Investigation of annual changes in phytoplankton community structure in a small artificial eutrophic pond was carried out from May 2002 to April 2003. A heavy bloom of Aphanizomenon flos-aquae var. klebahnii Elenk.(Cyanobacteria) persisted in most of the water column from June to the end of October. In November, the A. flos-aquae bloom suddenly crashed and green algae were predominant until the end of spring. Weekly monitoring suggested strong involvement of the changes in abiotic factors in the cyanobacterial bloom degradation. To clarify the effects of pH, water temperature, and day length on the growth of A. flos-aquae, laboratory batch experiments were conducted. The results showed that A. flos-aquae could not grow below pH 7.1 and 11°C, and the growth tended to be suppressed under a 10L:14D photoperiod. pH, water temperature, and day length are vital factors in the growth of A. flos-aquae and, additionally, grazing by cyclopoid copepods also seemed important in bloom collapse.

Populations of the toxic, epiphytic dinoflagellate Gambierdiscus toxicus Adachi et Fukuyo are associated closely with Jania sp. on Hitiaa and Papara fringing reefs in Tahiti. Small populations were also observed to be associated with Amphiroa sp. and Halimeda opuntia (L.) Lamouroux. The cells attached themselves to the thallus by means of a short thread. When the thalli were irradiated, the cells began to detach from them and swim around the branches. The swimming cells stopped and attached to substrata when a disturbance occurred. The attached cells began to swim within a short time under light conditions when the thallus of Jania sp. were placed near the attached cells. Amphiroa sp. and H. opuntia also induced this re-commencement of swimming of the attached cells. These observations suggest that G. toxicus usually swims around macroalgal thalli on coral reefs. When sudden disturbance or strong water motion occurs, they attach to the surface of macroalgae and are not dispersed. Soon after water motion becomes slow, the cells begin to swim into the water around the thalli. The epiphytism of G. toxicus is different from epiphytic pennate diatoms, most of which adhere to the thallus all the time. The population of G. toxicus is maintained as an association to a limited number of species of macroalgae which support the re-commencement of swimming after disturbance.

Seasonal variations in the cell volume, number of cells in a colony and trichome length of nine bloomforming 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.

No abstract

Toxic cyanobacterial blooms have occurred in the near-shore waters of the North Basin of Lake Biwa, Japan, since 1994, and have been attributed to deterioration of water quality in the enriched littoral zone of the lake. From 1997 onwards, the bloom-forming cyanobacteria have been observed with increasing frequency in the deep offshore waters of the North Basin. In the present study, we examined the mechanisms responsible for these bloom populations in the main body of the lake. Specifically, we addressed the hypothesis that buoyant, nutrient-replete colonies of cyanobacteria are generated inshore, are advected offshore by large-scale horizontal transport processes, and subsequently accumulate in the downwelling center of large surface gyres that characterize the overall circulation pattern in the epilimnion of the North Basin. Diel variations of Microcystis biomass at the center and the edge of the Lake Biwa gyre were monitored at 6-h intervals on August 23-24, 2000, and the horizontal distribution of buoyant Microcystis was determined on October 6. The hydrodynamic structure of the first gyre was determined over the preceding 2 days by an on-board Acoustic Doppler Current Profiler (ADCP). The gyre was characterized by a counterclockwise horizontal current that could potentially advect material large distances offshore, a downwelling current near the center of the gyre, and an upwelling current at the edge of the gyre, caused by the radial pressure gradients. The biomass of Microcystis near the water surface was greater at the center than at the edge of the gyre, and the biomass at 5 m depth at the edge of the gyre was greater than that at the water surface or at the thermocline near the edge of the gyre. The results are consistent with the gyre-Microcystis hypothesis, and show the potential for accumulation of large concentrations of cyanobacteria in deep offshore lake environments that are normally considered unsuitable for cyanobacterial blooms.