Effects of macrophytes and environmental factors on sediment denitrification in a subtropical reservoir

Sediment denitrification plays an important role in nitrogen removal in aquatic systems. However, the importance in nitrogen removal in reservoirs, with a focus on seasonal differences of conditions such as macrophyte beds and environmental factors, is less well understood. This study examined sediment denitrification rate (D_n), and their potential controlling factors were determined in both macrophyte beds and deeper waters in the subtropical reservoir. The mean D_n in the reservoir annually was 18.0 ± 6.3 (mean ± S.E.) mmol N m^-2 d^-1, with significant seasonal variation (p < 0.01), i.e. 43.2 ± 12.8, 6.7 ± 6.3, and 4.0 ± 2.2 mmol N m^-2 d^-1 in winter, spring and summer respectively. There were no statistical differences in D_n between shallow waters with macrophyte beds and deeper waters without macrophyte beds, although macrophyte beds had higher denitrification rates in summer. The D_n rates were significantly correlated with temperature, conductivity, dissolved oxygen, pH, nitrate-nitrogen concentration (NO₃^--N) (p < 0.01) and turbidity (p < 0.05). Linear regression models demonstrated environmental variables explained between 36% and 76% of the variation in D_n. The correlation with NO₃^--N concentrations suggests that it may be a limited factor for D_n. Annual nitrogen removal of the reservoir by a combination of sediment and water denitrification was totally estimated to be 370 t N with an annual removal efficiency of approximately 11%. Nitrogen removal was much higher in winter than other seasons, with about 305 t N removed, accounting for 12% of the total nitrogen inputs. Therefore, denitrification appears to play a minor role throughout much of the year, but in winter months when nitrate accumulates, it may play a more major role.