Which common algicide's effectiveness decreases with increasing water alkalinity?

Copper sulfate is a widely used algicide that acts by releasing copper ions into the water, which inhibit the growth of algae. However, the effectiveness of copper sulfate decreases in waters with higher alkalinity. This occurs because elevated alkalinity leads to an increased concentration of bicarbonate and carbonate ions, which can react with copper ions, effectively binding them and reducing their availability for algal control. Additionally, the presence of these ions can lead to the formation of less reactive copper compounds, diminishing the algicide's potency.

In contrast, other options may not face the same challenges in alkaline conditions. Copper chelate, for example, is often utilized for its stability and retains effectiveness in various alkalinity levels due to its chelation properties, which help protect copper ions from being antagonized by other ions. Endothall and sodium carbonate peroxyhydrate also maintain relative effectiveness in different water conditions, thus illustrating why copper sulfate is particularly impacted by increasing water alkalinity.