The correct answer is option 2: Blue-green algae. Waterblooms, also known as algal blooms, are a rapid increase or accumulation of algae in freshwater or marine ecosystems. These blooms can occur naturally, but they are also often caused by human activities that contribute to excessive nutrient levels in the water, such as increased runoff from agricultural or urban areas.

Blue-green algae, also known as cyanobacteria, are a type of bacteria that can photosynthesize like plants. They are often responsible for causing harmful algal blooms, which can have negative impacts on aquatic ecosystems. Blue-green algae can deplete oxygen in the water, block sunlight, and release toxins that can harm other organisms in the ecosystem. The excessive growth of blue-green algae is often fueled by high levels of nutrients, such as nitrogen and phosphorus, in the water. These nutrients can come from sources like fertilizer runoff, sewage discharge, or industrial waste.

Alert - correct answer should be option 3: Fishing trawlers. Fishing trawlers are not responsible for waterblooms.