Drought and Water Flow: Key to River Nutrient Health

At the Florida-Georgia border, the Chattahoochee and Flint rivers converge to form the Apalachicola River, which delivers essential freshwater and nutrients to the Apalachicola Bay ecosystem.

A recent study, led by Ebrahim Ahmadisharaf, an assistant professor at the FAMU-FSU College of Engineering, explored how droughts and water flow in the Lower Apalachicola River region affect the levels of nitrogen and phosphorus, two nutrients critical for a healthy aquatic environment. The study was published in Water Research.

Ahmadisharaf, who also works at the Resilient Infrastructure & Disaster Response Center (RIDER), emphasized that understanding how ecosystems react to changes is vital for their effective management. By regulating water systems, it is possible to prevent long-term negative effects on the environment.

The research team used data collected over 20 years by the Apalachicola National Estuarine Research Reserve, a federally supported organization, and streamflow information from a U.S. Geological Survey gauge. They used these to investigate the relationship between nutrient levels and drought conditions through various stages and recovery periods.

Phosphorus Trends

The study focused on dissolved inorganic phosphorus levels, which increased slightly at the onset of droughts while the variability in these levels decreased. As droughts worsened, phosphorus levels fluctuated more and generally decreased. However, after the droughts, when water flow improved, phosphorus levels surged due to the "flushing" effect, where nutrients are washed into the streams. This phenomenon had long-term effects on the river system, with a 35% rise in phosphorus levels during high flows between 2003 and 2021, leading to potential problems like excess nutrient accumulation, increased microorganism growth, and lower levels of dissolved oxygen downstream.

Nitrogen Dynamics

The research also looked at dissolved inorganic nitrogen, which showed more varied responses to drought. The effects depended on how severe the drought was and whether it occurred during wet or dry seasons. After droughts, nitrogen levels rebounded, but their behavior in streamflows changed—nitrogen concentrations during low flows became higher than during high flows, which was the opposite of what had been observed before the droughts.

In an ecosystem, like in medicine, the right balance of nutrients is key. While nitrogen and phosphorus are essential for the growth of plants and animals, too much can lead to algae blooms, which deplete oxygen in the water and produce toxins. Ahmadisharaf noted that the quick rise in phosphorus after droughts could temporarily overwhelm downstream ecosystems, causing algae blooms, fish deaths, and potential health risks for humans.

This study gives a more detailed understanding of how the Apalachicola River's ecosystem responds to drought conditions. Ahmadisharaf highlighted that because climate change affects the frequency, severity, and duration of droughts, these findings are crucial for developing strategies to improve climate resilience and protect coastal water quality.