Grazing impact on autotrophic picoplankton in two south andean lakes (Patagonia, Argentina) with different light:nutrient ratios.

BALSEIRO, E.G; QUEIMALINOS, CLAUDIA P; MODENUTTI, BEATRIZ E

REVISTA CHILENA DE HISTORIA NATURAL

Sociedad Chilena de Biología

Año: 2004 vol. 77 p. 73 - 85

0716-078X

Andean ultraoligotrophic lakes are environments with high light:nutrient ratios. In these lakes a particularplanktonic food web has been noticed, constituted by large mixotrophic ciliates which share and compete forfood resources with nanoflagellates and cladocerans. Clearance rates on autotrophic picoplankton ofnanoflagellates, the ciliate Ophrydium naumanni and cladocerans were compared through grazing experimentsin lakes Moreno Oeste and Rivadavia. The lakes exhibited significant differences in the light:nutrient ratioand had different crustacean and zooplankton compositions. In lake Moreno Oeste the metalimnion wasincluded in the euphotic zone resulting in an illuminated layer where deep chlorophyll maxima developed. Onthe contrary, in lake Rivadavia the illuminated layers were restricted to the epilimnion and no deepchlorophyll maxima were observed. In lake Moreno Oeste, the contribution to total bacterivory of the ciliateO. naumanni and the cladoceran Ceriodaphnia dubia was observed to increase at 30 m depth, due mainly tothe 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 wereconsiderably high (one order of magnitude higher than those obtained for O. naumanni and C. dubia). In lakeRivadavia, nanoflagellate grazing rates were lower and the relative impact of the nanoflagellate assemblagewas comparable to those of O. naumanni and Daphnia cf .commutata. The observed difference in clearancerates of the nanoflagellate assemblage probably would reflect an increase in the phagotrophy where lightenergy is higher relative to phosphorus. In lake Moreno Oeste where light is not limiting, the observedincrease of the phagotrophy by protists may be due to a higher requirement of limiting elements.