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

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.