Final answer:
Domestic wastewater discharged without proper treatment can deplete dissolved oxygen in a river or lake due to excess nutrients such as phosphorus and nitrogen. This can result in the death of aquatic organisms and the formation of dead zones.
Step-by-step explanation:
In a river or lake, if domestic wastewater is discharged without proper treatment, it can deplete the dissolved oxygen. This can happen due to excess nutrients, such as phosphorus and nitrogen, which enter the ecosystem from fertilizer runoff and sewage.
Excessive growth of microorganisms, such as algae, occurs when there is an influx of these nutrients. As these microorganisms die and decompose, they consume oxygen from the water, leading to a depletion of dissolved oxygen. This process can result in the death of aquatic organisms, such as shellfish and finfish.
Overall, improper treatment of domestic wastewater can cause oxygen depletion in aquatic ecosystems, impacting the health of the water's ecosystems and leading to dead zones in lakes and at the mouths of major rivers.