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Rotifers are widespread in virtually all aquatic habitats and play an important role in the pelagic food web. As is the case with ciliates, most of the more than 2000 rotifer species are suspension feeders, grazing upon bacteria, small algae and heterotrophic protists and serve as important food for higher trophic levels (Gliwicz, 2003).
The genus Cephalodella (Monogononta: Notommatidae) is among the most species-rich genera of the phylum Rotifera, with approximately 190 described species (Segers, 2007). This genus is characterised by great phenotypic similarity, rendering taxonomic identification difficult (Nogrady & Pourriot, 1995). As a consequence, our understanding of the distribution and ecology of many species is blurred by misidentifications and doubtful records, and new species are to be expected even from well-explored regions (Jersabek, Weithoff & Weisse, 2011). Jersabek et al. (2011) recently described the new species Cephalodella acidophila Jersabek, Weithoff & Weisse from highly acidic mining lakes (pH < 3) in East Germany (Lake 130) and northern Austria (Lake Langau). This species occurs in man-made habitats at low to moderate abundance (Weithoff et al., 2010; Moser & Weisse, 2011b). In the Lake Langau, the population density ranged from five individuals per litre in summer to 22 individuals per litre in autumn; the peak was recorded when temperature was below 10 °C (Moser & Weisse, 2011b). Phytoplankton biomass in the mixolimnion of this lake is relatively constant, with seasonal variations ranging from 0.17 to 0.36 mg C L−1 (Moser & Weisse, 2011b); higher phytoplankton biomass was recorded in the monimolimnion of L. Langau.
As is the case for natural volcano lakes, man-made acidic mining lakes are extreme aquatic habitats characterised by iron, sulphate and heavy metal concentrations several orders of magnitude higher than in circumneutral lakes (summarised in Geller, Klapper & Salomons, 1998). Accordingly, biodiversity in these lakes is greatly reduced, and rotifers may represent the dominant or even sole metazoan taxon in the simplified planktonic food web (Gaedke & Kamjunke, 2006; Weithoff et al., 2010; Moser & Weisse, 2011b). The present investigation is part of a larger project investigating whether the pelagial of acidic mining lakes is primarily colonised by pH specialists or generalists (Moser & Weisse, 2011a,c; Weisse et al., 2011) and to determine what factors control these plankton communities. Secondly, we used acidic mining lakes as a suitable ecosystem model to test for the significance of strong habitat selection (Weisse et al., 2011).
General aspects of the ecology of Cephalodella sp. (a closely related strain to C. acidophila) such as growth, reproduction and feeding have been investigated with a strain isolated from Lusatia, East Germany (Kamjunke et al., 2004; Weithoff, 2004, 2005, 2007; Weithoff & Wacker, 2007). The strain used in these studies was an as yet unidentified species that was originally misidentified as C. hoodi.
The response of the rotifer community to decreasing pH has been studied mainly in moderately acidified (pH 4–7) North American lakes (Yan & Geiling, 1985; Brett, 1989; Frost et al., 1998). From these studies and similar investigations in Sweden (Berzins & Pejler, 1987), it is known that the occurrence and abundance of rotifers in lakes are confined by pH (Wallace et al., 2006; Segers, 2007). The distribution of rotifers in highly acidic (pH < 3) lakes has been investigated in Lusatia, East Germany (reviewed by Deneke, 2000). The effect of pH on various life history parameters has been studied experimentally with five Brachionus species over the pH range 5–10 (Yin & Niu, 2008). An experimental investigation on the pH response of rotifers at lower pH is, to our knowledge, still lacking.
We used a C. acidophila clone isolated from a small acid mining lake at Langau, Austria, to investigate whether this species is an acidophil (i.e. specifically adapted to the harsh environmental conditions prevailing in acid mining lakes). Alternatively, C. acidophila may be acidotolerant, taking refuge from competitors or predators that are more dominant in circumneutral or weakly acidified lakes. Since the investigations cited earlier revealed that seasonal temperature variation in these small lakes may range from 0 to 30 °C and food is generally scarce, we investigated the interactive effect of pH, temperature and food level on the fitness of the rotifers. Our hypothesis was that the combined stress effect of low pH, extreme temperature and limiting food supply would confine the ecological niche of C. acidophila to a relatively narrow habitat range compared to more common rotifers. The significance of the interactive effect of the two abiotic factors for the width of the realised niche width has already been demonstrated for the dominant flagellate species in these acidic mining lakes (Moser & Weisse, 2011a). We used life-table experiments to monitor the survival, reproduction and hatching success of the rotifers.