Final answer:
Low O2 levels in rivers impair the water's ability to sustain fish life, leading to fish kills and disrupting the entire ecosystem. This can affect the food chain and the overall quality of the water, leading to a domino effect on various species.
Step-by-step explanation:
The low levels of O2 in rivers can have detrimental effects on marine life, most notably leading to large-scale fish kills. This occurs because warmer water from sources such as industrial discharge lowers the water's ability to hold dissolved oxygen, a condition necessary for fish and other aquatic organisms to survive. Consequently, fish die off in large numbers, which disrupts the ecosystem balance.
Predators that rely on fish for food will start to suffer from lack of prey, and there could be an increase in the populations of scavengers and decomposers that break down dead matter. The disruption can have a chain reaction, affecting birds, mammals, and even plants that are part of the river ecosystem. Moreover, as the fish die off, there can be a significant change in the water quality, potentially leading to algae blooms, further oxygen depletion, and altered pH levels, impeding the survival of other aquatic species.