North American Veterinary Licensing Examination (NAVLE) Aquatics Practice Exam

Seasonal variation in oxygen levels in lakes is primarily influenced by both temperature changes and algal blooms.

Temperature plays a significant role because as the water warms in the spring and summer, it can hold less dissolved oxygen compared to cooler temperatures. In addition, temperature stratification occurs, where warmer surface water forms layers above colder, deeper water. This stratification can limit the mixing of water layers, reducing the oxygen transfer from the surface to the deeper areas, affecting overall oxygen availability in various parts of the lake.

Algal blooms, which can occur in response to nutrient runoff, particularly during warmer months, also significantly impact oxygen levels. When these blooms die off and decompose, they consume a substantial amount of dissolved oxygen in the process, leading to lower oxygen levels, especially in the hypolimnion or deeper layers of a stratified lake.

Thus, the correct answer is rooted in the understanding that both temperature changes and algal blooms directly affect the oxygen dynamics within the aquatic environment, contributing to the seasonal variation observed in lake ecosystems.