Regional distribution and anion component characteristics were investigated for iodine springs in the Hokkaido prefecture, Japan, over the required periodic spring water analysis between 2009-2019. Almost all iodine springs were located in the band along the Japanese Sea side of northern Hokkaido, to the northern and central districts of the Ishikari Plain and the eastern boundary of the plain. These iodine springs were nearly pumped up from wells more than 1000 m deep, reaching the stratum of the Neogene period. Iodide ion concentrations ranged 10-15 mg kg^-1 in approximately half the iodine springs, with the highest concentration being 76.2 mg kg^-1. The majority of iodine springs were characterized by a temperature <34℃, pH of 7.0-8.0, and a dissolved matter concentration (without gaseous components) of 20-30 g kg^-1. There were significant differences in temperature and pH observed between the iodine springs and highly saline non-iodine springs. In the former, the dominant ion concentrations of chloride, sulphate, hydrocarbonate, and bromide were 10-12.5 g kg^-1, <10 mg kg^-1, 100-500 mg kg^-1, and 25-50 mg kg^-1, respectively. A difference in sulphate ion concentration of the four anions was found between iodine springs and the highly saline non-iodine springs. The low sulphate ion concentration of the former suggest that the iodine springs originated from fossil seawater. However, the bromide to chloride ion and the boron to chloride ion ratios were higher in many of the iodine springs than in fossil seawater. This has been attributed to a possible inflow of bromide ions and boron into fossil seawater. Further research is needed to determine the mechanism of forming chemical composition of the iodine springs.

　In this study, the effects of a reed planting project with construction of breakwaters in Lake Shinji were investigated on surface sediments using chemical analysis. Compared with that in areas with and without breakwaters at Rokuonji, the sediment inside the breakwaters exhibited higher organic carbon, nitrogen, and total phosphorus concentrations. At the western shore where the reeds were planted, organic carbon, nitrogen, and total phosphorus levels in the surface sediment were higher on the offshore side than inside the water area in the reed community. Furthermore, the total sulfide concentration inside the breakwaters at Rokuonji was approximately 60 times higher than that of Torigasaki as a reference point.
　In a lake where the bottom surface is disturbed by waves, the grain size is normally sand, and an aerobic environment is maintained. However, comparison between the inside and outside of the breakwater in the Rokuonji area showed that the organic matter and sulfide levels increased and the bottom sediment became more fine-grained in the lakeshore, which is inside the breakwaters. These results suggest that the reeds did not directly cause organic pollution of the surface sediment in Lake Shinji; rather, the change in hydraulic conditions caused by the construction of the breakwater was the causative factor.

　The IPCC (2021) reported that global air temperature has increased by 1.09℃ from the period 1850-1900 to 2011-2020 along with increasing precipitation. Similar increases in air temperature and precipitation have been observed in Japan. Earlier studies on the effects of global warming on inland waters have focused on the thermal effects on physical and chemical processes as well as organisms, and little attention has been given to the thermal effects on ecosystems. Recently, increasing concerns have been raised regarding the effects of global warming and associated heavy rain on inland water ecosystems. In this special issue, seven articles are presented from scientific researches on the impacts of heavy rain induced flooding on the Ozegahara mire ecosystem performed at the 4 th Scientific Research of Ozegahra Mire (2017-2019). I hope that the present seven articles contribute to a deeper understanding of the ecosystem processes in Ozegahara mire under the impacts of flooding.

　The Ozegahara mire is the largest peatland (approximately 8 km²) on the mainland of Japan. The mire is located on an amphitheater-shaped basin surrounded by mountains that rise 2,000 m asl, and the effects of climate change and flooding on the mire ecosystems were studied. The water level of Kaminoohorigawa was highest between 2016 and 2017, exceeding 7 m from the bottom of the stream after heavy rainfall in autumn 2016, and approximately 2 m from the bottom of the stream after heavy rainfall in early spring 2017. Water level of ground surface in Kamitashiro in snowmelt season was 0.2 m, and that at Kenkyumihonen was 1.6 m in snow coverage. The water level during the snowmelt season at Kenkyumihonen was very high at several times. The water level of the bog pool of KA1-08 increased in March 2018 due to surface water and groundwater inflow. The bog pool KA1-04 had early snowmelt due to groundwater inflow from the middle bog pool. Heavy snow flooding in torrents occurred in May 2019 in the Kamitashiro area. After the flooding, the distribution of the muddy water in the bog pool was obtained by air photography by UAV. A high content of inorganic particles was detected in the peatland sediment of 0.9 m depth layer and estimated by historical flooding a millennium ago. Radio cesium from the Fukushima Dai-ichi Nuclear Pwer Plant accident was detected as 5,919 Bq m^-2 at Nakatashiro and 8,594 Bq m^-2 at Kamitashiro. The radioactivity of Kamitashiro in the flooding area was higher than that of Nakatashiro in the non-flooding area because of contaminants from the hydrographic basin.

　The Ozegahara mire is the largest mire (altitude; about 1400 m, length; 6 km, width; 2 km, area; 7.6 km²) in Honshu of Japan. Floods over mire have frequently occurred in the Ozegahara mire under recent climatic changes, resulting pouring flooded waters into many pools. Shore aquatic invertebrates were collected at 12 pools in the Kamitashiro of the Ozegahara mire in October 2018, to estimate the influence of floods on population of macroinvertebrates. The animals were collected by the unit-time sampling method using a hand net. We grouped pools into two types, 5 pools receiving occasional floods (OFPool) and 7 pools receiving frequent flooding (FFPool) based on observation of muddy pools, images by a drone, altitude of pools and invaded fishes into pools by a recent heavy flood (accumulated rainfall; 84 mm, 20-21 May 2019) struck to this mire. Twenty-five taxa appeared in OFPool and 26 taxa in FFPool, determining 27 taxa in all pools studied. Average abundance of order level taxa except Hydrachnellae were low in FFPool, especially total abundance of invertebrates showed significant difference between OFPool and FFPool, and Diptera population also. Hydrachnellae had also an inclination of low abundance in FFPool. Chironomidae (Diptera) indicated significantly low population in FFPool, especially Tanypdinae, indicating the strong influence of floods on this taxon. Biomass of families excluding Chironomidae did not indicate significant difference between OFPool and FFPool. Appearance of Notonecta reuteri reuteri, Chaoborus sp. and Setodes sp. were rare in FFPool. Flooded waters disturbed vigorously shoreline of pools. It is estimated that the lowering of the shore aquatic invertebrate abundance is probably caused by loss of animals in pool water, detritus, dead leaves and stems with attached algae and fungi with animals accumulating at the pool shore. Predation by fishes invaded to pools through floods may also decrease population of shore aquatic invertebrates.

　The Ozegahara mire is the largest raised peat bog in central Japan. Nymphaea tetragona and Nuphar pumila var. ozeensis were the dominant floating-leaved plants in the bog pools. N. tetragona is known to have specific spatial distributions in pools. Forty pools in the Kamitashiro area were surveyed in August 2017 to document the present status of macrophytes. To clarify the factors that cause these specific spatial distributions of N. tetragona in the pools, the growth status of N. tetragona (coverage index, number of floating leaves, and width of leaf blades) and bottom sediment properties (fabric matter, ash, and total phosphorus content) were measured in three study pools in the Kamiatashiro area in August 2018. The survey revealed that an increase in N. tetragona and a decrease in N. pumila var. ozeensis has continued since the 1970s. The results from the three study pools showed that there were no clear differences in the growth status of N. tetragona (coverage index and number of leaves) and in the bottom sediment properties (fabric matter, ash, total phosphorus content, and the thickness of the decomposed peat) between the shore side bottoms and at the central bottoms in the study pool where N. tetragona was distributed all over the bottom (overall type). On the other hand, an obvious increase in the growth status of N. tetragona and the values of the bottom sediment properties were observed toward the central bottoms in the study pools, where N. tetragona was distributed all over the pool, avoiding the shore side bottom (overall without shore side type). The results indicated that the absence of N. tetragona on the pool bottoms was attributed to sediment nutrient infertility.

　Cladoceran fauna were studied in four pools in the Kamitashiro of the Ozegahara mire from May 2018 to November 2019. Eight species belonging to eight genera were identified in the samples collected. Diaphanosoma brachyurum was widely distributed widely in the pools in the Kamitashiro. The species appearing between the pools were highly similar, suggesting the possibility that the eggs of Cladocera had been resting, but were diffused by snowmelt floods. The appearance time and population density of Cladocera vary greatly depending on the pool, and the annual variation is large, suggesting that the pools in the Ozegahara mire is an unstable environment.
　Some of the pools, which are thought to have been invaded by fish owing to the flood, had extremely low population densities of Cladocera. This was potentially due to the invasion of the pool by fish, which would have a devastating effect on Cladocera.

　To clarify the genetic characteristics of the white-spotted char, Salvelinus leucomaenis, which inhabits the Shimoyosaku tributary (which is isolated from the main stream during the normal stage of water), we performed mitochondrial DNA cytochrome b region analysis and microsatellite analysis of 35 individuals sampled from both groups of the isolated tributary (Shimoyosaku) and the main stream. mtDNA cytochrome b region analysis confirmed both groups to have only one haplotype that has been observed in populations of the North Kanto area. Genetic diversity of the isolated group was lower than that of the mainstream group, and gene divergence was greater between groups, suggesting that gene flow between groups was extremely low.

　Aquatic humic substance (AHS) in bog pools is one of the important carbon constituents in peatlands. To understand AHS dynamics in bog pools, we measured the quantity and quality of AHS in surface water from 39 pools in Ozegahara Mire, central Japan, in August 2015 and 2016. The mean carbon concentrations in dissolved organic matter (DOM) and AHS were 6.9 mg-C L^-1 and 5.0 mg-C L^-1, respectively, and AHS formed about 70 % of total DOM. Two AHS-like components were detected using a EEM–PARAFAC: Oze-1 (Ex/Em < 252 and 336/485 nm) and Oze-2 (Ex/Em < 252 and 309/406 nm). Although AHS, Oze-1, and Oze-2 concentrations were significantly lower in 2015 than in 2016, AHS proportions and the Oze-1/Oze-2 ratio did not differ significantly between years. Therefore, the percentage of AHS in DOC and the relative AHS composition are likely balanced in terms of supply and removal. Additionally, AHS, Oze-1, and Oze-2 concentrations were significantly positively correlated with electrical conductivity and bacterial numbers, indicating that AHS was one of main dissolved materials and considered to be a crucial energy source for bacteria. These concentrations were also positively correlated with the ratio of pool perimeter to pool area (P/A), and negatively correlated with dissolved oxygen (DO), pool area and pool perimeter. These results suggested that the bog pools with longer perimeter contacting the surrounding mire for pool area would be subjected to more AHS seepages, and that photooxidative AHS degradation in the surface water would be promoted by increased pool size. Such the steady state between AHS loading and removal would determine the AHS contents in the bog pools.

　As a part of the 4^th Scientific Research of Ozegahara, physicochemical characteristics and nitrogen metabolism of the peat soil were studied at 39 sites in Ozegahara (July 2017–August 2018). The survey data indicated that the peat soils almost saturated with water were slightly acidic and exhibited high denitrification activity in the soils containing appreciable amounts of NH₄⁺ and NO₃^-. The wet hollow mire sites of the bank-hollow complex near the Yoppi River are characterized by abundant occurrence of Myrica gale var. tomentosa on the mires with abundant available phosphorous in the soils. In this bank-hollow complex, significant correlations were detected among nitrogen metabolic activities, NO₃^- concentration, redox potential, and water level. We also found a significant correlation between nitrogen fixation in the rhizome system of The Myrica and its shrub height in hollow mires. Analysis of these survey data suggested that the active nitrogen fixation activity of The Myrica at the hollow mires under flood-associated phosphorus supply might be a possible cause of the recent extensive increase in The Myrica abundance in Ozegahara. Further detailed studies are needed on the process of flood-associated phosphorus supply to Ozegahara and its effects on nitrogen fixation activity in the rhizome system of The Myrica, as well as nitrogen cycle in the Ozegahara ecosystem.