How does untreated sewage threaten freshwater ecosystems?

Untreated sewage threatens freshwater ecosystems by introducing harmful pollutants and pathogens, disrupting biodiversity and water quality.

Untreated sewage, often containing a cocktail of pollutants, poses a significant threat to freshwater ecosystems. These pollutants can include organic matter, nutrients such as nitrogen and phosphorus, heavy metals, pharmaceuticals, and a variety of pathogens. When these substances enter a freshwater ecosystem, they can have a range of detrimental effects.

Organic matter in sewage can lead to a process called eutrophication. This occurs when the organic matter decomposes, consuming oxygen in the water. The resulting low oxygen levels, or hypoxia, can lead to the death of aquatic organisms, particularly fish and invertebrates, which require oxygen to survive. This can disrupt the balance of the ecosystem, reducing biodiversity and altering the food web.

Nutrients such as nitrogen and phosphorus can also contribute to eutrophication. They stimulate the growth of algae and other aquatic plants, which can form dense blooms that block sunlight from reaching other plants and organisms. When these plants die and decompose, they further deplete the oxygen levels in the water. This can lead to 'dead zones', areas where the lack of oxygen makes it impossible for most aquatic life to survive.

Heavy metals and other toxic substances in sewage can accumulate in the bodies of aquatic organisms, leading to poisoning and death. These substances can also biomagnify up the food chain, affecting predators and potentially humans who consume contaminated fish or shellfish.

Pharmaceuticals and personal care products, often present in sewage, can have subtle but significant effects on aquatic organisms. For example, hormones and endocrine disruptors can interfere with reproduction and development, leading to population declines and changes in community structure.

Pathogens in sewage, including bacteria, viruses, and parasites, can cause disease in aquatic organisms and pose a risk to human health. They can also alter the balance of microbial communities in the water, with potential knock-on effects on nutrient cycling and other ecosystem processes.

In summary, untreated sewage can threaten freshwater ecosystems in multiple ways, from depleting oxygen levels and disrupting biodiversity, to introducing harmful substances and pathogens. These impacts can have far-reaching consequences, affecting not only the health and functioning of the ecosystem itself, but also the services it provides to humans, such as clean water and food.