Temporal variability in the concentration of toxic heptapeptide microcystin was studied ( ) ( ) during the warm season of four years 1991 -1994 in a hypertrophic lake Lake Suwa in central ( ) ( Honshu, Japan. Lake water samples ca. 5 L were filtered to separate intracellular microcystin cell ) ( ) fraction from extracellular microcystin filtered lake water fraction . These fractions were analyzed to measure the total quantity of microcystin in lake water. Total amounts of extra-and intracellular microcystin were measured with high performance liquid chromatography. Concentrations of intracellu-( ) lar microcystin usually exceeded concentrations of extracellular microcystin 24 out of 26 times . High concentrations of intracellular microcystin were found during the exponential growth phase of the blooms, whereas concentrations of extracellular microcystin were highest at the end of the blooms.( ) However, concentrations of extracellular microcystin remained very small -4 g / L compared to the levels of intracellular microcystin. The relatively higher percentages of microcystin in filtered lake water ( ) )20% at the end of blooms suggests that release of microcystin from cells occurs during senescence and the decomposition period of Microcystis cells.

Seasonal changes in species composition of Microcystis and concentrations of toxic heptapeptide microcystins, were investigated in Lake Suwa from June to October in 1991. Microcystins-RR and -LR were the main components of the toxins contained in bloom samples of Microcystis and a very little quantity of -YR was detected through this period. The high amounts of microcystins were estimated during the exponential growth phase of the bloom from June 11 to July 20. The highest concentrations of microcystins-RR and -LR were estimated as 121 and 81.6 pg/lOO mg cells on July 20, 1991, respectively. While the higher amounts of the toxins were estimated when M . aeruginosa predominated, the lower amounts were estimated during the predominate period of M. viridis. Since the dissolved inorganic nitrogen (DIN) concentration may well affect the dominance of M. aeruginosa and M. viridis, the production of the toxins by Microcystis may be associated with the DIN concentration. 0 1993 John Wiley & Sons, Inc.

1. Field and laboratory experiments were conducted to examine the effect of larval retreats of the net-spinning stream caddisfly Hydropsyche orientalis on abundances on flow exposed stone surfaces of nymphs of the mayfly Ephemerella setigera, which prefer slow flow conditions, and to determine whether the construction of retreats ameliorated the habitat and made it more suitable for E. setigera. 2. In a field experiment, artificial substrates with retreats of H. orientalis had higher E. setigera abundances than substrates lacking retreats. In addition, abundances of E. setigera nymphs increased significantly with those of H. orientalis larvae on the upper surface of boulders in streams. 3. The drift loss of E. setigera from plates, with and without retreats, was investigated along a current velocity gradient in a laboratory channel experiment. Nearly all E. setigera nymphs remained on the plates with retreats, even at the highest current velocity. In contrast, on the plates without retreats, the drift loss of E. setigera nymphs increased as the current velocity increased. 4. These results suggested that the habitat amelioration by H. orientalis retreats provided a refuge location for E. setigera nymphs and increased their abundances on stone surfaces exposed to flow forces.