Seagrass protection goals clearer thanks to Charlotte Harbor study

Credit all photos to Conor Goulding/Mote Marine Laboratory

Seagrass beds — vital habitat for Florida wildlife — may decline if poor water clarity obscures their sunlight. Now, resource managers in Charlotte Harbor have new tools to keep water clarity on target, thanks to a study by Mote Marine Laboratory and Janicki Environmental, Inc.

The peer-reviewed study was funded by the Charlotte Harbor National Estuary Program (CHNEP) and published in the summer 2016 volume of Florida Scientist.

The study provides new mathematical modeling tools that are helping CHNEP and partners produce a concise and easy-to-understand Water Clarity Report Card for resource managers, scientists and members of the public who aim to safeguard Charlotte Harbor’s green jewel: more than 62,000 acres of seagrass habitat.

Seagrass beds shelter and feed small invertebrates, shellfish, sport fish, manatees and more. Resource managers track the health of Charlotte Harbor’s 14 estuary segments, in part, by monitoring changes in water quality and clarity and resulting changes in seagrass coverage. Clarity describes how much light reaches a given depth without being absorbed, scattered or otherwise made unavailable to seagrasses. Clarity is critical but tricky to monitor in shallow estuary waters, where the changing angular distribution of light can challenge even high-quality measuring gear.

To address this challenge, the new study presents an Optical Model, which translates water quality sampling results — water color, turbidity (cloudiness from particles), and chlorophyll from algae — into water clarity values, describing how much light will be blocked from seagrasses at a given depth. Using water quality data from water samples, the model can estimate clarity more consistently than available instruments can measure it in the field. The new study also provides the Water Clarity Reporting Tool produced by Janicki Environmental, Inc., which compares present water clarity with a past reference period, detecting improvements and declines.

“We’ve provided a way to model water clarity more reliably at depths where we want seagrasses to be, to determine if the criteria for light are being met,” said Dr. Kellie Dixon, the senior scientist at Mote who developed the new, Charlotte Harbor-specific optical model. “If seagrasses are retreating, we need to know if that is due to a change in water clarity — for instance, from freshwater with brown color or runoff carrying fertilizer and other nutrient sources into Charlotte Harbor.”

Dixon continued: “If you’re trying to monitor water clarity by measuring light in the field, a lot can go wrong — for instance, your boat can tip sideways and reflect unwanted light where you’re trying to measure. However if you collect a jug of water, then laboratory analyses can accurately detect color, chlorophyll and turbidity, and we can use that to model light.”

To create the new optical model, Dixon analyzed years of water quality sampling and light-sensor data from Charlotte Harbor, using only the most reliable data. Using the finished Optical Model, Charlotte Harbor-based scientists can plug in their own water quality data — color, turbidity and chlorophyll — to calculate water clarity with more accuracy and precision than ever before.

“This is a wonderful tool for monitoring water clarity,” said Judy Ott, program scientist with CHNEP. “Kellie fine-tuned existing modeling work to account for the details of how light is absorbed as it goes around the water column, for waves distorting the light and for the angle of the sun. This is the first time we’ve had a good enough tool to go back in time, to use our water quality data going back to the ‘60s and calculate water clarity historically.”

Dixon’s model is also the first to represent how Charlotte Harbor’s water quality affects the individual wavelengths within sunlight, such as blue, green and red. That’s important: seagrasses need specific wavelengths of light.

Ott said that fine-tuning water clarity information helps to fine-tune management. “Seagrass growth depends, in part, on the percentage of light available; they may grow deeper if the water is clear enough. In Charlotte Harbor, our targets for seagrasses and estuary health are based on water quality and clarity. If we anticipate changes in water flow or quality, we can use the optical model to estimate how that will affect water clarity, which will let us focus on projects most protective and restorative of our resources.”

To see how water clarity fares from year to year, the public can visit the Water Clarity Report Card on the Charlotte Harbor Water Atlas website presented by CHNEP and the University of South Florida. For example, the Report Card shows that Pine Island Sound  had stable and often good water clarity over the past decade, whereas another area called West Wall had declining clarity for the past four years and cautionary scores before that. Scores for most sites vary among years or every few years. Researchers monitor these annual changes for longer-term trends.

Water quality data, used to model clarity, are collected each month by the Coastal Charlotte Harbor Monitoring Network. Partners including CHNEP, the Florida Fish and Wildlife Conservation Commission, City of Cape Coral, Florida Department of Environmental Protection, Lee and Charlotte counties collect and analyze about 65 randomized samples each month across Charlotte Harbor, from Lemon Bay through Estero Bay. Field scientists continue to monitor light in the water with increasingly sophisticated gear, which in turn, helps to check and refine models such as Dixon’s.

Dixon has previously calibrated the Optical Model for other Florida estuaries. Next, she hopes the model will shed more light on water clarity trends in Mote’s underwater “backyard,” Sarasota Bay.

Under reduced lighting, Mote Chemist Susan Launay measures a sample for chlorophyll filtration & analysis.
Samples ready for the analysis of turbidity (a measure of how much light is scattered).