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Water Surrounding Sanibel and Captiva Classified as Impaired Due to Nutrient Enrichment

October 27, 2021

The Florida Department of Environmental Protection’s (DEP) 2020-2022 Biennial Assessment Draft classifies the waters in Pine Island Sound, San Carlos Bay, and the J.N. “Ding” Darling National Wildlife Refuge as “verified impaired” because they exceed nitrogen and chlorophyll a standards designed to support a healthy ecosystem.


The DEP evaluates water quality data collected for waterbodies within 29 watershed basins throughout the state. If there is enough data to meet the state’s requirements, DEP will determine if the waterbody meets water quality criteria consistent with the federal Clean Water Act. The criteria are meant to protect the health of fish and wildlife and ensure that waterbodies are swimmable and fishable and, in some cases, meet standards for shellfish harvesting and drinking water.


In 2020, the DEP analyzed data, including new data provided by SCCF  (Sanibel-Captiva Conservation Foundation), which indicate that the waters immediately surrounding both Sanibel and Captiva islands do not meet water quality standards. When waterbodies are identified as impaired, the DEP will develop a plan for reducing the pollutant of concern. In this case, the plan would focus on reducing the amount of nitrogen entering the waterbodies from the adjacent land uses.


In addition to the nutrients nitrogen and phosphorus, DEP considers concentrations of chlorophyll a in the water. Chlorophyll a is an indicator of how much algae growth is occurring. High levels of chlorophyll a indicate waters are enriched with nitrogen and phosphorus (eutrophic) and the ecosystem can become over productive, leading to oxygen depletion, fish kills, harmful algae blooms (HABs), and other ecological problems.


“The sad reality is that most waterbodies throughout Florida are currently impaired, with a large majority of them being impaired for nutrient pollution. Despite all the work that our communities on Sanibel and Captiva have done to preserve natural areas and reduce nutrient runoff from the landscape, the quality of our coastal waters continues to decline,” said SCCF Environmental Policy Director James Evans. “This is in part due to the influence of freshwater discharges from the Caloosahatchee and Peace rivers, but local nutrient sources also continue to impact water quality.”


The waters on the Gulf of Mexico side of Sanibel and Captiva are not classified as impaired because DEP still does not have enough data to determine their status. However, since 2018, the SCCF Marine Laboratory has been systematically collecting water quality data during quarterly sampling cruises in the Gulf aboard the R/V Norma Campbell. This data is entered into DEP’s database and will eventually be used to assess the status of the Gulf waters adjacent to the islands. “What we have discovered so far in the Gulf of Mexico has surprised us a bit,” said SCCF Marine Laboratory Research Associate Mark Thompson. “Even though the Gulf has much more water to dilute pollutants entering from adjacent sources, levels of nitrogen and chlorophyll a near the islands often exceed criteria established for Pine Island Sound.”


SCCF will be working with DEP to evaluate water quality in the nearshore Gulf waters. “Our Marine Lab will continue to document and track nutrient loads entering local waterbodies to assist in identifying solutions to reduce nutrient sources and improve water quality,” said Thompson.


The DEP is accepting public comment on the assessment until Nov. 10. Learn more about the assessment at Watershed Assessment Section | Florida Department of Environmental Protection.


The devastating red tide events of 2018-2019 resulted in more than 425 tons of dead marine life being removed from Sanibel. The Sanibel-Captiva Chamber of Commerce reported more than $47 million in economic losses between July and October. A closer assessment of the nutrients that contribute to red tide and other harmful algal blooms (HABs) will provide valuable tools to researchers in determining the factors that contribute to destructive blooms.


The large-scale HABs of recent years have made the links between water pollution, human health, quality of life, and the economy quite clear. Southwest Florida has experienced the health and economic impacts of closed beaches, aerosolized toxins, and dead and dying fish and wildlife scattered along the coast. All coastal communities must work together to improve water quality.


Sanibel has set an example of the change that can occur when a community makes a concerted effort to improve water quality. By conserving natural lands, reducing the use of fertilizer, eliminating turfgrass, planting native plants, minimizing and/or eliminating impervious surfaces, and converting the island from septic to centralized sewer, Sanibel has demonstrated a significant commitment to improving water quality. Despite extensive efforts by the citizens of Sanibel, more work needs to be done to improve local water quality.


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