Continuity effects on rotifers and microcrustaceans caused by the construction of a downstream reservoir in a cascade series (Iguaçu River, Brazil)

Abstract

This study evaluated changes in the community of rotifers and microcrustaceans associated with the construction of a large and dendritic reservoir in Iguaçu River (Brazil), as the last reservoir of a sequence of five cascading systems. Differences were clear between pre-filling and post-filling phases for organisms and some of environmental variables. In the pre-filling phase, the community was more homogeneous along the downstream river gradient, and spatial compartmentalization in the new reservoir was common during the post-filling phase. From 140 identified taxa, 10 species occurred exclusively in the pre-filling phase and 32 in the post-filling phase. After completion of the fifth reservoir filling-up, opportunistic, pioneer and fast-developing species quickly dominated, and downstream of the fourth reservoir the pre-filling decreasing gradient of richness, diversity and evenness disappeared. Richness of rotifers and cladocerans, cladocerans diversity, and evenness of rotifers, cladocerans and copepods were generally higher in the post-filling phase. A non-metric multidimensional analysis based on a presence/absence matrix depicted a homogeneous and dense group of species associated to the pre-filling phase and a second, dispersed group related to the post-filling phase. Spearman correlations pointed out significant positive effects of transparency on rotifer species richness in the post-filling phase, and negative effects on the microcrustacean richness in the pre-filling phase. Dam construction caused disruption of the downstream lotic gradient along the series of dams, leading to the development of distinct species in lentic spatial compartments. Efeitos contínuos sobre rotíferos e microcrustáceos causados pela construção de um reservatório a jusante de uma série em cascata, Rio Iguaçú, Brasil Resumo Esse estudo analisou os efeitos da construção de um reservatório largo e dendrítico no Brasil sobre a comunidade de rotíferos e microcrustáceos, sendo o último reservatório de uma cascata de cinco. Houve claras diferenças entre as fases pré e pós-enchimento para os organismos e para algumas variáveis ambientais. Na fase pré-enchimento, a comunidade foi mais homogênea no gradiente longitudinal no rio a jusante, e na fase pós-enchimento houve compartimentalização espacial no novo reservatório. Dos 140 táxons identificados, dez espécies ocorreram exclusivamente na fase pré-enchimento e 32 na fase pós-enchimento. Após o enchimento, espécies oportunistas, pioneiras e de rápido desenvolvimento foram dominantes, e desapareceu o gradiente de decréscimo a jusante do quarto reservatório para a riqueza, diversidade e equitabilidade durante a fase pré-enchimento. A riqueza de rotíferos e cladóceros, a diversidade de cladóceros, e a equitabilidade de rotíferos, cladóceros e copépodes foram elevadas na fase pós-enchimento. A análise de escalonamento multidimensional não-métrico sobre a matriz de presença e ausência mostrou um grupo homogêneo e denso de espécies e meses de amostragem na fase pré-enchimento, e outro grupo d...

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