Resumo O fósforo é considerado o principal fator responsável pela eutrofização de lagos e reservatórios. No presente estudo, utilizou-se um modelo transiente de mistura completa para analisar a dinâmica do fósforo em 40 lagos e reservatórios localizados nas regiões Nordeste, Sudeste e Centro-Oeste do Brasil. Os resultados con- firmaram a dependência do coeficiente de decaimento de fósforo com o inverso da raiz quadrada do tempo de residência hidráulica. No entanto, em virtude da elevada temperatura da água dos lagos e reservatórios do Nordeste, esse coeficiente assumiu valores cerca de duas vezes superiores àqueles observados em regiões de clima tropical. Por outro lado, devido à alta variabilidade de temperatura dos lagos e reservatórios do Sudes- te, o coeficiente oscilou entre valores extremos observados em regiões de clima temperado e semiárido. Em virtude disso, propôs-se uma equação geral para calcular o coeficiente de decaimento de fósforo como uma função não apenas do tempo de residência hidráulica, mas também da temperatura da água. Após validação e análise de sensibilidade do modelo proposto, o mesmo foi aplicado a um reservatório hipotético sujeito a variações temporais de volume, vazão, carga e temperatura da água. Espera-se que este modelo simplificado possa auxiliar na gestão de lagos e reservatórios brasileiros. Palavras-chave: Eutrofização. Temperatura. Qualidade da água. Abstract Phosphorus is considered the main factor responsible for eutrophication of lakes and reservoirs. In the present study, a transient complete mixing model was used to analyse phosphorus dynamics in 40 lakes and reservoirs located in the Northeast, Southeast and Central-West regions of Brazil. The results confirmed the dependence of the phosphorus decay coefficient on the inverse of the square root of the hydraulic residence time. However, due to the high-water temperature of the lakes and reservoirs of the Northeast, this coefficient assumed values about two times higher than those observed in regions of tropical climate. On the other hand, due to the high temperature variability of the lakes and reservoirs in the Southeast, the coefficient oscillated between extreme values observed in temperate and semiarid regions. As a result, a general equation was proposed to calculate the phosphorus decay coefficient as a function not only of the hydraulic residence time, but also of the water temperature. After valida- tion and sensitivity analysis of the proposed model, it was applied to a hypothetical reservoir subject to temporal variations of water volume, flow, load and temperature. This simple model will potentially assist in the management of Brazilian lakes and reservoirs. Keywords: Eutrophication. Temperature. Water quality.

Field surveys were carried out to investigate the surface jet flows and the resulting circulation patterns generated by diffused aeration in a shallow lake. In conrast to previous studies, the experimental conditions included point-source bubble plumes with very high air flow rates (100-400 L/min) relative to the shallow water depth (1.5 m). The results indicate that the surface jet velocity can be described by linear profiles. The decay of this velocity with distance displayed a similar behaviour to that reported in the literature, but the surface jet spread much faster under the present conditions. Combining our results with published data allowed for the generation of dimensionless correlations for the velocity and depth of the surface jet. A good agreement between the total circulation flow rates predicted by using the proposed correlations and integral modelling was also obtained. Lastly, a simple returning flow model was proposed to describe the circulation flow patterns induced by the bubble plumes. The results were also applied to assess the impact of circulation on vertical algae migration, which is important for water quality management.