The co-regulation of nitrate and temperature on denitrification at the sediment-water interface in the algae-dominated ecosystem of Lake Taihu, China

Abstract

Sediment denitrification is a dominant mechanism for nitrogen removal and can help to minimize lake eutrophication. However, the spatio-temporal variability of denitrification rates and its controlling factors in sediments of large shallow lakes are poorly understood. In this study, we investigated the controlling factors on the temporal and spatial variability of denitrification rates in Meiliang Bay in Taihu Lake, China, to determine the contribution of denitrification on the total lacustrine nitrogen budget. We collected 18 intact monthly sediment cores and an additional 36 seasonal sediment cores from January 2013 to January 2014. Cores were collected from the inner and outer sections of Meiliang Bay for analysis of denitrification rates and sediment properties. We also collected in situ surface water samples for water quality analysis. Denitrification rates at the sediment-water interface (SWI) were measured using acetylene inhibition techniques and intact sediment core incubation. We used a t test to determine the differences in water quality and sediment properties between the two sites and a one-way ANOVA to identify seasonal differences in denitrification rates, water quality, and sediment properties. We also applied Pearson’s correlation, distance-based redundancy analysis (db-RDA) and random forest model to identify the relationships between denitrification rates and environmental factors. Denitrification rates ranged from 0.76 to 40.94 μmol N m−2 h−1 and 0.13 to 52.55 μmol N m−2 h−1, with annual mean values of 19.97 and 17.15 μmol N m−2 h−1 for the Inner and Outer Bay, respectively. Sediment denitrification rates in the Inner and Outer Bay showed similar seasonal variability, with the highest values in spring and summer and the lowest values in autumn. Nitrate addition was shown to significantly increase denitrification rates in summer and autumn (P < 0.05); however, carbon addition showed no significant influence on denitrification rates in the four seasons. Our results imply that denitrification rates were nitrate-limited in summer and autumn. Distance-based redundancy analysis (db-RDA) and random forest model showed that denitrification rates were mainly determined by nitrate, temperature, and chlorophyll a (Chla), and that nitrate in the water column was the most important predictor of denitrification rates. In general, denitrification rates showed significant seasonal variability in Meiliang Bay due to the co-regulation of both water temperature and nitrate concentrations. The dominance of each controlling factor on denitrification rates varied in different seasons. Based on our calculations, nitrogen removal by denitrification accounted for approximately 10.7% of the total nitrogen input to Taihu Lake. Therefore, we suggest the need for effective measures to reduce external nitrogen inputs of to Lake Taihu to prevent on-going eutrophication.