Lake Taihu, China
Updated: Mar 28, 2019
Nitrogen (N) and phosphorus (P) are important nutrients in aquatic ecosystems, often co-limiting primary production. Non-point source N loads from agriculture are a main driver of eutrophication in aquatic systems, which is often manifested as hypoxia, loss of biodiversity, and cyanobacterial harmful algal blooms. Cyanobacterial blooms are particularly problematic because they often produce toxins (e.g., the Toledo water crisis in 2014), compete for nutrients with other microbes, and indicate unhealthy aquatic systems.
Lake Taihu is China’s third largest freshwater lake (Taihu translates to “Great Lake” in Mandarin). Approximately 40 million people live in its watershed, including the city of Shanghai, and rely on the lake for fishing, tourism, and domestic use. However, due to industrial development and urbanization in the watershed, Taihu has shifted from a mesotrophic lake to a hypereutrophic lake experiencing severe cyanobacterias blooms. These blooms have been associated with toxin producing Microcystis spp.,which can form surface scums for up to 10 months per year. Non-N2 fixing cyanobacteria, such as Microcystis, thrive on chemically reduced N forms, such as ammonium and urea.
For my dissertation project, I was interested in measuring ammonium cycling rates and nitrification rates in Lake Taihu, and investigating ammonia oxidizer community structure. The first step of nitrification, ammonia oxidation, is carried out by ammonia-oxidizing archaea (AOA) or bacteria (AOB). Nitrification can be closely coupled in time and space to removal of N via denitrification, thus it is an important step in the N cycle. The natural removal of excess N in eutrophic systems is crucial to help mitigate high N loads, and substrate availability for denitrification can depend on nitrification. In eutrophic systems, ammonia oxidizers must compete for ammonium with cyanobacteria and other microbes. This competition could help determine microbial community structure and bloom severity.
In our publication, we show that nitrification rates were significantly lower when the Microcystis bloom was present and relatively high under no bloom conditions. These results suggest that nitrifiying microbes are poor competitors for ammonium and are generally outcompeted by Microcystis.
Many studies of nitrifier community composition have shown that AOB are ubiquitous and found in soils, marine, and freshwater ecosystems (Schleper 2010), while AOA are generally accepted to be abundant in the open ocean (Francis et al., 2005; Molina et al., 2010; Newell et al., 2011). In hypereutrophic Taihu, AOA exceeded AOB on all sampling occasions. This was surprising given the oligotrophic nature of AOA.
We also show that internal recycling of ammonium via regeneration is the main mechanism sustaining the blooms in the late summer, and it exceeds external inputs of N from the river loading. We concluded that to mitigate harmful algal blooms, external nutrient loadings into the lake must be reduced so that N can be efficiently removed via nitrification/denitrification.
Hampel, J.J., McCarthy, M.J., Gardner, W.S., Zhang, L., Xu, H., Zhu, G. and Newell, S.E., 2018. Nitrification and ammonium dynamics in Taihu Lake, China: seasonal competition for ammonium between nitrifiers and cyanobacteria. Biogeosciences, 15(3), p.733. https://doi.org/10.5194/bg-15-733-2018