In eutrophic lakes, oxygen is usually limited beyond what depth?

• A. Little oxygen beyond 30 feet
• B. Full oxygen to bottom
• C. No oxygen issues
• D. Oxygen abundant throughout

Answer: (A)

In eutrophic lakes, deeper waters tend to become short on oxygen because a lot of organic material ends up decomposing there and the lake often stays stratified, so that oxygen from the surface isn’t mixed down to the bottom. The surface gets plenty of light and photosynthesis, but when the surface blooms die off, microbes consume the dissolved oxygen as they break down the dead matter. Since the water doesn’t mix well between the warm upper layer and the cooler deeper layer, the oxygen near the bottom isn’t replenished. That’s why oxygen is usually limited beyond a shallow depth, such as around 30 feet. If oxygen were present to the bottom or abundant throughout, it would imply a well-mixed or non-eutrophic system, which isn’t typical of eutrophic lakes.