Nitrification and ammonium dynamics in Taihu Lake, China: seasonal competition for ammonium between nitrifiers and cyanobacteria
- 1Department of Earth & Environmental Sciences, Wright State University, Dayton, OH, USA
- 2The University of Texas at Austin, Marine Science Institute, Port Aransas, TX, USA
- 3Taihu Laboratory for Lake Ecosystem Research, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China
Abstract. Taihu Lake is hypereutrophic and experiences seasonal, cyanobacterial harmful algal blooms. These Microcystis blooms produce microcystin, a potent liver toxin, and are linked to anthropogenic nitrogen (N) and phosphorus (P) loads to lakes. Microcystis spp. cannot fix atmospheric N and must compete with ammonia-oxidizing and other organisms for ammonium (NH4+). We measured NH4+ regeneration and potential uptake rates and total nitrification using stable-isotope techniques. Nitrification studies included abundance of the functional gene for NH4+ oxidation, amoA, for ammonia-oxidizing archaea (AOA) and bacteria (AOB). Potential NH4+ uptake rates ranged from 0.02 to 6.80 µmol L−1 h−1 in the light and from 0.05 to 3.33 µmol L−1 h−1 in the dark, and NH4+ regeneration rates ranged from 0.03 to 2.37 µmol L−1 h−1. Nitrification rates exceeded previously reported rates in most freshwater systems. Total nitrification often exceeded 200 nmol L−1 d−1 and was > 1000 nmol L−1 d−1 at one station near a river discharge. AOA amoA gene copies were more abundant than AOB gene copies (p < 0.005) at all times; however, only abundance of AOB amoA (not AOA) was correlated with nitrification rates for all stations and all seasons (p < 0.005). Nitrification rates in Taihu Lake varied seasonally; at most stations, rates were highest in March, lower in June, and lowest in July, corresponding with cyanobacterial bloom progression, suggesting that nitrifiers were poor competitors for NH4+ during the bloom.
Regeneration results suggested that cyanobacteria relied extensively on regenerated NH4+ to sustain the bloom. Internal NH4+ regeneration exceeded external N loading to the lake by a factor of 2 but was ultimately fueled by external N loads. Our results thus support the growing literature calling for watershed N loading reductions in concert with existing management of P loads.