[H-TT16] Development and application of environmental traceability methods

[HTT16-P09] Nutrient dynamics in recharged paddies : Focusing on Water-Aquatic Insect Relationships

*Hirota KATSUDA¹, Jing ZHANG¹, Osamu INAMURA², Kou OURA¹, Yuika KITAZAWA¹ (1.UNIVERSITY OF TOYAMA, 2.Uozu aquarium)

Keywords:carbon stable isotope, nitrogen stable isotope, water stagnation, Aquatic insect

Recently, the use of snowmelt water in winter and changes in land use in Toyama Prefecture has caused a decline in the groundwater level. It has also been noted that the amount of terrestrially derived nutrients transported to Toyama Bay has decreased due to climate changes such as less snow and more rain and improved wastewater treatment technology (Katazakai and Zhang, 2021). In Toyama Prefecture, groundwater recharge using fallow paddies is being promoted as a countermeasure to the declining groundwater level. In addition, as an adaptation measure to compensate for reduced land-derived nutrients, verification of the effects of nutrient addition is underway, focusing on groundwater recharge. Water bodies such as fallow fields are inhabited by a wide variety of organisms, and material exchange between land and water is mediated by these organisms. Therefore, a detailed understanding of nutrient cycling with ecological considerations is necessary. However, knowledge of food chains and material cycling in water stagnation systems is limited. In this study, we selected three ponds of different depths, the Betsumata Recharge Pond and Nature Observation Pond in Betsumata Valley, Uozu City, Toyama Prefecture, and measured the stable isotope ratios (δ¹³C and δ¹⁵N) of carbon and nitrogen in aquatic organisms to determine their food web structure. Aquatic insects were collected in June and October 2023 using hand net. Plants, sediments, lichens, and algae were also collected as potential food sources. Water from each pond was also collected and measured for major dissolved constituents, hydrogen oxygen stable isotope ratios, and nutrients. There were no significant differences in the δ¹⁵N of aquatic insects in each pond. The δ¹³C of some species differed between survey periods. Even within the same species, δ¹³C differed significantly among individuals. This difference might be caused due to the differences in the proportion of food utilized by each individual. The basic producers, plant δ¹³C and δ¹⁵N, were classified by species. Sediment δ¹³C and δ¹⁵N were comparable in relatively deep ponds. In general, the δ¹³C and δ¹⁵N allowed us to understand the food chain of each pond. In the presentation, the relationship between these food chains and nutrients will be discussed.