SPATIAL AND TEMPORAL DYNAMICS OF A METALIMNETIC CRYPTOMONAS PEAK

A metalimnetic peak of Cryptomonas phaseolus was studied during several stratification periods in Lake Ciso (Spain). We described the seasonal, spatial and diel dynamics of the peak, and tested the likelihood of several hypotheses to explain the mechanism of formation of the peak and the factors controlling the actual depth of the peak. The metalimnetic peak generally accounted for >60% of the lake phytoplanktonic biomass. Cryptomonas phaseolus was shown to be active at depth, growing at the metalimnion during the spring and remaining there, in a dynamic equilibrium where losses balanced production, through the stratification period. The upper limit of sulfide distribution delimited the exact depth of the peak at noon hours. The peak did not move following optimal light intensities; rather, it followed the depth oscillations of the oxygen-sulfide interface. Low sulfide concentrations were apparently tolerated and the protist moved inside the sulfide-rich part of the lake at night and during the winter. Metalimnetic algal populations behave differently from epilimnetic ones. A single species accounts for most of the biomass and production, and persists during all the stratification period. This indicates that the mechanisms currently believed to account for succession (competition and predation) do not operate efficiently in metalimnetic peaks.