In Lake Suwa, various eutrophication control measures have been taken to reduce external loads of phosphorus and nitrogen. As a result, the composition of the phytoplankton in the lake has also changed. Since 1999, blue-green algae have decreased, and Mougeotia have appeared August–December in 2011. DNA analysis of the 18S rRNA region of Mougeotia revealed that Mougeotia in Lake Suwa come from two phylogenetic clades. A genetic distance （p-distance） of 4% indicated they are strongly differentiated and should be recognized as different species. This work has shown that there are two phylogenetic clades of Mougeotia in Lake Suwa, both of which are different from M. scalaris clades, and that many strains isolated from Lake Suwa and Lake Biwa are the same phylogenetic clades. The longer cell length of Mougeotia in January 2012 than in November 2011 and the form of zygospores in January 2012 suggest it should be classified as M. elegantula. Mougeotia generally occurs in oligotrophic to mesotrophic lakes, and was first found in Lake Suwa in the 1960s. The reappearance of Mougeotia in Lake Suwa in 2011 suggests that, as a result of water quality improvement, the lake is transitioning from a blue-green algae dominated eutrophic lake to a mesotrophic lake.
The monomictic Ishitegawa Reservoir (surface area: 0.50 km2; total capacity: 1.28 × 107 m3; mean retention time: 0.33 y) is located on the Ishite River (N 33ﾟ53', E 132ﾟ50') in Ehime Prefecture, Japan. In this reservoir, summer chlorophyll a concentrations (Chl-a) have been observed to decrease gradually from year to year, but the cause remains unclear. To determine the cause, we first examined a monthly data set from January 1983 to January 2003 that contained measures of water temperature (WT), pH, Chl-a, total phosphorus and total nitrogen taken at 0.5 m water depth at the main station in the reservoir (Sta. R). We found two significant long-term linear regressions of pH against date: a positive relationship when pH≤ 8.10 (R2 = 0.099, p < 0.001, n = 123) and a negative relationship when pH > 8.10 (R2 = 0.164, p < 0.001, n = 118). This pattern was also found at a station near the head of the reservoir where river inflow occurs (Sta. H). Low pH (≤ 8.10) was observed mainly in the circulation period (October – March) and high pH (> 8.10) was found during times of stratification (April – September). Next, we examined the relationship of pH = 8.1 with other limnological parameters by using a monthly data set of dissolved inorganic carbon (DIC), dissolved oxygen, and main cation and anion concentrations from February 1993 to January 2003. Concentration of free CO2 (dissolved CO2 + H2CO3) was calculated from DIC, pH, WT and ionic strength by using the dissociation equations of carbonic acid. pH was negatively related to the log partial pressure of gaseous CO2 in equilibrium with free CO2 (R2 = 0.968, p < 0.001, n = 120). At pH 8.1, free CO2 was in equilibrium with atmospheric CO2 (360 μatm partial pressure). Chl-a concentrations during the stratification period (pH > 8.10) decreased from year to year following the gradual decrease in the free CO2 concentration and the gradual increase in pH in the preceding circulation period. This data suggests that partial carbon limitation of phytoplankton growth may have occurred under high pH (> 8.10). In addition, plastic-coated sheet fences that were installed across the upper 5 m of the water column near Sta. H during several intervals in the study period formed a strong interflow of river water below the fences, and may have strengthened carbon limitation at Sta. R.
We documented the distribution and life history of the tapeworm, Proteocephalus tetrastomus (Cestoda, Proteocephalidea, Proteocephalidae) in the pond smelt, Hypomesus nipponensis in Japan. Infection with P. tetrastomus was confirmed in pond smelt populations from 19 of 34 lakes we sampled. The distribution of P. tetrastomus ranged from Hokkaido to the Kinki region, and was not significantly biased to either freshwater or brackish, nor to any trophic status of the lakes. There was no significant negative relationship between the abundance of tapeworms and the condition factor of infected fish in any of the lakes. Year-round sampling in Lake Ogawara revealed that P. tetrastomus grew and developed in the pond smelt from spring through summer. Gravid worms were present from June through December, and small juveniles were first documented in July. The number of juveniles increased up to late autumn and remained high during winter. These seasonal changes suggest that P. tetrastomus is basically univoltine, and the generation turnover occurs primarily from summer to autumn.
Water, salt and phosphorus balance of the 19 years was calculated in Lake Shinji, a shallow brackish lake in Japan. Average annual freshwater inflow of 19 years into Lake Shinji was in the range of the 1.25 ～ 2.35 × 109m3, the average was 1.77 × 109m3. Reverse flow from the Lake Nakaumi located downstream of the Lake Shinji was in the range of 0.32 ～ 0.84 × 109m3, the average was 0.49 × 109m3, corresponding to 27.7% of the freshwater inflow. Retention time in consideration of the amount of freshwater inflow and reverse flow from the Lake Nakaumi were from 47.5 to 76.2 days, and 59.4 days on average. The percentage of annual deposition of TP to inflow TP was in the range of -23.6 to 69.3%, and 20.9% on average. In addition, there was a positive relationship (r = 0.71) between annual TP inflow and sedimentation rate, settlement amount was small drought year when inflow is low. Phosphorus concentrations peaked by released from the sediment in August-September around becomes a normal value almost in November-December. We calculated the ratio of phosphorus resettle to the sediment for the duration from the peak of the TP stock to become normal value, using water amount of outflow from Lake Shinji, inflow load and TP concentration of Lake Shinji. TP sedimentation rate was in the range of 8.8 to 65.6%, and 45.1% of phosphorus which released in summer was resettled to the bottom of the lake again in average, and it was considered to be involved in the release of the following year. In the process of phosphorus concentration decreases, SRP was greatly reduced, but the fact that changes in the PP was hardly seen. The SRP reduction occur when DO at the sediment surface increased at the same time. It was considered that reduction of phosphorus in the water column that released from sediment when there was anoxic is attributed to the SRP adsorption to the sediment surface where become aerobic. At this time sediment is deficiency state of phosphorus because it is after releasing SRP, it was believed to adsorb SRP easily.
Measurements of nitrogen and phosphorus bioaccumulation in the leaves, stems, rhizomes, and spikelets of reeds (Phragmites australis) in the Yatsu Tidal Flat were carried out monthly from June 2013 to January 2014. The reeds increased in length, in stem and rhizome circumference, and in biomass until October. After November, the reeds started to senescence and biomass decreased. Storage of nitrogen and phosphorus in the leaves and stems was higher from August–October than during other months, while storage in the rhizomes (at 10 cm depth from the sediment surface) was low throughout the investigation period. Aboveground nitrogen and phosphorus storage in the leaves, stems, and spikelets accounted for more than 90 % of stores of these nutrients in this study.