Variação espacial e temporal dos fatores limnológicos em riachos da microbacia do rio São Francisco Verdadeiro Spatial and temporal variation i of limnological factors in streams of the micro-basin of the São Francisco Verdadeiro river

Com objetivo de caracterizar espacial e temporalmente os riachos da microbacia do rio São Francisco Verdadeiro, Marechal Cândido Rondon - PR, em função de fatores limnológicos, foram estabelecidas seis estações de coleta em três riachos (Curvado, Ajuricaba e Arroio Fundo), sendo uma estação na foz e outra na nascente. As coletas de água foram obtidas entre junho de 2005 e maio de 2006, totalizando 72 amostras. Foram analisadas as seguintes variáveis: temperatura, pH, condutividade elétrica, oxigênio dissolvido, turbidez, material em suspensão, fósforo total dissolvido, ortofosfato, nitrogênio total e nitrogênio amoniacal. De acordo com a análise de variância, houve baixa variabilidade entre as estações de coleta, porém a variação temporal demonstrou a influência do ciclo hidrológico. A análise de componentes principais (ACP) discriminou o sistema com base nos maiores valores registrados para as variáveis limnológicas, ficando separados os períodos influenciados pelas secas e pelas chuvas. A maior parte das variáveis está associada com o período de chuvas, no entanto há concentrações esporádicas de nutrientes na seca e menores valores de oxigênio dissolvido, demonstrando a dificuldade de diluição nesse período, causada pela diminuição da chuva e, consequentemente, do volume de água dos riachos.<br>In order to characterize spatially and temporarily streams of the São Francisco Verdadeiro river micro-basin in Marechal Cândido Rondon, State of Paraná, Brazil, according to limnological factors, six sampling stations were established in three streams (Curvado, Ajuricaba and Arroio Fundo) - one station at the mouth of the river and other at the spring of the streams. Water samples were collected between June 2005 and May 2006, totalizing 72 samples. The following variables were analyzed: temperature, pH, electric conductivity, dissolved oxygen, turbidity, suspended matter, total dissolved phosphorus, orthophosphate, total nitrogen and amoniacal nitrogen. According to ANOVA (variance analysis) it was observed low variability among the sampling stations however the temporal variation demonstrated the influence of the hydrologic cycle. The Principal Components Analysis (PCA) discriminated the system based on the largest values registered for the limnological variables separating the periods influenced by droughts and rains. Most of the variables are associated with the rain period; however, sporadic concentration of nutrients in the drought and smaller dissolved oxygen values indicates a dilution difficulty in this period, caused by decrease of rain and consequently decrease of water volume in the streams

OPINION Endless summer: internal loading processes dominate nutrient cycling in tropical lakes

1. Fossil diatom assemblages deposited in more than a dozen African lakes roughly 9500 years BP were dominated by a single planktonic species, Stephanodiscus astraea (Ehrcnb.) Grun. (although realistically this is likely to be a species complex). These diatoms flourished when lake-levels were maximal. Data are included from many of (he large African lakes, and others extending from Lake AbhÉ0, Ethiopia, to Lake Cheshi, Zambia. 2. Because the ecological physiology of Stephanodiscus species is well known one can predict the nutrient regime that must have existed when Stephanodiscus bloomed. Owing to competition for resources Stephano-discus species dominate when the supply ratio of silicon to phosphorus (in moles) in the epilimnion is relatively low (Si:P∼1). Consequently, lakes dominated by S. astraea are often hypereutrophic. 3. We propose a series of hypotheses to explain why tropical lakes have decreasing Si:P ratios as lake-levels increase, primarily owing to internal P-loading processes in the epilimnia. These observations appear to contradict present conceptions of the fundamental relationships governing nutrient loadings to and within lakes. Tropical lakes appear to have had increasing epilimnetic phosphorus loading as lake-levels increased. In contrast, large, deep lakes in the temperate zone are usually oligotrophic, with high Si:P ratios. 4. Our major conclusion is that regeneration rates are greater than removal rates for phosphorus in tropical lakes as compared to temperate lakes, especially where epilimnelic mixing exceeds 50 m. Biological control of the elemental cycles dominate in tropical lakes, whereas nutrient cycles in temperate lakes are dominated by physical processes for a large part of the year. This results in major differences in the fundamental mechanisms of nutrient regeneration and their relationships to morphometric features of lakes in the two regions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71789/1/j.1365-2427.1990.tb00280.x.pd