Aim To test the relevance of competition and predation for the two most abundant and frequent cladoceran populations, the medium-sized Ceriodaphnia richardi Sars, 1901 and the large-sized Daphnia gessneri Herbst, 1967, in a tropical shallow lake. Methods Laboratory experiments and weekly samplings in the lake for a year were performed to evaluate: density fluctuations and reproduction; predation on juveniles and adults by larvae of Chaoborus brasiliensis Theobald, 1901; competitive interactions and possible implications of competition and predation on the fluctuations of populations in the lake. Juveniles and adults of both cladocerans were offered to larvae instar IV of chaoborid, for testing selective predation in laboratory experiment. Competitive interaction between the two cladocerans was tested in experiments, with high (0.70 mg C.L-1) and low (0.25 mg C.L-1) concentrations of the edible chlorophycean Desmodesmus spinosus Chodat. Results C. richardi and D. gessneri were more abundant in the cool season, when their densities were inversely correlated, and when invertebrate predators, C. brasiliensis and the water mite Krendowskia sp., were less abundant. Despite higher concentrations of food and larger number of offsprings produced by the cladocerans in the warm season in the lake, their densities were lower, coincident with the increase of invertebrate predators, mainly chaoborid larvae. C. brasiliensis preyed on adult C. richardi and on young D. gessneri. In the competition experiments, the densities and the population growth rates of C. richardi were higher than those of D. gessneri. The competition results support the “small body size” and the “rmax” hypotheses. The exclusion of D. gessneri by C. richardi occurred only in experimental conditions, while they coexist in the lake, but without reaching their carrying capacity. Conclusions Predation is the key factor influencing the populations of cladocerans, whereas competition seems to play a secondary role, probably restricted to some periods of lower food concentration in the cool season.

The occupation of the littoral and limnetic zones by planktonic species has led to questions about whether littoral macrophytes would be a suitable habitat or refuge. These questions guided our field and laboratory studies on the spatial occupation and the food suitability for microcrustaceans of a shallow tropical lake. Replicated zooplankton samples were collected over 18 days, at intervals of 3 days, at three zones (the middle and edge of the stands of the emergent macrophyte, Ludwigia sp., and the limnetic zone). Some species, such as the cladocerans Diaphanosoma birgei and Daphnia ambigua, showed preference for the littoral zone and the limnetic zone, respectively. Copepodites, and adult copepods Thermocyclops decipiens and Tropocyclops prasinus meridionalis are mainly limnetic dwellers. The size of the cladocerans Ceriodaphnia richardi and Daphnia gessneri did not differ between the zones, nor did the clutch size of the former species; smaller individuals of D. ambigua were found within macrophyte stands. In laboratory experiments, the seston of the three zones did not negatively affect the development of the cladocerans C. richardi and D. birgei. On the contrary, the seston of the middle of the macrophyte stands promoted better growth rates of both cladocerans and some of the life table parameters of D. birgei than the seston of the other two zones. Food quantity and quality are not negative factors to microcrustaceans in the littoral and limnetic zones. The preference or indifference of microcrustaceans to habitats may be part of their adaptive strategies regarding other factors, including predation.

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Notonectids are widely distributed in freshwaters and can prey on zooplankton in temperate lakes. However, its role in structuring the zooplankton community is unknown in tropical lakes. Thus, our objective was to study the notonectid Martarega uruguayensis in a Brazilian tropical shallow lake to evaluate its potential as a zooplankton predator. Its horizontal distribution was analized in the lake throughout one year in fortnightly samplings. Backswimmers were more abundant (mean density 162.9 ± 25.8 ind.m-2) in the cool-dry season, with a strong preference by the littoral zone (mean density 139.9 ± 17.5 ind.m-2). Laboratory experiments were undertaken with young and adult notonectid and the two most abundant cladocerans, Daphnia gessneri and Ceriodaphnia richardi, as prey. Predation by backswimmers in the laboratory showed that only juveniles fed on microcrustaceans (mean ingestion rate of 1.2 ± 0.2 Daphnia and 1.0 ± 0.2 Ceriodaphnia per predator per hour), without size selectivity. Adult insects probably have difficulties in detecting and manipulating small planktonic organisms. On the other hand, young instars might influence zooplankton community, especially in the littoral zone of the lake. This study does contribute to a better understanding of trophic interactions in tropical shallow lakes and is the first to investigate the predation of a notonectid on microcrustaceans from Lake Monte Alegre. Distribuição e flutuação de notonectídeos (Notonectidae) em um lago tropical raso e a predação sobre microcrustáceos Resumo Notonectídeos são amplamente distribuídos em águas continentais e podem predar zooplâncton em lagos temperados. Porém, seu papel na estruturação de comunidades zooplanctônicas é desconhecido em lagos tropicais. Então, nosso objetivo foi estudar o notonectídeo Martarega uruguayensis em um lago tropical raso brasileiro para avaliar seu potencial como predador do zooplâncton. Sua distribuição horizontal foi analisada no lago durante um ano em coletas quinzenais. Os notonectídeos foram mais abundantes (densidade média 162,9 ± 25,8 ind.m-2) na estação fria-seca, com nítida preferência pela zona litorânea (densidade média 139,9 ± 17,5 ind.m-2). Foram feitos experimentos de laboratório com jovens e adultos de notonectídeo e as duas espécies de cladóceros mais abundantes, Daphnia gessneri and Ceriodaphnia richardi, como presas. No laboratório, somente os jovens predaram os microcrustáceos (taxa média de ingestão 1,2 ± 0,2 Daphnia e 1,0 ± 0,2 Ceriodaphnia por predador por hora), sem haver seletividade. Os insetos adultos provavelmente têm dificuldade em detectar e manipular pequenos organismos planctônicos. Por outro lado, os estádios jovens podem influenciar a comunidade zooplanctônica, especialmente na zona litorânea do lago. Esse estudo contribui para uma melhor compreensão sobre interações tróficas em lagos tropicais rasos e é o primeiro a investigar a predação de um notonectídeo sobre microcrustáceos do Lago Monte Alegre. Palavras-chave: predação, Heteroptera, insetos aquáticos, zooplâncton, Lago M...