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

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. Un debat existe concernant la necessite de reduire l'azote (N), en plus du phosphore (P), pour controler l'eutrophisation. Des experiences de fertilisation a l'echelle des lacs et les pratiques de restauration des lacs a hautes latitudes ont demontre que le P est le principal facteur de regulation du phytoplancton total. Reconnaissant le caractere limite des donnees a grande echelle pour les zones climatiques plus chaudes, nous avons mene une experience de fertilisation dans quatre etangs situes dans le bassin subtropical du fleuve Yangzi (Chine). L'abondance du phytoplancton total dans un etang recevant du P (+P) etait semblable a celle d'un etang recevant du N et du P (+N+P). Les deux avaient plus de phytoplancton qu'un etang ne recevant aucun ajout de nutriment (temoin). La concentration d'azote total (TN) dans l'etang +P augmentait avec l'apparition de cyanobacteries fixatrices de N. L'abondance du phytoplancton total etait semblable pour les etangs sans ajout de P (+N, temoin), et l'abondance de phytoplancton dans ces deux etangs etait plus faible que dans l'etang +N+P. Ces resultats montrent que le P, et non le N, determine l'abondance de phytoplancton total et que la fixation de N compense les deficiences en N dans les lacs subtropicaux. Cette experience appuie la notion voulant que les efforts d'attenuation de l'eutrophisation devraient principalement porter sur la reduction du P. [Traduit par la Redaction]
Introduction Eutrophication of lakes is a global environmental problem, associated with algal blooms, fish kills, ecosystem degeneration, and toxic algae, and hence represents a hazard to drinking water supplies. Many [...]