You may know Lake Erie as a favorite fishing spot or home to the best beaches in Ohio but every summer it has a major problem. Over the past few decades, hypoxic algae blooms have become a huge problem for the lake. Algae blooms are caused by phosphorus runoff. This runoff comes from local farms who use phosphorus-rich fertilizer on their crops every Spring. The phosphorus runoff is rapidly consumed and causes an explosion of cyanobacteria in Lake Erie. This alga has been known to produce a dangerous chemical called microcystin, which is a toxin known to be harmful to humans and animals. The consumption of contaminated water has been shown to have a negative effect on the local wildlife populations. The toxin causes liver failure and can lead to death.
What about the local fish populations?
Lake Erie algal blooms, August 2011
Many individuals count on the summer months for a time of bountiful harvest of Lake Erie’s most famous fish. Unfortunately, these algae blooms create hypoxic dead zones. These are areas of the lake that the algae have completely taken over. The hypoxic dead zones are devoid of any oxygen making it impossible for any living organism to live in that area of the lake. Algae blooms keep increasing in size, some years taking up 700 square miles of the lake. This has resulted in a slight decline in fish populations as they are getting trapped in these dead zones. 
References
The United States Environmental Protection Agency. Harmful Algal Blooms. Retrieved from https://www.epa.gov/nutrientpollution/harmful-algal-blooms (Links to an external site.)
Watson, S. B., et al. (2016). Harmful algae: The re-eutrophication of Lake Erie: Harmful algal blooms and hypoxia. Elsevier, 56 (1): 44-66). 10.1016/j.hal.2016.04.010
Images
Tom Archer, (August 19, 2011) Michigan Sea Grant. Retrieved from https://www.flickr.com/photos/miseagrant/8551122735/in/album72157630018693025/
Jeff Schmaltz, (August 3, 2014) NASA observatory. Retrieved from https://earthobservatory.nasa.gov/IOTD/view.php?id=84125