The resilience of coral reef ecosystems can be negatively affected by local stressors, including poor water quality resulting from land-based sources of pollution. A recent study by NCCOS and partners found that excessive nutrients from inland waterways are jeopardizing the health of Florida’s Coral Reef system.
Using machine-learning statistical modeling, the researchers tested for relationships between water quality from 29 reef sites across south Florida from 2018 to 2021, and measured land-based outflow from the coastal inlets, rainfall, and wind over the same time period. This allowed the team to determine direct links between inland water sources associated with four major inlets between Hillsboro Inlet in Pompano Beach and Government Cut, a manmade shipping channel servicing PortMiami.
Significant correlations were found between higher freshwater outflow from coastal inlets, rainfall, and wind, and increasing concentrations of all water quality analytes examined at the reef sites, indicating that water from inland sources is impacting the coral reef ecosystem. High nutrients come from a variety of contaminants, such as human waste, animal farming byproducts, and fertilizers. These contaminants are worsened by the hardening of surfaces that don’t allow for natural soil filtration to occur. Rainfall washes nutrients (nitrogen, phosphorus), heavy metals (arsenic, zinc, copper), and other chemicals into the waterways, which are connected to the ocean. Abnormally high levels of pollutants coming from the canals stress corals, affecting their bleaching tolerance, growth, and reproduction, while promoting overgrowth by competitors like algae and sponges.
Three key takeaways of the study:
- Pollution from the land is washing into the near-shore waters. And the flow from inlets, plus rain and wind, cause spikes in pollutants, such as excess nitrogen and phosphorus.
- The inlet flows were the main culprit in the changes in nitrogen, whereas higher phosphorus was associated with rainfall, an indicator of surface runoff.
- Nitrates can be reduced on the reefs if inland water flow is kept below 75 cubic meters per second.
The results provide a scientific basis for future management of land-based sources of pollution in the region to reduce coral reef stressors. The study identifies specific, actionable tipping points for water management. For example, nitrate levels peaked when the inlet flow over the previous three days hit a very specific range: between 75 and 100 cubic meters per second. Since canal flows are controlled, water managers can use this tipping point in the flood planning and canal releases to improve water quality on the coastline and beaches. The study also illustrates that actions are needed to better manage rainfall runoff before it reaches the waterways. For example, developing drainage systems that collect runoff and facilitate the natural filtration through the ground into the aquifer would help improve local water quality.
This work is authorized by the The Coral Reef Conservation Act and the Coastal Zone Management Act.