Abstract

In a study from October 1989 to January 1992, a dense population ofCryptomonas cf. erosa (maximum density 38 000 cells mL^–1) consistently developed at the chemocline of Lake Arcas, central Spain,where sharp physical and chemical gradients occurred during stratification. This population developed following the establishment of vertical water stratification and declined when the lake was near autumnal mixis. Population growth in situ, attributable to photosynthetic adaptation to low light intensities rather than to phagotrophy, causes these algal maxima. The population densities and net growth rates of C. cf. erosa in situ were influenced by the coexisting phototrophic prokaryotes at the chemocline, because their shading effect and their role in phosphorus accumulation may impede nutrient diffusion from hypolimnetic waters. C. cf. erosa cells optimize their physiological characteristics by migrating downwards into sulfide-rich waters during the evening to reach higher phosphorus availability, and most migrate upwards in the morning and afternoon to attain sulfide-free waters, where they can photosynthesize without sulfide inhibition. The Cryptomonas population of Lake Arcas suffers intense grazing by aerobic and anaerobic ciliates either above or below the oxic–anoxic boundary. Consequently, grazing pressure on Cryptomonas can not be significantly reduced by migration to anoxic waters.