Influence of phytoplankton fractions on growth and reproduction of tropical cladocerans

The influence of two seston fractions, < 20 μm (nanoplankton) and ≥ 20 μm (microplankton), 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 μm 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 (r) 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 (g) in the microplankton was significant for M. micrura. Fecundity (eggs/total female) 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.