Effect of N, P concentration on growth rate

The total nitrogen, phosphorus, biomass, pH, dissolved oxygen in and temperatures of three eutrophic waters were investigated in the rapid-growth season of phytoplankton (July-October). Chloromonas rosae was cultivated in water samples from three eutrophic waters and diluted water samples from Lake Donghu to determine the effect of N and P concentrations on the growth rate. The relationship between the biomass and the N and P concentrations, analyzed by regression, showed that phosphorus was the limiting factor for algae growth in eutrophic water. The relationship between the growth rate and the P concentration can be described with the regression equation y=0.0806ln(x)+0.4658, (R2=0.889). The growth rate increased linearly with the increase of P concentration when it was below 0.05 mg/L. It increased less when the P concentration was above 0.05 mg/L, and was almost unchanged when the P concentration exceeded 0.2mg/L. The P concentration corresponding to the growth rate “0” (deduced from the regression equation) was 0.003mg/L, close to the minimal P concentration of poorly nutrient lakes. This indicated that the regression equation was representative. The average values for plankton cellular N and P in the three eutrophic waters were 53% and 85%, respectively. To evaluate the levels of eutrophy, N and P, both in the water and in the plants, must be considered. The biomass of phytoplankton is controlled by concentrations of both dissolved and cellular N and P. The following linear regression equations describe the relationship between biomass and N: y=10.687x−7.8304, (R2=0.950), and between biomass and P, y=122.11x−12.069, (R2=0.991). They exemplify the absolute and relative aspects of growth-limiting factors with Redfield values. We conclude that the only way to prevent eutrophication is to maintain a balance between the input and output of nutrients and to remove excessive dissolved N and P in the water.