Oxygen molecules that are not bonded to any other element are considered free, or non-compound, elements. When free oxygen is present in water, we call it dissolved oxygen. Dissolved oxygen is available for fish and aquatic plants to breathe in, much like humans take in oxygen from the air.
Lake Pontchartrain typically has high levels of oxygen at the surface due to wave action, but these dissolved oxygen levels may decrease with depth below the surface, in a process known as an oxycline. The Mississippi River is generally well oxygenated throughout its depth, although it may have an oxycline at times in areas that are very deep.
Too much algae in water doesn’t only cause problems due to its possible toxic effects, it can also affect other important water quality characteristics important to aquatic life. Algae depend on nutrients and sunlight to grow. Too many nutrients in water can cause algae to grow quickly and fill the water column with floating algae. All land and water-based plants produce dissolved oxygen (DO) during a process called photosynthesis, which is a process driven by sunlight. During the day, heavy algae growth can cause spikes in DO in water as seen on August 25, 2021 in the graph below. However, as some of these algae begin to die, large numbers of microorganisms begin to feed on these dead algae. Since these microorganisms use oxygen during the process of breaking down dead algae, and no photosynthesis occurs during overnight hours, this can cause a significant drop in DO in the overnight and early morning hours. The below graph shows a drop of DO from a high to 13.4mg/L at 2:30pm to a low of 4mg/L at 5:30am. DO levels of 4mg/L or lower make it difficult for lake fish to breathe if such conditions last too long, and can cause fish kills.