A3-hectare lake in the american midwest suddenly has succumbed to an algal bloom. what is the likely cause of eutrophication in freshwater ecosystems, such as this one?

Run off of nutrients enriched water into the lake

Explanation:

When a lake receives nutrients from the nearby agricultural land then it becomes nutritionally enriched due to increase in the concentration of nitrogen and potassium.  

Both nitrogen and potassium are essential nutrients responsible for enhancing crop and plant growth.  

Therefore, when lake water becomes enriched algal boom occurs thereby covering the top surface of lake and hence making the lake water devoid of oxygen. This entire process is collectively termed as eutrophication