Microscopy of algae and biomass determination
The abundances of Anabaena, B. submarina and R. salina in the experiments and in the cultures were determined from 2 ml (start) and 50 ml (final) samples, respectively, preserved with acid Lugol’s solution. At least 250 B. submarina or R. salina cells were counted from at least 25 fields. For Anabaena, at least 300 filaments were measured for length per sample. All samples were counted in Utermo¨hl chambers at 125x magnification using a Leica microscope (Utermo¨hl, 1958). The microscopy counts of all algae were converted to carbon according to Olenina et al. (2006) and growth rates of phytoplankton were calculated according to Levasseur et al. (1993).