Hisanori Kagawa

Distribution pattern of the dinoflagellateCeratium hirundinella (O. F. Müller) Bergh in a reservoir

Horizontal distribution of the dinoflagellateCeratium hirundinella (O. F. Müller) Bergh in the Ishitegawa Reservoir, Ehime Prefecture, Japan, was investigated. Water quality was also surveyed. It was observed that the population ofC. hirundinella exponentially decreased in number from the head of the reservoir to the dam site. Further investigation proved thatC. hirundinella initiated growth at the head of the reservoir, and later gradually expanded downstream. It was found during the period of increase in water temperature that the cell density ofC. hirundinella at the uppermost station exponentially increased.

Predicting the summer chlorophylla concentration in a reservoir based on the environmental conditions of the preceding spring

From a monthly survey over a ten-year period (1983–1992) of the Ishitegawa Reservoir, Japan, two multiple regression equations describing the mean chlorophylla (Chl-a) concentration at 0.5 m depth during July, August and September (CS) and the maximum Chl-a concentration in the photic zone, including its subsurface maximum during this period (CM), were obtained. The conductivity at 0.5 m depth in May and the mean air temperature during May or during May and June were used as independent variables. These equations were calculated using seven years of data (1983–1988 and 1992). From 1989 to 1991 two impermeable plastic-coated sheet fences were installed across the upper end of the reservoir along the top 5 m of water column. The equations were used to determine the effectiveness of this flow regulation on the summer Chl-a concentration. In 1989 and 1991, when the fences were in place from June to August, no effects were found on the observed CS and CMvalues. In 1990, when the fences had been in place since October 1989, the observed values were lower than the predicted values.

Contribution of dissolved calcium and magnesium to phytoplanktonic particulate phosphorus concentration at the heads of two river reservoirs

At the heads of two river reservoirs, the Ishitegawa Dam and the Nomura Dam Reservoirs in Japan, the concentrations of phytoplanktonic particulate phosphorus (PP) were compared with those of dissolved calcium (Ca) and magnesium (Mg), using multiple regression analyses on the data taken from samples which registered more than 6.0 µg l−1 in chlorophyll a concentration. Of the 27 monthly samples, 16 from the Ishitegawa Reservoir and 17 from the Nomura Reservoir were used. A significant regression line, logPP = k1F + k2, was obtained, where k1 (> 0) and k2 were constants and F (named the Ca-Mg index) consisted of log(Ca/Mg) − 0.5 log(ca + Mg) in mol concentration in Ca and Mg, in common with the two reservoirs (r2 = 0.730 & 0.913).

Succession of phytoplankton species in indoor reservoir models varying in sediment composition and water inlet site

Using four continuous-flow indoor reservoir models, the combined effects of varying water inlet depth and sediment composition on phytoplankton species composition and its succession pattern were studied.Species which are known to bloom in eutrophicated reservoirs were dominant in the species succession in each tank. At any one time, a step-like pattern was observed in the composition of the dominant species of each tank, wherein all the species which appeared in a tank with a lower phosphorus concentration were included in the species composition of the tank with a higher phosphorus concentration in an additive relationship.

Chlorophyll sedimentation in a deep reservoir in Japan

Sedimentation of phytoplankton has an important ro le in lake ecosystems as a source o f nutrient regeneration and algal recruitment. Positive correlations are found among many lakes between epilimnetic chlorophyll concentration and sedimentary chlorophyll content (GORHAM et al. 1974 ), and between primary production and sedimentation of organic carbon (GALYEZ & NIELL 1993 ). However. in an interannual comparison in the Ishitegawa Reservoir, Japan, we could not find any significant correlations between epilimnetic chlorophyll concentration and sedimentary ch!orophyll content (KAGAWA et al. 2000). In the present study we reinvestigate this relationship, taking into consideration redistribution of bottom sediment.

Effects of plastic-coated sheet fences crossing the upper end of a reservoir on summer lake chlorophyll a concentration

Pre-reservoirs are used as a way of redueing nutrient loading in lakes and reservoirs (e.g. FIALA & VASATA 1982, HOEHN 1994). The dominant proeesses in norma! pre-reservoirs are nutrient uptake by algae and sedimentation, so surfaee release is reeommended at the outlet of a pre-reservoir (BENNDORF & PDTz 1987). However, as a eountermeasure against eutrophieation of a darn reservoir, KAGAWA (1989) proposed a method of subsurfaee release involving an impermeable fenee erossing the reservoir in the upper part near the inflowing river, extending down from the water surfaee to the upper hypolimnion. The intent was to form a strong interflow or underflow of river water below the fenee to faeilitate the release of the hypolimnetie water.