ABSTRACT
For many decades, polymorphism and its consequences have only been studied from the taxonomic point of view. Presently, interest has switched to the environmental causes of morphological variation and its consequences in the form and essence of the species. This study aimed at evaluating desmids morphological modifications of Staurastrum rotula Nordstedt during inter-annual succession patterns in two warm monomitic tropical lakes: Dom Helvécio (19°45′- 19°48′45″S, 42°33′45″W) and Carioca (19°45′20″S, 42°37′12″W). The effect of thermal stability and light and nutrients availability was based on samples collected monthly from January 2002 to December 2006 compared the morphological modifications. Results indicated that morphological variation, asexual reproduction, theratological forms, mucilaginous envelope and fungal infection were highest in Lake Dom Helvécio and coincided with the biomass increase of species with complex morphology between September and March (stratification period). The Zmix oscillation, wind and rainfall occurring at the end of the mixing period and beginning of the stratification were suggested as autochthonous and allochthonous disturbance agents, respectively, identified as inducers of asexual reproduction and consequently of the morphological variation. It was suggested that incidence of parasitism may act as a potential controlling agent for the Staurastrum rotula population size. It was concluded that morphological variation represents accidents in the original form, i.e. in the desmid species essence, promoting the existence of ecoforms, not of new infraspecific taxa.
Por muitas décadas, o polimorfismo e suas conseqüências foram estudados apenas do ponto de vista taxonômico. Atualmente, o interesse se volta para as causas ambientais das variações morfológicas e suas consequencias na forma e essência da espécie. O estudo teve como objetivo avaliar as modificações morfológicas de Staurastrum rotula Nordstedt durante padrões inter-anuais de sucessão em dois lagos tropicais monomíticos-quentes: Dom Helvécio (19°45′-19°48′45″S, 42°33′45″W) e Carioca (19°45′20°″S, 42°37′12″W). O efeito da estabilidade térmica e potenciais distúrbios sobre as modificações morfológicas foi baseado em amostras coletadas mensalmente de janeiro de 2002 a dezembro de 2006. Os resultados indicaram que a variação morfológica, a reprodução assexuada, as formas teratológicas, envelope mucilaginoso e infecção fúngica foram maiores no Lago Dom Helvécio e coincidiu com o aumento da biomassa de espécies com morfologia complexa entre setembro e março (período de estratificação). A oscilação Zmix, vento e chuva ocorrentes entre o final do período de mistura e início da estratificação foram sugeridos como agentes perturbadores autóctones e alóctones, respectivamente, identificados como indutores de reprodução assexuada e, consequentemente, de variação morfológica. Foi sugerido que a incidência de parasitismo atua no controle do tamanho da população Staurastrum rotula. A variação morfológica, portanto, representa acidentes na forma original, ou seja, na essência das desmídias, promovendo a existência de ecoformas, não de novos táxons infraespecíficos.
Subject(s)
Desmidiales/cytology , Lakes/chemistry , Brazil , Desmidiales/microbiology , SeasonsABSTRACT
Los procesos de transformación ambiental que se dan en la actualidad en un marco de cambio climático, generan modificaciones en los ecosistemas y comunidades biológicas, ?cómo responden las poblaciones a estos factores de tensión?, ?qué efectos se darían sobre la diversidad taxonómica y ecológica? Se analizó la variación de la composición taxonómica y estructura del zooplancton en función de los cambios ambientales en un reservorio tropical ubicado a 6° 02' 18" N y 73° 29' 16" O. Semanalmente se realizaron muestreos en la zona limnética del sistema durante cuatro meses que abarcaron los periodos seco, de transición y de alta precipitación. Se estableció un alto grado de variabilidad temporal asociada al poco tiempo de retención hidráulica estimado en ocho días. Se registraron nueve especies en el zooplancton, entre ellas Keratella tropica tropica y Thermocyclops decipiens que fueron las más abundantes y constantes. Los valores de diversidad H' y riqueza S hallados se consideran bajos, lo que corresponde a una comunidad poco madura vinculada a un ambiente físico fluctuante, evidenciado en los elevados coeficientes de variación de la conductividad eléctrica y la transparencia Secchi. La fluctuación del volumen del sistema en lapsos cortos de tiempo, genera cambios significativos en la expresión física del sistema con efecto directo sobre la composición y estructura del zooplancton. En general el modelo de respuesta del zooplancton en este reservorio, se ajusta a los supuestos de la hipótesis de disturbio intermedio.
Processes of environmental transformation that currently occur in the climatic change context generate changes in ecosystems and biological communities. ?how populations respond to these stressors? ?what effects could occur on taxonomic and ecological diversity? The taxonomic composition and structure of the zooplankton was analyzed with relationship to environmental changes in a tropical water reservoir located at 6° 02' 18" N and 73° 29' 16" W. During four months, samples were taken weekly covering stations of low, medium, and high precipitation. A high degree of temporal variability was established, it associated with a short hydraulic retention time estimated at eight days. Nine species of zooplankton were collected, of which Keratella tropica tropica and Thermocyclops decipiens were the two most abundant and constant species. Found values of H' diversity and S richness were considered low, corresponding to a little mature community associated with a fluctuating physical environment and supported by high variation coefficients of electrical conductivity and Sechhi disk transparency. Drastic variations on the system volume in short time lapses generate important changes in the physical expression of system with a direct effect on composition and structure of the zooplankton. In general, the response model of the zooplankton in the reservoir according to the statement by the intermediate disturbance hypothesis.