Oxygen in Marco Island Waterways at Historic Low

During the month of June 2023, oxygen levels in the Marco waterways were the lowest in recorded history. The average oxygen level was 3 mg/L. This is well below the level that indicates healthy conditions at 5 mg/L.

Lowest readings were at the Swallow sampling location (1.3 mg/L) and Landmark (1.9 mg/L). Of the 14 sampling locations, 13 are considered hypoxic and none of the samples were considered healthy.

Dissolved Oxygen (DO), which is the amount of free oxygen dissolved in water, is an essential component of the aquatic environment. The most important and commonly used measurement of water quality, it indicates a waterbody’s state of health - that is, the ability to support aquatic life. A vast array of aquatic organisms depends on the presence of adequate levels of DO for survival. Generally, waters with DO concentrations of 5 mg/L or higher can support a well-balanced, healthy biological community.

In ocean and freshwater environments, the term "hypoxia" refers to low or depleted oxygen in a water body. Hypoxia is often associated with the overgrowth of certain species of algae, which can lead to oxygen depletion when they die, sink to the bottom, and decompose.

Oxygen levels in the Marco Island waterways have been declining since 2019. The root cause was determined to be phosphorus in the reuse water distributed by the sewage treatment plant. Phosphorus is an element of fertilizer that feeds algae in the waterways.