Ecology of Cryptomonas at the chemocline of a karstic sulfate-rich lake

In a study from October 1989 to January 1992, a dense population of Cryptom onas cf. erosa ( maximum density 38 000 cells mL(-1)) consistently develope d at the chemocline of Lake Arcas, central Spain, where sharp physical and chemical gradients occurred during stratification. This population develope d following the establishment of vertical water stratification and declined when the lake was near autumnal mixis. Population growth in situ, attribut able to photosynthetic adaptation to low light intensities rather than to p hagotrophy, causes these algal maxima. The population densities and net gro wth rates of C. cf. erosa in situ were influenced by the coexisting phototr ophic prokaryotes at the chemocline, because their shading effect and their role in phosphorus accumulation may impede nutrient diffusion from hypolim netic waters. C. cf. erosa cells optimize their physiological characteristi cs by migrating downwards into sulfide-rich waters during the evening to re ach higher phosphorus availability, and most migrate upwards in the morning and afternoon to attain sulfide-free waters, where they can photosynthesiz e without sulfide inhibition. The Cryptomonas population of Lake Arcas suff ers intense grazing by aerobic and anaerobic ciliates either above or below the oxic anoxic boundary. Consequently, grazing pressure on Cryptomonas ca n not be significantly reduced by migration to anoxic waters.