The focus of this study is to investigate if microcrustaceans undergo diel horizontal migration (DHM) in a tropical shallow lake on a yearly basis and analyse the adaptive value regarding predation. Abundance of invertebrate predators, chaoborid larvae and water mites, and microcrustaceans (cladocerans and copepods) were evaluated on a monthly basis in three stations located on a transect during the day and at night. Both invertebrate predators were predominantly pelagic. Cladocerans did not undergo significant DHM, distributing indistinctly onshore and offshore or being mostly pelagic. Nauplii, copepodites of two copepod species and adults of Tropocyclops prasinus meridionalis Kiefer were mostly distributed offshore, and did not perform DHM. The limnological features (temperature, dissolved oxygen, pH, and conductivity) were suitable for the organisms in both zones of the lake. Algal food concentration was a little lower in the littoral than in the limnetic zone during the day, but it seems to be suitable for the organisms. However, as the algae quality was not evaluated, it is not possible to be conclusive concerning its influence. The results indicated that DHM was not performed by the microcrustaceans and is not, therefore, a strategy for decreasing predation by both invertebrates, Chaoborus brasiliensis Theobald and Krendowskia sp., on a yearly basis in this shallow lake.Keywords: Chaoborus, water mites, cladocerans, copepods, food, DHM. Distribuição horizontal de microcrustáceos e predadores ao longo do ano em um lago neotropicalResumo O foco deste estudo é verificar se microcrustáceos apresentam migração horizontal diária (MHD) em um lago raso tropical, durante um ano, e o valor adaptativo com relação à predação. A abundância de predadores invertebrados, larvas de caoborídeos e ácaros aquáticos, e de cladóceros e copépodos foi avaliada mensalmente em três estações de coleta localizadas em um transecto, de dia e à noite. Ambos os predadores invertebrados foram predominantemente pelágicos. Os cladóceros não realizaram MHD, distribuindo-se indistintamente nas zonas litorânea e pelágica, ou foram mais abundantes na zona limnética. Os náuplios e copepoditos de duas espécies de copépodos e os adultos de Tropocyclops prasinus meridionalis Kiefer ocuparam principalmente a zona limnética, não migrando horizontalmente. As características limnológicas (temperatura, oxigênio dissolvido, pH e condutividade) foram adequadas nas duas zonas do lago. A concentração de alga foi um pouco menor no litoral do que na zona limnética durante o dia e parece ser adequada para os organismos; contudo, como a qualidade das algas não foi avaliada, não é possível ser conclusivo sobre sua influência. Os resultados indicaram que os microcrustáceos não realizaram MHD, a qual não constitui, portanto, uma estratégia para diminuir a predação dos dois invertebrados, Chaoborus brasiliensis Theobald e Krendowskia sp., em uma base anual, neste lago raso.

A comparison between two studies on diel vertical migration of two cyclopoid copepod species, in Lake Monte Alegre, undertaken in 1985/86 and 1999, revealed a change in their migratory behavior. In summer, during a period of marked stratification with low dissolved oxygen near the bottom, the organisms avoided the deepest layers, and migration was nocturnal or undetectable, in both periods. On other occasions, with partial or total circulation in the lake, a weak twilight migration of copepodites and adults in 1985 was replaced by the reverse in 1999. Differences were found among stages, with the weakest or undetectable migration being observed for nauplii. The migratory pattern change for copepodites and adults might be related with a recent predation pressure increase by Chaoborus larvae on copepods, after the virtual disappearance of their main cladoceran prey.