Revista Chilena de Historia Natural 77 (1): 73-85, 2004
RESEARCH ARTICLE
Grazing impact on autotrophic picoplankton in two south Andean lakes (Patagonia, Argentina)
with different light:nutrient ratios
Andean ultraoligotrophic lakes are environments with high light:nutrient ratios. In these
lakes a particular planktonic food web has been noticed, constituted by large mixotrophic ciliates which share and compete for food
resources with nanoflagellates and cladocerans. Clearance rates on autotrophic picoplankton of nanoflagellates, the ciliate
Ophrydium naumanni and cladocerans were compared through grazing experiments in lakes Moreno Oeste and
Rivadavia. The lakes exhibited significant differences in the light:nutrient ratio and had different crustacean and zooplankton
compositions. In lake Moreno Oeste the metalimnion was included in the euphotic zone resulting in an illuminated layer where deep
chlorophyll maxima developed. On the contrary, in lake Rivadavia the illuminated layers were restricted to the epilimnion and no deep
chlorophyll maxima were observed. In lake Moreno Oeste, the contribution to total bacterivory of the ciliate O. naumanni
and the cladoceran Ceriodaphnia dubia was observed to increase at 30 m depth, due mainly to the vertical
distribution of both species. On the contrary, the grazing rates of the nanoflagellate assemblage, dominated by the mixotrophic
Chrysochromulina parva, did not change along the water column and were considerably high (one order of magnitude
higher than those obtained for O. naumanni and C. dubia). In lake Rivadavia, nanoflagellate grazing rates
were lower and the relative impact of the nanoflagellate assemblage was comparable to those of O. naumanni and
Daphnia cf .commutata. The observed difference in clearance rates of the nanoflagellate assemblage
probably would reflect an increase in the phagotrophy where light energy is higher relative to phosphorus. In lake Moreno Oeste where
light is not limiting, the observed increase of the phagotrophy by protists may be due to a higher requirement of limiting
elements.
Key words:
planktonic bacterivory,
ultraoligotrophic lakes, mixotrophy, light:nutrient ratio, Ophrydium naumanni, nanoflagellates
