Can small cladocerans depress rotifer densities? A field study.

MODENUTTI, BE; BALSEIRO, EG; QUEIMALIÑOS, CP

Barcelona

Congreso; XXV SIL Congress; 1992

The population fluctuations of rotifers and small cladocerans were analysed in Laguna Ezquerra, a small temperate lake of the South Andes (Argentina). The varying phytoplankton concentration would determine alternative situations in this relationship. According to food availability (measured as biovolume of algae less than 20 µm) the whole period was divided in two phases. During the first phase a direct non-linear relation was obtained between log of cladoceran biomass and algal biovolume. This implies that cladocerans are food limited, since their biomass depends on food resource. A clear negative relationship between log cladoceran biomass and log rotifer density was obtained (R2 = 76%). Under conditions of food limitation this relationship would imply a negative effect of cladocerans over rotifers or mutual suppression. A direct relation between resources and rotifer density is unlikely since they share resources with cladocerans. An inverse relation between rotifers and total available resources was in fact found. Rotifers respond positively to increased food supply when it is accessible and this happens when cladoceran biomass drops. An inverse relation between residuals of the log cladoceran biomass-biovolume relation and log rotifer density was obtained (r = 0.55; p < 0.001; with 4 days of time lag r = 0.6). In the second phase the biovolumen greatly increased. Both rotifer density and cladoceran biomass responded positively. A positive relation between log cladoceran biomass and rotifer density was obtained (p < 0.05). Neither cladoceran biomass nor rotifer density was significantly related to biovolumen (p > 0.05). Our results indicate that the different relationships established between rotifers and small cladocerans are a consequence of varying food levels.