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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 [...]

Mollusk biomass is usually expressed as wet mass with shell, but this expression fails to represent real biomass due to the high calcium carbonate content in shells. Tissue dry mass and ash free dry mass are relatively close to real biomass. However, the determination process of these two parameters is very complicated, and thus, it is necessary to establish simple and practical conversion methods for these two parameters. A total of six taxa of freshwater snails (Bellamya sp., Alocinma longicornis, Parafossarulus striatulus, Parafossarulus eximius, Semisulcospira cancellata, and Radix sp.) common in the Yangtze Basin were selected to explore the relations of their five shell dimension parameters, dry and wet mass with shells with their tissue dry mass and ash free dry mass. The regressions of the tissue dry mass and ash free dry mass with the five shell dimension parameters were all exponential (y=axb). Among them, shell width and shell length were more precise (the average percentage error between observed and predicted value being 22.0% and 22.5%, respectively) than the other three parameters in the conversion of dry mass. Wet mass with shell could be directly converted to tissue dry mass and ash free dry mass, with an average percentage error of 21.7%. According to the essence of definition and the errors of conversion, ash free dry mass would be the optimum parameter to express snail biomass. [ABSTRACT FROM AUTHOR]