This year, while all eyes and focus were along our coast, something was happening in the Harbor that went mostly unnoticed. And no, it wasn’t red tide. Red tide cannot grow in salinities less than 18 practical salinity units, and the entire upper Harbor was much fresher than that all summer long because we had a lot of rain — not just over Charlotte Harbor, but up the rivers as well. In fact, the Peace River was near flood stage for a while.
Along with the fresh water inflow, we also experienced very warm temperatures. The combination of fresh water and plentiful heat caused many parts of Charlotte Harbor to become stratified, with denser salt water on the bottom and lighter fresher water on the top. This does not normally happen in Florida, where our estuaries tend to be well mixed because the fresh water moves so slowly due to our lack of elevation. When the water stratifies, the bottom waters often become very low in dissolved oxygen, a condition known as hypoxia.
Oxygen enters water from the atmosphere and through photosynthesis by aquatic plants and algae. Diffusion of oxygen from the atmosphere occurs across the air-water interface and may be enhanced through turbulent mixing caused by winds and waves. This happens because wind and waves increase the surface area of water in contact with the atmosphere. A greater surface area to water volume ratio provides more opportunity for oxygen transfer. (This is the same effect that aquarium or bait bucket aerators have.)
Aquatic plants and algae also play a role in contributing dissolved oxygen to aquatic systems by converting sunlight energy into usable energy required for supporting aquatic life. This process, known as photosynthesis, occurs during daylight hours. Photosynthesis is conducted by submerged and emergent plants as well as macro- and micro-algae.
While plants and algae contribute dissolved oxygen, they also use it. Because photosynthesis occurs only during daylight hours and respiration (or use of oxygen) occurs day and night, dissolved oxygen levels are typically highest in the mid to late afternoons and lowest right before sun up in the morning.
Fish, crabs and other aquatic organisms also consume oxygen through aerobic respiration, and bacteria and fungi use oxygen during the process of breaking down dead plants and animals. Other factors that affect dissolved oxygen include temperature (warmer water holds less oxygen), the amount of dissolved salts (saltier water holds less oxygen) and atmospheric pressure (lower pressure, less oxygen in the water).
When consumption exceeds production, dissolved oxygen becomes depleted. Increases in consumption may occur as the result of too many plants or algae (remember, respiration occurs 24/7, but photosynthesis only occurs during the day), stratification or turnover, high water temperature, increased levels of organic material entering the water (e.g., septic tanks or manure), decomposition of organic matter (e.g., plant or algae death) or by certain chemicals that remove oxygen directly.
Although oxygen levels can drop at any time of the year, they are more prone to drop in the summer. Oxygen molecules in warm water are unstable and vibrate. As a result, warm water is less able to hold oxygen gas in solution.
Dissolved oxygen can be measured using a portable meter or test kit and is typically measured as a concentration using milligrams of dissolved oxygen per liter of water. It may also be reported as the percent saturation. A measurement of 5 mg/L is recommended for optimal fish health. Between 2 and 4 mg/L is considered hypoxic, and fish become stressed.
A pretty significant portion of the deeper bottom areas from the lower Peace River down to Cape Haze were hypoxic this summer due to stratification. Fortunately, we rarely see any significant fish kills from hypoxia events that happen in the Harbor — probably because there are so many other places for critters to go. But when hypoxia occurs in lakes, ponds, or at the ends of canals, dead fish are frequently the result. I guess this is just another good reason to be thankful for cooler fall weather.
Betty Staugler is the Charlotte County extension agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. The Florida Sea Grant College Program supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at email@example.com or 941-764-4346.