Zooplankton of fishless ponds of northern Patagonia: insights into predation effects of Mesostoma ehrenbergii

TROCHINE, C., BALSEIRO, E. & MODENUTTI, B.

INTERNATIONAL REVIEW OF HYDROBIOLOGY

Wiley-VCH

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

1434-2944

The turbellarian predator Mesostoma ehrenbergii, a common inhabitant of fishless ponds of northern Patagonia, can consume prey larger than 1 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 composition and 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 higher species richness. The surveyed ponds showed distinctive crustacean zooplankton assemblages and sizes, but all were dominated by calanoid copepods of the genus Boeckella. Our results indicated that M. ehrenbergii consumed the whole size range of offered prey, from ~0.8 mm (Ceriodaphnia dubia) to ~6 mm (Parabroteas sarsi). Predation rates were higher for intermediate-bodied copepods (~ 1.5 mm) and C. dubia (0.8 mm), but 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 might be important to determine the true effect Mesostoma on zooplankters. The structural complexity created by macrophytes in Patagonian fishless ponds may also help account for the lack of a strong predation effect of Mesostoma in the field survey.