Researchers examine impact of drought and water volume on Apalachicola River nutrients

Near the Florida-Georgia border, the Chattahoochee and Flint rivers meet and become the Apalachicola River, which carries fresh water and nutrients downstream to Apalachicola Bay.

New research led by FAMU-FSU College of Engineering Assistant Professor Ebrahim Ahmadisharaf examined how drought and water volume in the Lower Apalachicola River watershed affect nitrogen and phosphorus, nutrients essential for a healthy aquatic ecosystem. The study was published in Water research.

“In watershed systems like this, which are subject to upstream regulations, knowing how the ecosystem responds to changes helps us manage it effectively,” said Ahmadisharaf, who is also a researcher at the Resilient Infrastructure & Disaster Response Center (RIDER). “We can regulate the system to avoid negative consequences, including some that could be long-lasting.”

The research team examined 20 years of nutrient data collected by the Apalachicola National Estuarine Research Reserve, a nationally protected natural area funded by the National Oceanographic and Atmospheric Administration and managed by the Florida Department of Environmental Protection.

The researchers also analyzed streamflow data from a U.S. Geological Survey gauge to characterize drought and river flow conditions, which they compared to water nutrient readings using statistical analyses. This allowed them to study the impact of droughts and river flow regimes on nutrients during different drought phases and over short- and long-term periods after the droughts ended.

Phosphorus content

One of the nutrients the researchers studied was dissolved inorganic phosphorus. At the onset of droughts, phosphorus levels tend to increase slightly, and the range of these levels typically narrows. As droughts continue and worsen, the variability of phosphorus levels increases and the average level declines. After droughts, when water flows increase, phosphorus levels in streams rebound quickly due to the “flushing” effect, in which nutrients are washed into streams from land. Three consecutive stream flow droughts in 20 years have had long-term impacts on phosphorus export. For example, phosphorus levels increased by 35% during high flows from 2003 to 2021, threatening the downstream estuary with excess nutrient levels, increased growth of microorganisms, and lower dissolved oxygen levels.

Nitrogen levels

The researchers also looked at changes in dissolved inorganic nitrogen. The impact of drought on nitrogen levels was more variable, with changes related more to the severity of the drought and whether it occurred in the wet or dry season. Nitrogen levels rebounded after the droughts ended, but their dynamics within streamflow patterns changed. For example, nitrogen levels in low flows became higher than those in high flows. Before and during droughts, the researchers observed the opposite trend.

In an ecosystem, as in medicine, the right dosage makes all the difference. Nitrogen and phosphorus are essential nutrients for plant and animal growth. But too much of either leads to problems like harmful algae blooms, which deplete dissolved oxygen and produce toxins.

The rapid increase in phosphorus after droughts could lead to a temporary excess in the downstream ecosystem that would cause algal blooms, fish kills and lead to human health problems, Ahmadisharaf said.

The results of this research help researchers better understand the Apalachicola River and its watershed. The effects of drought can be location-specific, so it is essential to look at the river in detail.

“These results help us better understand how to carefully manage nutrient levels, especially during and after droughts, to avoid ecological problems,” Ahmadisharaf said. “Given that climate change affects the timing, severity and duration of droughts, this study is important for addressing climate resilience from a coastal water quality perspective.”