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Abstract: In order to understand the role of nitrification and denitrification in the accumulation of nitrous oxide (N2O) in the hypolimnetic water of brackish Lake Nakaumi, the effects of dissolved oxygen (DO) concentration on these activities were investigated by incubation experiments. N2O was produced during the oxidation of NH4+ to NO2− in nitrification and during the reduction of NO3− to N2 in denitrification. N2O-producing activity by nitrification (N2ON) increased markedly with decreasing concentrations of DO. Low DO (10%–30% saturation) induced high N2ON. In contrast to nitrification, N2O-producing activity by denitrification (N2OD) decreased with decreasing concentrations of DO. Little N2O was accumulated during denitrification under low-level conditions of DO (10%–30%), because of further reduction of N2O to N2. It can therefore be assumed that N2O produced as the by-product of nitrification is concurrently reduced to N2 by denitrification under low-DO conditions. This would result in no substantial accumulation of N2O during active nitrification in the hypolimnetic water of Lake Nakaumi.

Abstract: Diel variations in urea decomposing activity in the euphotic zone of brackish Lake Nakaumi were measured under fixed light intensity. The decomposition rate of urea was 17 to 44 μ mol urea m−3 h−1 in the light and 10 to 27 μ mol urea m−3 h−1 in the dark. Higher decomposition rates were obtained in the upper euphotic zone. A clear diel periodicity in the urea decomposition rate was observed, with high rates from 1200 to 1600 and low rates from 0000 to 0400. Chlorophyll a specific decomposing activity ranged from 12 to 21 μg urea C mg chl.a −1 h−1 in the light and 7 to 13 μg urea C mg chl.a −1 h−1 in the dark. In the light, high values were obtained from 1600 to 2000 and low values from 0400 to 0800. The diel change in specific decomposing activity exhibited a similar pattern to that of the photosynthetic assimilation number, following the diel change in photosynthetic activity.

Abstract: The hydrazinium reduction technique has so far been inapplicable to the determination of nitrate in brackish water and seawater due to interference from magnesium ions. We developed a pretreatment method for brackish water and seawater samples for the determination of nitrate based on the hydrazinium reduction technique. Magnesium ions in water samples were converted to a precipitate of a complex with oxine (8-quinolinol) at pH 9.5, and then the precipitate was centrifuged at 3000 rpm for 20 min. The extra oxine in the resulting sample (the supernatant liquid), which inhibits the reduction of nitrate to nitrite, was removed using a Sep-Pak C18 cartridge. Thus we achieved the preparation of a magnesium-free water sample. Using the hydrazinium reduction technique with the proposed pretreatment method, nitrate in brackish water and seawater as well as fresh water was quantitatively determined with high accuracy.

Abstract: The influence of brackish phytoplankton cell classes upon the response of urea decomposition was investigated in Lake Nakaumi. The urea decomposition rate was 5 to 350 μmol urea m−3 h−1 in the light and 3 to 137 μmol urea m−3 h−1 in the dark. The urea decomposition rates in the light were obviously higher than in the dark. An extremely high rate (350 μmol urea m−3 h−1) was observed in Yonago Bay. The rate in the smaller fraction (<5 μm) exceeded that in the middle (5–25 μm) and larger fractions (>25 μm). The chlorophyll- and photosynthesis-specific rates for urea decomposition in the light were 0.5 to 3.9 μmol urea mg chl.a −1 h−1 and 0.3 to 1.3 μmol urea mg photo.C−1. The specific urea decomposing activities were higher in the smaller fraction than in the other two fractions. The present results suggest that in brackish waters urea decomposition occurred with coupling to the standing crop and photosynthetic activity of phytoplankton.

The significance of a halocline for nutrient cycles in a shallow coastal lagoon, Lake Nakanoumi, was investigated. The study was made possible when the northern part of the lagoon was enclosed by polder dikes prior to land reclamation for agricultural purposes. The disappearance of the halocline allowed vertical mixing to extend to the whole depth and vertical gradients in water properties then decreased remarkably. Despite the disappearance of the halocline, dissolved oxygen content of bottom waters decreased considerably during summer months. Summer peaks in inorganic nitrogen and phosphate were still observed after disappearance of the halocline.

Twenty-four patients with myasthenia gravis of Ossennan’s generalized type underwent extended thymectomy through a sternal-splitting approach. Their clinical responses to thymectomy and postoperative changes in anti-acetylcholine receptor antibody titers were evaluated. The followup time ranged from 1 month to 7 years and 7 months (average, 36 months). Six patients (25%) had remissions and 17 patients (71%) were improved after operation. The preoperative anti-acetylcholine receptor antibody titers dropped significantly after operation (p < 0.001). The postoperative reduction in these titers correlated with the time course after operation (p < 0.05). Their postoperative reduction was significantly greater in the six patients having remissions than in the 15 having marked (p < 0.02) and the six having moderate improvement (p < 0.005). This study has revealed that anti-acetylcholine receptor antibody titer in plasma declines progressively after thymectomy, and the postoperative reduction of this titer correlates with the clinical effect of thymectomy.

Abstract: In the monthly zooplankton survey in Lake Nakaumi, Shimane Prefecture, conducted since July 1997, Cyclopina kiraensis occurred in the samples in July 1997 and July 1998. This species had not been recorded since the original description from the mouth of the Yahagi-furukawa River, Aichi Prefecture, in 1984. This paper is the second record of the species and is the first report of the species from the coast of the Sea of Japan. We describe environmental conditions of the habitat and the characteristics distinguishing this species from the morphologically allied species, Paracyclopina nana, which co-occurred in Lake Nakaumi.