Zheng et al.’s (2021) article entitled ‘Dominant Contribution of a Lake’s Internal Pollution to Eutrophication During Rapid Urbanization’ explains how algae blooms may come about in artificial urban lakes which were generated from rapid urbanisation. They studied Tonggu Lake in Chongqing City, China which is about 10 years old that became hyper-eutrophic. When the district was undergoing urbanisation, the lake was formed via a dam that collected water from streams that accumulated in a forest valley.
Eutrophication refers to the addition of excess nutrients to a water body. Zheng et al. (2021) explain that pollution can be from external discharge or through the internal release of excess nutrients (e.g. phosphorous, nitrogen which contribute to eutrophication) from the lake’s sediment.
In comparison to natural lakes, artificial lakes are susceptible to eutrophication and harmful algae blooms because they “lack complete aquatic food chains and usually have slow flow rates and long retention times that lead to a slow removal rate for endogenous pollutants such as sediments” (p. 904). Pollutants in artificial lakes are more able to accumulate in the water body without being removed via carbon and nutrient cycles as compared to natural lakes. Hence, artificial lakes have weak resistance to internal pollution.
For example, in Tonggu Lake, the excessive phosphorous and nitrogen which built up and polluted the lake caused eutrophication and promoted frequent algae blooms (Zheng et al., 2021). Within just 10 years, Tonggue Lake became ultra-eutrophied. The dominant algal species, or cynobacteria, in this lake was the toxic Microcystis sp., which is indicative of phosphorous being the primary cause of the algae bloom.
This is because usually, Microcystis sp. ‘fixes’ nitrogen molecules in the air (i.e. convert them into ammonia in the soil/aquatic body) when there is an excess of nitrogen in the air and phosphorus is the limiting agent (Zheng et al., 2021). While nitrogen fixation with excess nitrogen causes algae blooms, phosphorous as a limiting agent would limit the eutrophication. However, the high total phosphorous and optimal soluble reactive phosphorous (SRP) concentration for algae growth in Tonggu Lake resulted in rapid eutrophication and strong algae bursts (Zheng et al., 2021).
The strong correlation between algae growth and phosphorus is seen from the following diagrams:
Interestingly, the “contribution of internal sources to eutrophication exceeded those of the external sources” in Tonggu Lake (Zheng et al., 2021, p. 908). Even if pollution by external sources is reduced, water quality would not show a significant improvement since the internal pollution is still affecting the lake, as with Tonggu Lake. The ecosystem would still suffer a limited ability to function. Research on how much artificial lakes are affected by internal versus external pollution is hence important to manage the water system.
Fun fact: my blog background is a picture of an algae bloom in a lake!
References:
Zheng, X., Liu, G., Yang, W., Peng, X., Liu, H., Li, H., & Li, W. (2021). Dominant Contribution of a Lake’s Internal Pollution to Eutrophication During Rapid Urbanization. Bulletin of Environmental Contamination and Toxicology, 107(5), 904–910. https://doi.org/10.1007/s00128-021-03236-1
