What environmental issue results from increased acidifying substances in lakes?

The increase in acidifying substances, such as sulfuric and nitric acids resulting from pollution, leads to a significant reduction in the pH levels of water in lakes. This change in acidity can harm aquatic life in several ways. Many fish and other organisms are sensitive to pH levels; a more acidic environment can lead to stress, reduced reproduction rates, and even mortality in sensitive species. Acidification can also disrupt the availability of essential nutrients and minerals, further exacerbating the difficulties faced by aquatic organisms.

In contrast, the other options do not reflect the consequences of increased acidification accurately. Decreased visibility is not a direct consequence of acidity; enhanced fish populations contradict the detrimental effects of acidification, and while increased algae growth might occur in some contexts, it doesn't apply specifically to acidification issues and is usually associated with nutrient pollution rather than acidity itself. Therefore, harming aquatic life is the most accurate result of increased acidifying substances in lakes, as it encompasses the various negative impacts on the ecological health of these environments.