Harmful cyanobacteria blooms is a relevant and pressing issue worldwide. This is usually attributed to increases of dissolved inorganic nitrogen and phosphorus as well as suitable growth conditions.
Vancouver Lake of Washington, USA is a large (~9.3 km2), shallow (mean depth ~1.2 m) floodplain lake that is well known for swimming, bird watching, boating, and fishing. In the past, Vancouver Lake was clear, moderately deep (6-8m) that was flushed during spring and fall. However, since the early 20th century, many forms of rapid urbanisation like hydroelectric dam construction as well as land reclamation took place which caused the uncontrolled sediment load and nutrient load. In the 1960s, there were already cyanobacteria blooms and poor water quality. By the 1980s, the lake had shallowed to an average of 1m in depth.
Currently, the lake depth remains at an average of 1m. The water quality continues to be poor, with high levels of dissolved nitrogen and phosphorus, turbidity and pH. According to the Public Health officials, the lake continues to struggle with cyanobacteria because it is shallow, nutrient-rich without any outlets or freshwater sources and experiences very few rain events but is constantly being exposed to sunlight and warm temperatures. Such algal blooms forces the closure of the lake and swim beach periodically on and off during summer season, in order to protect the health of its visitors.
Vancouver Lake during Cyanobacteria blooms
One interesting fact is that several weeks before cyanobacteria outbreaks, the dissolved inorganic nitrogen availability decreases, hence favouring the N-fixing cyanobacteria to give them an advantage for rapid growth. This was exactly what Prof Taylor mentioned in his freshwater pollution lecture!
References:
Rollwagen-Bollens, G., Lee, T., Rose, V. and M. Bollens, S., 2018. Beyond Eutrophication: Vancouver Lake, WA, USA as a Model System for Assessing Multiple, Interacting Biotic and Abiotic Drivers of Harmful Cyanobacterial Blooms. Water, 10(6).
Photo credits:
https://katu.com/news/local/health-advisory-issued-for-vancouver-lake-over-cyanotoxins-again