During morphological studies of microorganisms using SEM, chemical processing of samples often results in artificially modified microstructures. The water freeze-drying method is a unique process that can be used to prepare SEM samples without using artificial chemicals. The results of freshwater cyanobacteria sample preparation using this method were compared to those of samples fixed in advance using glutaraldehyde and samples where no fixation was performed. The internal structures of the fractured samples were also observed. The comparison between samples confirmed that, in many species, the surface shape and internal structures were well preserved without using chemical fixation, and that cell components, such as thylakoids in Oscillatoria and gas cells in Microcystis, could be observed in a shorter time than using conventional methods. We found that the mucilaginous sheath of Microcystis aeruginosa and Microcystis viridis were partitioned by bulkheads, and contained cells. The time required for specimen preparation using the water freeze-drying and fracturing method is approximately 1 hour. The method not only preserves fine structures well, it also has the advantage of simple and safe sample preparation compared to conventional methods.

　Harmful algal blooms of certain nuisance cyanobacteria are observed almost every year in Lake Oshima-ohnuma, located in Hokkaido, Japan. However, an unusual milky-white bloom was observed in September 2015. The causative organism of this milky white bloom was an algae-lysing protist, Asterocaelum sp., feeding on Dolichospermum planctonicum, which caused the cyanobacterial bloom. The bloom of the cyanobacterium D. planctonicum was recognized (8.6 × 10³ cells mL^-1) before the milky-white bloom and the number of Microcystis aeruginosa, a cyanobacterium, increased (1.2 × 10⁴ cells mL^-1) after the milky-white bloom. Microscopic observation revealed seasonal fluctuations in the cell numbers of the algae-lysing protist and cyanobacteria. Quantification in this study confirmed the hitherto-known information that members of the genus Asterocaelum feed on D. planctonicum and cause the milky-white bloom. The observation on the number of Asterocaelum sp. as well as that of cyanobacteria is newly proposed to understand the mechanism of harmful algal blooms in freshwater ecosystems better.

　I investigated the life cycle and habitat of the larvae and pupae of Rhyacophila nigrocephala in middle reaches of the Kino-kawa River, Nara, Japan. Monthly occurrence of larvae, pupae, and adult suggested that this species was bivoltine: the wintering generation started in November 2014 and terminated in May and June 2015, whereas the non-wintering generation started in June and terminated in October 2015. The larvae of this species were predominately found in hyporheic habitat of the riverbed. However, a few of the first instar larvae, which hatched from eggs just before sampling, were collected from the surface of cobbles in June. The later stage of the fifth instar larvae and most pupae also were collected from the surface of cobbles. These results indicate that this burrowing caddis larvae used the surface of the riverbed temporally after hatching from eggs and before pupation.