is indirectly confirmed by a significant dependence of the concentration of total phosphorus on the water color. An equation for calculating the chlorophyll a concentration using the total phosphorus value for water bodies of Karelia not affected by anthropogenic influence has been proposed for the first time. An overestimation of the real trophic status of high-color waters, determined based on the total phosphorus value, can lead to an unreasonably high permissible anthropogenic phosphorus load on water bodies in Karelia.