Final answer:
Power plant discharge most likely causes thermal pollution, characterized by increased water temperatures and reduced dissolved oxygen, adversely affecting aquatic ecosystems.
Step-by-step explanation:
The discharge by a power plant would most likely cause thermal pollution. When water used to cool a power plant is released back into the environment at a higher temperature, it has a lower concentration of dissolved oxygen, which aquatic organisms require for survival. This concept relates to thermal pollution, where the discharged water, being warmer and less dense, tends to float above the cooler water, creating a barrier that inhibits the dissolution of atmospheric oxygen into the water body. Consequently, this could lead to stress or death among fish and other aquatic organisms, as many are cold-blooded and unable to cope with significant changes in environmental temperature. Additionally, thermal pollution can disrupt the ecological balance of the ecosystem, though it does not directly lead to eutrophication, which is typically caused by excessive nutrient loads from sources like agricultural runoff.