Feeding of Boeckella gracilipes (Copepoda, Calanoida) on ciliates and phytoflagellates in an ultraoligotrophic Andean lake

In Andean lakes a distinctive assemblage of large mixotrophic ciliates (>80 μm), dominated by the peritrich Ophrydium naumanni Pejler and the heterotrich Stentor araucanus Foissner & Wölfl, has been previously reported (Wölfl, 1995; Modenutti, 1997; Queimaliños et al., 1999; Modenutti et al., 2000). These two species have been found only in large, deep ultra-oligotrophic lakes (area >5 km2, Zmax ≈100 m), where they develop large populations. In previous studies of Lake Moreno Oeste (Queimaliños et al., 1999; Modenutti et al., 2000), O. naumanni dominated the ciliate assemblage, showing a marked vertical distribution pattern during the early summer months. The symbiotic Chlorella of this ciliate caused a deep chlorophyll maximum at 30 m depth, contributing up to 90% of the total photosynthetic biomass (Queimaliños et al., 1999). In spite of the great importance of this source of energy for the planktonic food web, direct predation on Ophrydium cells, which would cause Chlorella mortality, has not been observed. Freshwater calanoid copepods (Epischura and Diaptomus) can efficiently remove several species of ciliate oligotrichs (Burns and Gilbert, 1993). Therefore, by feeding selectively at high rates, calanoids may suppress populations of some ciliates and thereby influence microzooplankton community structure (Burns and Gilbert, 1993). Recently, top-down effects that extended to the level of flagellates and bacteria have been studied in two species of Boeckella, B. hamata and B. dilatata, in New Zealand lakes (Burns and Schallenberg, 1996, 1998). These results revealed a stronger grazing pressure on protozoa exerted by copepods than by cladocerans (Burns and Schallenberg, 1998). In Araucanian lakes of Argentina and Chile, Boeckella gracilipes Daday is the most important zooplankton in terms of frequency of occurrence and density (Zúñiga, 1988; Bayly, 1995; Modenutti et al., 1998). Therefore, we expect that this copepod would graze on O. naumanni in Andean lakes. In the present study we analyse vertical copepod distribution in relation to phytoplankton and ciliates in the water column of Lake Moreno Oeste. Through laboratory experiments and studying mouth-part morphology