To understand the effects of climate change on lake water quality, we quantitatively determined long-term changes in water quality by simulating future water quality predictions for Lake Kojima in Okayama Prefecture, Japan. For future meteorological data, we used data from two scenarios, Shared Socioeconomic Pathways (SSP) 1-2.6, which has a small degree of climate change, and SSP5-8.5, which has a large degree of climate change. As a result of simulating the current 5 years and the next 30 years until 2050, there is a period in which the concentration of COD increases significantly, and the magnitude of the concentration change is equivalent to 5% of the average of the current period. This degree is about the same as the improvement target in the lake water quality conservation plan, and it is considered that the effects of the water quality conservation measures by the local government will be offset. In addition, regarding chlorophyll a, which represents the amount of phytoplankton, the number of days when the concentration of Chl.a exceeds 70 μg L^–1 will be larger than that in the current situation.

We aimed to evaluate the impacts of the water environment and nitrogen removal on paddy fields planted with forage rice in the research field of the Prefectural University of Kumamoto. Water balance (inflow: irrigation and precipitation; outflow: surface runoff, evapotranspiration, and underground seepage) and nitrogen dynamics (inflow: irrigation, precipitation, and fertilizer; outflow: surface runoff and underground seepage; others: plants and uncollected fractions of nitrogen) were investigated for two years (2019 and 2020) in paddy fields planted with forage rice. We found that most of the nitrogen load to the water environment in paddy fields planted with forage rice was attributable to nitrogen outflow caused by groundwater seepage, and that the sums of groundwater seepage and surface runoff were 9.1 kg N ha^-1 in 2019 and 34.1 kg N ha^-1 in 2020, which are very small compared with the nitrogen inflow in paddy fields. Higher than 90% of the nitrogen inflow was removed in paddy fields planted with forage rice. In 2020, 184.8 kg N ha^-1 (about 40% of the total nitrogen inflow) was removed by denitrification and 163.1 kg N ha^-1 (about 40% of the total nitrogen inflow) by forage rice harvest.

Compared with those in lakes and sea areas, the increase in the amount of organic matter (internal production) caused by algal growth in rivers is poorly understood. Therefore, to clarify the effect of internal production in rivers on water quality, we investigated the stagnant section of Hanamune River, where significant algal growth is observed, focusing on flow conditions and the elemental composition of particulate matter from which algae are fractionated. As a result, we were able to classify algae observed in rivers into the proliferative and inflow/mixed types depending on pH and dissolved oxygen saturation level. In addition, elemental composition ratios suggested that algae accounted for much of the particulate matter in the stagnant section of Hanamune River. From this, the term ‘biochemical oxygen demand (BOD) derived from algae (BOD_-Algae) ’ was defined as the BOD originating from particulate matter (P-BOD) in the stagnant section of the Hanamune River. By using chlorophyll-a as an indicator of algal abundance, the formula［BOD_-Algae (mg L^-1) ］ = 0.24 × ［Chl-a (μg L^-1) ］^0.66) was derived. From the obtained results, it was possible to quantitatively understand the effect of river internal production on BOD.