Indirect mechanisms accelerated due to ultraviolet-B irradiation on nutrient cycling in a freshwater ecosystem

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/m2), low UV-B (0.55 W/m2) and non-UV-B. Every treatment supplied photosynthetic active radiation (PAR) of 220 μmol/m2/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 2.1 mg/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.