How can harmful algal blooms adversely affect water quality?

Harmful algal blooms (HABs) can significantly impact water quality, and the correct choice highlights one of the primary drivers of these blooms: the increase in phosphorus availability. When nutrient levels, particularly phosphorus, rise in water bodies—often due to agricultural runoff, wastewater discharge, or other forms of pollution—this can promote excessive growth of algae.

As these algal blooms proliferate, they can deplete dissolved oxygen levels when the algae die and decompose, leading to hypoxic conditions that harm aquatic life. Additionally, some algal species produce toxins that can be detrimental to fish, wildlife, and human health. The presence of these toxins can make water unsafe for drinking, recreation, and other uses. Therefore, the increase in phosphorus availability is a critical factor in the formation of harmful algal blooms and their subsequent effects on water quality.

In contrast, the other options do not reflect the impacts of harmful algal blooms accurately. For example, HABs do not decrease water temperature; in fact, they can increase the surface water temperature by reducing light penetration, which can further exacerbate the growth of these algae. Filtering pollutants from water is associated with healthy ecosystems and certain types of aquatic plants or organisms, not with HABs