Bentos como indicador de condiciones tróficas del sistema del río Paraná Medio

MARCHESE, M.; EZCURRA DE DRAGO, I.

Eutrofização na América do Sul: Causas, Consequências e Tecnologias de Gerenciamento e Controle.

Lugar: San Carlos, Brasil; Año: 2006; p. 297 - 316

This study synthesizes the results of the different types of environment of the Middle Paraná system (31º 42’ S – 60º 42’ W), including tributaries (Salado del Norte and El Dientuco Rivers), lower and higher-hierarchy secondary channels (Coronda, Santa Fe, El Vado and Correntoso Rivers) and floodplain lakes with direct (Los Matadores, La Cuarentena, El Puesto) and indirect connections (El Tigre, La Mira) with the rivers during different hydrological regimen, including events as the ENSO. According to the results obtained, the environments identified as eutrophic were those directly affected with sewage drainage. In the Salado del Norte and Santa Fe Rivers we can observe differences between upriver and downriver stations, being eutrophic in the banks nearest to the sewage drainages and recovering downstream. This coincide with the higher BOD5, phosphorous and chlorophyll a concentrations. A natural eutrophication process may be identified in minor secondary channels and floodplain lakes with indirect connection to the river during the lowest water stages. In secondary channels with lower discharges during the low-water phase there is very low or null water current, results in the impoverishment of benthic communities. In floodplain lakes with indirect connection with the river, large water residence time and high macrophyte cover, if the amplitude and magnitude of the lower phase is large there is a great accumulation of dead organic matter and consequently a strong depletion of oxygen concentrations reaching anoxia. Therefore, the species diversity has a strong decrease with development of more tolerant species such as Chrironomus xanthus, Limnodrilus hoffmeisteri and Branchiura sowerbyi. When the river level begins to rise there is a rejuvenation of the environments, being colonized again by a large species richness and density. The species assemblages characteristic of the extreme degree from low to high trophic state in environments with lower conductivity are Narapa bonettoi + Haplotaxis aedeochaeta + Myoretronectes paranaensis + Tobrilus sp.Þ Limnodrilus hoffmeisteri + Pristina americana + Dero multibranchiata + Dero sawayai + Chironomus xanthus + Pisidium sp. + Corbicula fluminea. In addition, in environments with high conductivity, other species assemblages are characteristic of the extreme trophic degree Glyptotendipes sp. + Polypedilum spp. + Cytheridae sp. I Þ Tubifex tubifex + Heleobia parchappei + Chironomus xanthus + Goeldochironomus sp. + Limnodrilus hoffmeisteri.