Zooplankton fluctuations in Jurumirim Reservoir (São Paulo, Brazil): a three-year study.

During three consecutive years, monthly samples of zooplankton were taken in the lacustrine (dam) zone of Jurumirim (São Paulo, Brazil). The seasonal effect on basic limnological features (thermal regime, oxygen distribution, phytoplankton biomass, etc.) was also examined. The influence of the seasonality on the fluctuation of the zooplankton composition and abundance was not clearly detected (low degree of recurrent patterns). Rotifers (32 taxa) were the most abundant organisms during almost the entire study period with some seasonal alternations in the maximum abundance peaks of the main taxa (Conochilus unicornis, Keratella americana, K. cochlearis and Hexarthra spp.), except for Polyarthra (mainly P. vulgaris). Only occasionally copepods were numerically dominant. Higher copepod abundance was positively associated to periods of increase in the water retention time. Among the Copepoda (10 taxa) the calanoids (mainly Notodiaptomus iheringi) were more abundant, especially in warmer periods. Conversely, cyclopoids had higher abundance in autumn and winter. The species Thermocyclops minutus and T. decipiens co-occurred, but the first attained higher abundance. Some evidence of co-existence strategies between both species are considered. Cladocera (17 taxa) was never numerically dominant and the main taxa (Bosmina spp., Ceriodaphnia spp. and Diaphanosoma spp.) occurred almost the whole study period and did not present a seasonal pattern of fluctuation. Diaphanosoma (mainly D. birgei) attained the highest abundance among cladocerans. Most organisms were always found at the surface, but they also occupy the whole water column, even in periods of stratified conditions and low oxygen concentration in the bottom layers. Among the main zooplanktonic taxa, only Hexarthra avoids deep layers. An exceptionally high concentration of Copepoda nauplii on the surface was influenced by low transparency, high concentration of phytoplankton at this layer and low oxygen concentration at the bottom. In periods of higher retention timevariability there was a more heterogeneous distribution of the zooplankton in the water column. The increase in the retention time seems also to favor the copepod development. Finally, some inter-decade changes are considered on the basis of zooplankton assemblage structure observations.

Zooplankton fluctuations in Jurumirim Reservoir (São Paulo, Brazil): a three-year study / Flutuações do zooplâncton na represa de Jurumirim (São Paulo, Brasil): um estudo de três anos

During three consecutive years, monthly samples of zooplankton were taken in the lacustrine (dam) zone of Jurumirim (São Paulo, Brazil). The seasonal effect on basic limnological features (thermal regime, oxygen distribution, phytoplankton biomass, etc.) was also examined. The influence of the seasonality on the fluctuation of the zooplankton composition and abundance was not clearly detected (low degree of recurrent patterns). Rotifers (32 taxa) were the most abundant organisms during almost the entire study period with some seasonal alternations in the maximum abundance peaks of the main taxa (Conochilus unicornis, Keratella americana, K. cochlearis and Hexarthra spp.), except for Polyarthra (mainly P. vulgaris). Only occasionally copepods were numerically dominant. Higher copepod abundance was positively associated to periods of increase in the water retention time. Among the Copepoda (10 taxa) the calanoids (mainly Notodiaptomus iheringi) were more abundant, especially in warmer periods. Conversely, cyclopoids had higher abundance in autumn and winter. The species Thermocyclops minutus and T. decipiens co-occurred, but the first attained higher abundance. Some evidence of co-existence strategies between both species are considered. Cladocera (17 taxa) was never numerically dominant and the main taxa (Bosmina spp., Ceriodaphnia spp. and Diaphanosoma spp.) occurred almost the whole study period and did not present a seasonal pattern of fluctuation. Diaphanosoma (mainly D. birgei) attained the highest abundance among cladocerans. Most organisms were always found at the surface, but they also occupy the whole water column, even in periods of stratified conditions and low oxygen concentration in the bottom layers. Among the main zooplanktonic taxa, only Hexarthra avoids deep layers. An exceptionally high concentration of Copepoda nauplii on the surface was influenced by low transparency, high concentration of phytoplankton at this layer ...

Durante três anos consecutivos, foram feitas amostragens mensais do zooplâncton na zona lacustre (barragem) do reservatório de Jurumirim. Os efeitos da sazonalidade nas características limnológicas básicas (regime térmico, distribuição do oxigênio, biomassa fitoplanctônica, etc.) também foram examinados. Um padrão de flutuação da composição zooplanctônica e da sua abundância não foi claramente detectado (baixo grau de recorrência interanual). Os rotíferos foram os organismos mais abundantes durante a maior parte do tempo, com alterações nos picos de abundância dos principais táxons (Conochilus unicornis, Keratella americana, K. cochlearis e Hexarthra spp.), exceto para Polyarthra (principalmente P. vulgaris). Apenas ocasionalmente, os copépodes foram numericamente dominantes, estando esse fato relacionado a períodos de aumento no tempo de residência da água. Entre os Copepoda (10 táxons), os calanóides (principalmente Notodiaptomus iheringi) foram mais abundantes, especialmente em períodos mais quentes. Por outro lado, os ciclopóides foram mais abundantes no outono e no inverno. As espécies Thermocyclops minutus e T. decipiens coocorreram, mas a primeira alcançou maior abundância. Algumas estratégias de coexistência entre ambas as espécies são consideradas. Cladocera (17 táxons) nunca foi dominante numericamente e os principais gêneros (Bosmina, Ceriodaphnia e Diaphanosoma) ocorreram durante quase todo o período de estudo e não apresentaram um padrão sazonal de flutuação. Diaphanosoma birgei foi o táxon mais abundante entre os cladóceros. Embora a maioria dos organismos ocupasse toda a coluna d'água, mesmo em períodos de estratificação térmica e baixas concentrações de oxigênio no fundo, foram sempre mais abundantes na superfície. Entre os principais táxons zooplanctônicos, apenas Hexarthra evitou camadas profundas. A alta concentração de náuplios de Copepoda na superfície foi influenciada pela baixa transparência, alta concentração do ...