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The influence of two seston fractions, Ͻ 20 m ͑nanoplankton͒ and Ն 20 m ͑microplankton͒, on growth and reproduction of cladoceran species with different sizes, from Lake Monte Alegre, was evaluated through individual growth and life table experiments. Size, shape and other features of the algae in the fractions were described. The procedure of filtering lake water through a 20 m net for seston fractionation caused mutual contamination. The smallest cladoceran species, Ceriodaphnia cornuta Sars and Moina micrura Kurz, produced larger clutch sizes and exhibited higher intrinsic rates of population growth ͑r͒ in the nanoplankton, despite contamination of their food by inedible algae. The largest species, Simocephalus mixtus Sars, produced larger clutch sizes in the microplankton. There were no differences in juvenile biomass growth between treatments for M. micrura and Daphnia gessneri Herbst, but lower value of the exponential growth rate ͑g͒ in the microplankton was significant for M. micrura. Fecundity ͑eggs/total female͒ of M. micrura was significantly lower in the microplankton, while D. gessneri did not reproduce in this fraction, at the end of growth experiments. Spines, colonies, cenobium, filaments, hard cell wall, and gelatinous sheaths, represented constraints to cladoceran reproductive performance, besides algae size. Microplankton contamination by nanoplanktonic algae, in the experiments, probably minimized the negative effect of inedible algae. Nanoplankton was more suitable for the smallest species and microplankton for the largest one.

SUMMARY 1. In this study, the effects of nutrient (N and P) deficiency and the importance of essential polyunsaturated fatty acids (PUFA) [eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] to tropical cladocerans, growth and reproduction were determined in a growth bioassay. 2. The animals were fed N/P‐sufficient, N‐deficient and P‐deficient algae, and also N and P‐deficient algae supplemented with fish oil emulsions rich in EPA and DHA. 3. Cladocerans showed different responses to nutrient‐deficient algae and also to supplements of fish oil emulsions. Moina micrura was most sensitive to P‐deficient alga and, surprisingly, grew better and produced more eggs in N‐deficient alga than in N/P sufficient alga. Ceriodaphnia cornuta was less sensitive, growing well in both N and P‐deficient algae. This species, however, had a lower clutch size in N‐deficient alga. On the other hand, Daphnia gessneri was the most sensitive to mineral limitation, showing decreased growth and clutch size in both nutrient‐deficient algae. 4. The PUFA supplements to nutrient‐deficient algae increased growth rates only for M. micrura and C. cornuta, suggesting that these fatty acids are important food requirements for these species.