Seasonal variations in the morphology of bloom-forming cyanobacteria in a eutrophic pond

Seasonal variations in the cell volume, number of cells in a colony and trichome length of nine bloom-forming cyanobacteria species were investigated in a small eutrophic pond from May to November 2005. The main genera of cyanobacteria were Microcystis and Anabaena, which formed a dense bloom from July to August. M. aeruginosa, M. viridis and M. wesenbergii were present throughout the study period. M. viridis dominated the Microcystis population (39.2-67.1% of total biovolume) during the pre-blooming period, but M. aeruginosa and M. wesenbergii dominated after July. M. aeruginosa was the dominant species from July to November, constituting 49.0-93.2% of the Microcystis population. Each Microcystis species could always be identified from the cell volume and the number of cells in a colony. The numbers of cells in colonies of M. aeruginosa, M. viridis and M. wesenbergii were in the ranges 37-444, 28-143 and 50-264, respectively. The Anabaena population consisted of three species-A. crassa, A. flos-aquae and A. reniformis. A. crassa and A. flos-aquae were typically present at higher densities than A. reniformis. These species also showed distinctive cell volumes. The number of cells in colonies of A. crassa, A. flos- aquae and A. reniformis were in the ranges 19-178, 18-113 and 29-143, respectively. Planktothrix raciborskii and Raphidiopsis mediterranea appeared in August and Aphanizomenon flos-aquae increased from late October, although these species were less abundant. Cell volumes of Microcystis and Anabaena and trichome length of P. raciborskii were positively correlated with water temperature. Small colonies of Microcystis and Anabaena remained small during the bloom period. In contrast, the trichome length of P. raciborskii seemed to depend more strongly on growth conditions.