Total phytoplankton abundance is determined by phosphorus input: evidence from an 18-month fertilization experiment in four subtropical ponds

Abstract

There is a heated debate over the necessity of nitrogen (N) reduction, in addition to phosphorus (P) reduction, for the control of eutrophication. Whole-lake fertilization experiments and lake restoration practices in high latitudes have demonstrated that P is the primary factor regulating total phytoplankton. Recognizing the limited large-scale evidence in warmer climatic zones, a fertilization experiment was conducted in four ponds located in the subtropical Yangtze River Basin, China. Total phytoplankton abundance in a pond receiving P (+P) was similar to that in a pond receiving both N and P (+N+P). Both had higher phytoplankton than a pond receiving no additional nutrient (Control). Total nitrogen concentration (TN) in the +P pond increased with the appearance of N-fixing cyanobacteria. Total phytoplankton abundance was similar in the ponds without P addition (+N, Control), and both ponds had lower phytoplankton levels than the +N+P pond. These results showed that P, not N, determines total phytoplankton abundance and that N deficiency is offset by N fixation in subtropical lakes. This experiment supports the idea that attention should be mainly focused on P reduction in mitigating eutrophication.