Zooplankton of Fishless Ponds of Northern Patagonia: Insights into Predation Effects of Mesostoma ehrenbergii.

TROCHINE, C.; MODENUTTI, BEATRIZ E; BALSEIRO, E.G.

International review of hydrobiology

Wiley Interscience

Lugar: Berlin; Año: 2008 vol. 93 p. 312 - 327

1434-2944

Size-selective predation by fish is often considered to be a primary force affecting large-bodied zooplankton. However, changes in the zooplankton size spectrum are still possible in fishless lakes where other predators are present. Previous studies in fishless ponds of northern Patagonia showed that the turbellarian predator Mesostoma ehrenbergii, a common inhabitant of these environments, can consume prey of >1 mm (1.5 mm to 2 mm). Because the feeding strategy of M. ehrenbergii includes mucus trapping and external digestion, this predator may exploit a broad range of prey sizes. We hypothesize that M. ehrenbergii could exert a strong effect on zooplankton body size spectra in Patagonian fishless ponds. We investigated this hypothesis by analyzing the crustacean zooplankton composition and size spectra in five fishless ponds of northern Patagonia, and we carried out experiments to assess predation rates of M. ehrenbergii on potential prey species from 0.8 mm to 6 mm. These ponds were colonized by macrophytes, which favored habitat heterogeneity, especially in the smaller ponds that had the higher richness. These ponds showed distinctive crustacean zooplankton assemblages and sizes, but calanoid copepods of the genus Boeckella dominated the zooplankton assemblages. Our results indicated that M. ehrenbergii may consume the whole range of prey spectra studied, from ~0.8 mm (Ceriodaphnia dubia) to ~6 mm (Parabroteas sarsi). Overall, the magnitude of M. ehrenbergii predation was high for intermediate-bodied zooplankters 0.8 mm to 1.6 mm (Boeckella gracilis, Acanthocyclops robustus and C. dubia). However, we did not find conclusive evidence that variations in size spectra of crustacean zooplankton are a result of M. ehrenbergii predation. We suggest that an interaction between prey body size and its evasion tactic it’s important to determine the true effect Mesostoma on zooplankters.