What Caused the Summer Replacement of Dominant Planktonic Algae in Lake Barato?

Abstract

Dense water blooms of blue-green algae (Microcystis aeruginosa and Apha-nizomenon flos-aquae) had been observed in hypertrophic Lake Barato in the 1980s, but have not reappeared since 1990. Instead of these water blooms, diatoms (Melosira spp. and Cyclotella spp.) have dominated in summer. Orthophosphate (PO4-P) had been detected throughout the year, but since 1990 has apparently become depleted during May and June. In 1991, PO4-P concentrations increased in August and/or September, as did the biomass of blue-green algae (Anabaena spiroides or Phormidium spp.). These observations suggested that diatoms adapted to higher phosphorus limitation under conditions of low water temperature and high silicate concentration. Diatoms thus continued their dominance over blue-green algae because the latter could not grow in early summer due to a lack of PO4-P, most of which was taken up by the diatoms.