Abstract
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.
Highlights
Nitrogen (N) and phosphorus (P) are important nutrients in aquatic ecosystems, often co-limiting primary production (Elser et al, 2007)
Results from the amoA gene copy abundance analysis show that ammonia-oxidizing archaea (AOA) were more abundant than ammonia-oxidizing bacteria (AOB) across all stations and seasons in Taihu Lake
This result does not support our original hypothesis, the results agree with previous studies in the water column and sediments in Taihu Lake (Zeng et al, 2012), which reported higher AOA abundance (4.91×105–8.65×106 copies g−1 sediment) than AOB (3.74×104–3.86×105 copies g−1 sediment) in Meiliang Bay
Summary
Nitrogen (N) and phosphorus (P) are important nutrients in aquatic ecosystems, often co-limiting primary production (Elser et al, 2007). Unavailable (except to diazotrophs) atmospheric N can be fixed to readily assimilable ammonium (NH+4 ) and biomass via N2 fixation (Vitousek et al, 2013). Fertilizer production from anthropogenic N fixation (the Haber–Bosch process) has changed N cycling and the global N budget over the last century. Nonpoint source N loads from agriculture are a main driver of eutrophication in aquatic systems, which is often manifested as hypoxia, loss of biodiversity, cyanobacterial harmful algal blooms (cyanoHABs; Paerl et al, 2016; Paerl and Paul, 2012), and other detrimental characteristics. CyanoHABs are problematic because they often produce toxins, compete for nutrients with other microbes and primary producers, and indicate unhealthy aquatic systems
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