Strain-related differences in bacterivory and demography of Diaphanosoma mongolianum (Cladocera) in relation to diet and previous exposure to cyanobacteria in nature

We compared the demographic variables and bacterivory of two strains of Diaphanosoma mongolianum from two water bodies in Spain, one without Microcystis (Maidevera in Zaragoza) and the other with dense Microcystis (La Albufera of Valencia). We hypothesized that the strain rarely exposed to Microcystis would be unable to grow on this cyanobacterial diet. We fed both strains Monoraphidium caribeum and Microcystis aeruginosa, together and separately, and compared their demographic variables. Monoraphidium caribeum was cultured in the laboratory on a defined medium, while the cyanobacteria were collected from La Albufera and sonicated before feeding the cladocerans (at 0.5?×?10⁶ cells ml^?1). We also tested the growth of D. mongolianum on bacterial diets by using seston (0–15 µm), bacterioplankton (0–3 µm) and mixed fractions (3–15 µm), from sieving Lake Albufera. We conducted population growth and life table demography experiments at 25 °C, using the two strains of D. mongolianum. Both strains had r (population growth rate) ranging from 0.05 to 0.3 d^?1, on all diets. The r was higher (0.18 d^?1) on the 0–15 µm seston compared to the mixed fraction (0.12 d^?1) although D. mongolianum also grew well on bacterioplankton (0.16 d^?1) alone. The response of the strains collected from two different water bodies was different to the test diets. We found that both strains of D. mongolianum could effectively utilize Microcystis for survival and growth, regardless of previous exposure to the cyanobacteria. The tested cladocerans could also grow well on small sized food particles (0–3 µm and 0–15 µm). Our results explain why D. mongolianum is common in eutrophic water bodies