While a small amount of algae in a system or area could be beneficial, an overgrowth of algae could cause low DO levels.

An overgrowth of algae in an aquatic system can indeed lead to low dissolved oxygen (DO) levels, making the statement true. When algae thrive, especially during a bloom, they can account for a significant increase in biomass. At night or during periods of decomposition, this biomass can consume a large amount of oxygen as it breaks down, leading to a decrease in the available DO for other organisms in the system.

Additionally, a high concentration of algae can block sunlight, reducing photosynthetic activity from aquatic plants and further contributing to lower oxygen production. This creates a situation where fish and other aquatic life can suffer from hypoxia, which is a deficiency in oxygen. Therefore, while some algae are beneficial for nutrient cycling and as a food source, excessive growth inhibits the overall health of the aquatic environment, confirming that low DO levels are a consequence of overgrown algae.