Which chemical is used as a contact herbicide for algae control?

Copper sulfate is widely recognized as a contact herbicide commonly used for algae control in aquatic environments. When applied, it acts quickly upon contact with the algae, disrupting their cellular processes and ultimately leading to cell death. This makes it highly effective for treating algal blooms in ponds, lakes, and other water bodies.

The functioning of copper sulfate as a herbicide is attributed to its ability to release copper ions in water, which are toxic to algae and contribute to their demise. Its application is particularly useful in managing various types of algae due to its fast-acting nature, making it a preferred choice among aquatic pesticide applicators who need to address algal problems efficiently.

In contrast, other listed chemicals serve different functions: imazapyr and triclopyr are more commonly used for controlling invasive terrestrial plants rather than aquatic algae, and 2,4-D is primarily known for controlling broadleaf weeds rather than targeting algae specifically. Therefore, copper sulfate's unique effectiveness as a contact herbicide for algae is what establishes it as the correct answer.