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 (NH
4+). We measured NH
4+ regeneration and potential uptake rates and total nitrification using stable-isotope techniques. Nitrification studies included abundance of the functional gene for NH
amoA, for ammonia-oxidizing archaea (AOA) and bacteria (AOB). Potential NH
4+ uptake rates ranged from 0.02 to 6.80 µmol L
−1 in the light and from 0.05 to 3.33 µmol L
−1 in the dark, and NH
4+ regeneration rates ranged from 0.03 to 2.37 µmol L
−1. Nitrification rates exceeded previously reported rates in most freshwater systems. Total nitrification often exceeded 200 nmol L
−1 and was > 1000 nmol L
−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 NH
4+ during the bloom.
Regeneration results suggested that cyanobacteria relied extensively on regenerated NH 4+ to sustain the bloom. Internal NH 4+ 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.