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

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 4+ oxidation, amoA, for ammonia-oxidizing archaea (AOA) and bacteria (AOB). Potential NH 4+ 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 NH 4+ 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 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.

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Hampel, Justyna J. / McCarthy, Mark J. / Gardner, Wayne S. / et al: Nitrification and ammonium dynamics in Taihu Lake, China: seasonal competition for ammonium between nitrifiers and cyanobacteria. 2018. Copernicus Publications.

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