What is a consequence of untreated wastewater on aquatic ecosystems?

Untreated wastewater can have severe negative impacts on aquatic ecosystems, and one of the most significant consequences is fish kills. When untreated wastewater is released into water bodies, it often contains high levels of pollutants such as nutrients (nitrogen and phosphorus), organic matter, pathogens, and toxic substances.

The influx of nutrients can lead to algal blooms, which can deplete oxygen levels in the water as algae die and decompose. This process, known as eutrophication, creates hypoxic (low oxygen) conditions that can suffocate fish and other aquatic life, leading to mass mortality events, or fish kills. Additionally, the presence of harmful pathogens and toxins can directly harm fish populations and other aquatic organisms, further exacerbating the decline in fish health and diversity in these ecosystems.

In contrast, the other choices reflect outcomes that do not represent the reality of untreated wastewater's impact. For instance, untreated wastewater does not typically lead to a reduction in water temperature or an improvement in water clarity, and it tends to decrease biodiversity, rather than increase it, as the adverse effects on various species can disrupt the balance of the aquatic community.