Articles | Volume 15, issue 3
Research article
06 Feb 2018
Research article |  | 06 Feb 2018

Nitrification and ammonium dynamics in Taihu Lake, China: seasonal competition for ammonium between nitrifiers and cyanobacteria

Justyna J. Hampel, Mark J. McCarthy, Wayne S. Gardner, Lu Zhang, Hai Xu, Guangwei Zhu, and Silvia E. Newell

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.

Short summary
Our paper highlights the importance of dual-nutrient management: nitrogen (N) and phosphorus (P) in lakes with cyanobacterial harmful algal blooms. Taihu Lake (China) experiences seasonal blooms due to increased input of N and P from field runoff. The main process investigated in our study, nitrification, is important for N removal through denitrification. We show that nitrification is less efficient during the blooms, due to competition for nutrients between N microbes and cyanobacteria.
Final-revised paper