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We studied the importance of environmental drivers for the seasonal dynamics of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production in surface waters of two tropical lakes in southeast Brazil (Carioca-CA; and Dom Helvé cio-DH), 2011 and 2012, using high frequency measurements of dissolved oxygen. Metabolic rates were approximately twice as high during the fully mixed winter periods, compared to the summer periods. For both lakes, GPP was approximately 30% lower during the warmer and dryer spring of 2012 compared to 2011. Seasonal changes in GPP and R were negatively correlated to water column stability (GPP r = -0.82, p < 0.001; R r = -0.80, p < 0.001). Periods with high stability coincided with warm waters which reduced mixing and internal inputs of nutrients from hypolimnetic waters. GPP was accordingly suppressed in both lakes during summer due to a combination of nutrient depletion and photo-inhibition, which was more pronounced during summer. These conditions were more prevailing during the warm and dry year of 2012, indicating ecosystem responses in carbon cycling to the ongoing regional climate changes.

This is a long-term study (1993-1998) on the effects of increasing eutrophication on the crustacean zooplankton structure in a eutrophic reservoir. The study first demonstrates that the eutrophication increase verified is a phenomenon well described by a single abiotic variable: total phosphorus. During the study period, zooplankton suffered extensive qualitative and quantitative changes. Significant size and biomass responses of certain zooplankton groups to the temporal variations of total phosphorus were found. These findings suggest that the variables size and biomass of major zooplankton groups should always be considered simultaneously in order to assess impacts caused by eutrophication on the structure of zooplankton.

The present study deals with the ecological impacts of the introduction of two alien species of piscivorous fish in several lakes of the Middle Rio Doce lake district in Minas Gerais, Brazil. It was demonstrated that these effects were not restricted only to the fish community. The introduction of the predatory red piranha Pygocentrus nattereri and the tucunaré Cichla cf. ocellaris caused not only a sharp decrease in the number of native fish species, but also major shifts in other trophic levels. Just after the fish were introduced, most lakes began to show conspicuous changes in phytoplankton species composition, in which Cyanophyceae gradually came to dominate. The zooplankton community lost several species, and in some cases, such as Lake Carioca, all the cladoceran species disappeared. On the other hand, invertebrate predators, represented by the dipteran Chaoboridae, boomed in the lake, with higher densities of exotic species, probably as a result of the "ecological release" by reduction of the original fish fauna. There was a general trend of species loss in different trophic levels. All these changes are apparently associated with decreases in water quality. The present situation in these lakes demands new approaches to the management and conservation of these ecosystems.Keywords: exotic species, eutrophication, trophic cascade, zooplankton, introductions.A cascata trófica invertida em comunidades planctônicas tropicais: Impactos da introdução de peixes exóticos no distrito de lagos do médio rio Doce, MG Resumo O presente estudo trata dos impactos ecológicos da introdução de duas espécies invasoras de peixes piscívoros em diversos lagos da região lacustre do médio rio Doce em Minas Gerais, Brasil. Demonstrou-se que estes efeitos não se restringiram às comunidade de peixes. A introdução dos predadores Pygocentrus nattereri (piranha-vermelha) e Cichla cf. ocelaris (tucunaré) não só causou uma forte redução no número de espécies de peixes nativos, como também mudanças nos níveis tróficos inferiores. Pouco depois das introduções, a maioria dos lagos começou a mostrar alterações na comunidade fitoplanctônica, tais como o aparecimento da dominância de Cyanophyceae. A comunidade zooplanctônica perdeu diversas espécies e, em alguns casos, houve o desaparecimento de todas as espécies de cladóceros limnéticos, como é o caso da lagoa Carioca. Por outro lado, predadores invertebrados, representados pelos dípteros da família Chaoboridae, floresceram nos lagos com maiores densidades de espécies exóticas de peixes, provavelmente como resultado da "liberação ecológica" causada pela redução da ictiofauna original. Além de uma Pinto-Coelho, RM. et al. 1026 Braz. J. Biol., 68(4, Suppl.): [1025][1026][1027][1028][1029][1030][1031][1032][1033][1034][1035][1036][1037] 2008 tendência geral de perda de espécies em diferentes níveis tróficos, outras mudanças estão aparentemente associadas com a redução da qualidade de água. Dessa forma, esses ecossistemas estão necessitando urgentemente de novas abordagens nas est...

ABSTRACT. Carioca Lake is located within the limits of the Rio Doce State Park, in the eastern part of the state of Minas Gerais. This park, one of the largest natural reserves of the Atlantic Rain Forest in Brazil, is a hotspot of tropical biodiversity. The purpose of this study was to update existing information on the bathymetry and morphometric features of the lake, using differential GPS (DGPS) technology for data collection, coupled to a digital echo sounder. The bathymetry was based on the acquisition of 1106 points, where the depths as well as the geographic coordinates were obtained. The new study allowed the refinement of existing primary and secondary morphometric data for this lake.

The effect of water transparency, dissolved oxygen concentration and the invertebrate predator Chaoborus brasiliensis on the day-night vertical distribution of the copepod cyclopoid Thermocyclops inversus was investigated in a shallow tropical reservoir, Nado Reservoir, Belo Horizonte, Brazil. Diel cycles were carried out over a period of 12 consecutive months, between October 1999 and September 2000. The different developmental stages of T. inversus exhibited diel vertical migration (DVM) and displayed a clear ontogenetic trend, with the amplitude of DVM increasing with the age of the organism, and ranging from 0.4 m to 0.8 m for nauplii, 0.4 m to 1.2 m for copepodite, and 1.1 m to 2.1 m for adults. We observed that seasonal changes in dissolved oxygen and C. brasiliensis directly influenced the vertical distribution of the copepod population in this reservoir. Furthermore, it was showed that the diurnal vertical migration is an important predator avoidance behavior since it diminished the spatial overlap between prey and its potential predator. This finding supports the hypothesis that the vertical migration is a defense mechanism against predation. Thus, T. inversus is able to remain in the anoxic layers during day light hours, and at night they move upwards avoiding hypolimnetic waters to escape from predation by Chaoborus.

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Global changes, such as climate and land use alterations, are expected to affect freshwater ecosystems, modifying environmental conditions with ultimate effects on carbon and nutrient cycling. While effects of these changes have been the focus of several studies in temperate lakes, remarkably little effort has been dedicated to understanding the behaviour and responses of tropical lakes despite their importance in tropical environments. In a factorial mesocosm study conducted in a tropical lake, we investigated how changes in environmental conditions, such as additions of inorganic nutrients and allochthonous organic matter, and shading, affect gross primary production (GPP), community respiration (R) and net community production (NCP) rates, and the occurrence of GPP light saturation and photoinhibition. GPP rates in the mesocosms were enhanced by additions of inorganic nutrients (+NP) and organic matter (+C) and decreased with increasing mean light available (Ī). Apparent photoinhibition was pronounced in full light treatments (+L) without additions of nutrients (−NP) and organic matter (−C), occurring in 91% of the experimental days. This supports previous evidence of the importance of photoinhibition in the upper mixed layer of the lake, especially during nutrient depletion and high light availability in the warmer rainy season. The main factor affecting metabolic rates was nutrients (NP), followed by organic matter (C). Effect of light (L) and the interaction between C and L were also significant for GPP and R rates. Addition of allochthonous organic matter (+C) with a high content of coloured dissolved organic matter (CDOM) stimulated GPP rates and reduced the occurrence of apparent photoinhibition and light saturation. This also increased R rates in the mesocosms. However, R was strongly coupled to GPP, indicating that changes in respiration were more related to autotrophic activity than to a direct and independent stimulation of heterotrophic communities. Our results strongly support recent evidence of the interactions between nutrients, organic matter and light conditions, which under a scenario of changes in the frequency and intensity of precipitation events and human‐mediated changes in land use, may affect the magnitude, variability and balance of metabolic processes in the productive upper mixed layers of tropical lakes.