Tampa Bay Sediments Reveal Hidden Toll of Fertilizer Wastewater Discharges

The Tampa Bay area is known for its diverse ecology, being situated along the western coast of Florida. Because of the ecosystems and wetland areas being important to the local economy and climate, it is paramount that these are well-preserved and free from pollutants. Unfortunately, beneath the sediment surface, fertilizer-industry wastewater releases are presenting a viable threat to the ecosystem in Tampa Bay. Recently, a team of researchers from the University of Florida investigated the extent of this problem. The study’s findings were published in Ecological Indicators (1).

Why is phosphorus important?

Phosphorus is an essential element for global food production (2). However, mining and fertilizer manufacturing generate massive quantities of phosphogypsum waste, which are byproducts that carry the risk of environmental contamination (1). When released into waterways, these wastewaters can intensify cultural eutrophication, a process in which nutrient overload accelerates algal blooms, depletes oxygen, and degrades aquatic ecosystems (1).

However, this is not the biggest challenge environmentalists face. The presence of “legacy” nutrients is the biggest concern. Legacy nutrients are nutrients stored in sediments from past discharges (1).

What did the researchers do in their study?

In their study, the researchers were able to show a detailed picture of how industrial phosphogypsum releases has driven long-term phosphorus accumulation and altered biogeochemical cycles within the estuary (1). The researchers collected sediment cores from two locations: Piney Point Creek, the site of a high-profile 2021 wastewater release, and Bishop Harbor, an area affected by multiple discharge events dating back to the early 2000s, to gain a better understanding of Tampa Bay’s sediment record over the past two decades (1).

Then, the researchers analyzed the sediment cores using an assortment of analytical techniques. Through bulk nutrient analysis and ³¹P nuclear magnetic resonance spectroscopy, and short-lived and stable isotopes, the researchers reconstructed the timeline of nutrient deposition and the chemical nature of the stored nutrients (1).

What results did they achieve in their study?

The initial findings revealed an abundance of legacy nutrients in the sediment cores. Total phosphorus levels ranged from 0.11 to 15.01 mg per gram, and total nitrogen reached 0.04 to 0.37% (1). According to the researchers, these values were comparable to those observed in hypereutrophic lakes severely impacted by agricultural and urban runoff (1).

The researchers also uncovered new information about sediment accumulation rates, and the results indicated that past discharge events were negatively impacting these areas. In Piney Point Creek, sediment accumulation was 3,064 to 23,990 g/(m² yr) while Bishop Harbor registered 13,092 to 46,706 g/ (m² yr) (1). These numbers are higher than what other researchers have found in other Florida estuaries.

And finally, the researchers uncovered information about how extreme weather could impact sediment accumulation. When Hurricane Ian struck in September 2022, storm-driven reverse surge and unusually low water levels caused sediment resuspension, allowing researchers to track nutrient shifts before and after the hurricane (1). Their findings suggest that disturbances such as storms can remobilize buried nutrient pools, potentially refueling eutrophication even in the absence of new pollution (1).

Current estuarine nutrient budgets and restoration initiatives treat pollution as a forward-looking problem, focusing heavily on reducing present-day nutrient inputs. However, the Tampa Bay sediment record demonstrates that legacy nutrients stored for years or even decades can continue to influence ecosystem health long after visible wastewater releases cease (1). Ignoring internal nutrient fluxes, the authors argue, could lead policy makers to underestimate the scope of intervention needed to restore impaired waters (1).

What are the key takeaways from this study?

There are a couple of key takeaways from this study. First, the study’s findings stress the importance of improving phosphorus sustainability across the fertilizer supply chain. Second, the study highlights the urgent need for solutions such as improved phosphogypsum recycling, reducing dependence on mining while minimizing environmental risk (1).

As the global agriculture industry continues to rely heavily on phosphorus-based fertilizers, the Tampa Bay sediment archives offer a cautionary tale: environmental impacts are not limited to the moment of discharge. They can be buried only to reappear years later. Through the use of spectroscopy, researchers are able to get a better understanding of how severe the problem is and, therefore, positively contribute to driving better environmental policies.

The Tampa Bay watershed is approximately 2,200 square miles (3). It covers a sizable area, and therefore, if it is not adequately preserved, its negative impact would be great. This study helps reveal the extent of the problem, and why industry practices need to continue to become more environmentally sustainable.

References

1. Chappel, A. R.; Kennery, W. F.; Waters, M. N.; et al. Coastal sediments record decades of cultural eutrophication in Tampa Bay, Florida, USA. Ecol. Ind. 2025, 172, 113329. DOI: 10.1016/j.ecolind.2025.113329
2. Cordell, D.; Drangert, J.-O.; White, S. The Story of Phosphorus: Global Food Security and Food for Thought. Glo. Environ. Change 2009, 19 (2), 292–305. DOI: 10.1016/j.gloenvcha.2008.10.009
3. Tampa Bay Estuary Program, About the Bay. TBEP.org. Available at: https://tbep.org/about-the-bay/ (accessed 2025-11-19).