title: Indirect mechanisms accelerated due to ultraviolet-B irradiation on nutrient cycling in a freshwater ecosystem.
abstract: This study aimed to observe the mechanisms accelerated in nitrogen cycling in shallow freshwater when exposed to ultraviolet-B (UV-B) irradiation. The experiment included three treatments exposing Vallisneria gigantea Graebner to high UV-B (1.1 W/m(2)), low UV-B (0.55 W/m(2)) and non-UV-B. Every treatment supplied photosynthetic active radiation (PAR) of 220 micromol/m(2)/s and the experiment was conducted for 12 weeks. The results showed a significant reduction in total nitrate by 95.9% in the high UV-B treatment (RM ANOVA, F((2, 15))=3123.02, P<0.001) compared to non-UV-B (control) treatment, which showed an average reduction of 50.9%. Additionally, there was a significant reduction (RM ANOVA, F((2, 15))=1695.59, P<0.001) in water total nitrogen (TN). Dissolved oxygen (DO) significantly (RM ANOVA, F((2, 63))=207.71, P<0.001) decreased to a minimum of 2.1mg/l in high UV-B treatment at the end of 12 weeks. The reduced DO was caused by the use of oxygen in the system for photo-oxidation of dissolved organic carbon (DOC) as well as consumption by bacterial respiration processes. A decline in the DO in overlying water enhanced denitrification and retarded nitrification in UV-B-exposed treatments. Significant increase in dissolved ammonia (RM ANOVA, F((2, 15))=2056.28, P<0.001) in water under UV-B exposure was due to photo-oxidation and bacterial decomposition of organic nitrogen in the system. Our study was able to identify UV-B induced mechanisms to alter the natural balance of nitrogen, oxygen and dissolved carbon in shallow freshwaters.
authors.affiliation: Department of Environmental Science and Technology, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan.
authors.forename: Kasige
authors.initials: K
authors.lastname: Anusha
authors.forename: Takashi
authors.initials: T
authors.lastname: Asaeda
mesh_terms: D001610:Beta Particles; D002244:Carbon / Q000378:metabolism; D007700:Kinetics; D019668:Liliaceae / Q000528:radiation effects*; D009566:Nitrates / Q000378:metabolism / Q000528:radiation effects; D009930:Organic Chemicals / Q000378:metabolism / Q000528:radiation effects; D010788:Photosynthesis; D018520:Plant Shoots / Q000378:metabolism / Q000528:radiation effects; D014466:Ultraviolet Rays; D014867:Water / Q000378:metabolism
chemical_list: D009566:Nitrates; D009930:Organic Chemicals; D014867:Water; D002244:Carbon