Low Dissolved Oxygen in Coastal Plain Rivers: A Water Quality Problem or Natural Condition?

We depend on clean water for daily life around our home, for farming, and for manufacturing.  We also value it for recreational activities such as fishing, boating, or just enjoying the outdoors.  However, some human activities can impair the health of streams and rivers for the people that live around them and depend on them.  It is for these reasons that government agencies began monitoring the quality of our nation’s waterways in the late 1800s.

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A water quality sensor showing a dissolved oxygen concentration of 2.39 mg/L in a tributary of the Alapaha River in early April. The GA standard for dissolved oxygen is a minimum of 4 mg/L.

In 1972, Congress passed the Clean Water Act – legislation that requires all the nation’s waters be fishable and swimmable.  Essentially, this means that the nation’s waters must be clean enough to sustain healthy aquatic plant and animal life.  To meet this goal, the law requires the states to establish water quality standards – the maximum permissible concentration of various pollutants that a water body can tolerate without jeopardizing the aquatic ecosystem.  The law also requires that any waters found not meeting these standards must be brought up to standards by reducing sources of pollution.

Aquatic organisms require oxygen and they take it in a dissolved form – DO – from the water, DO levels in a river can affect the types of animals found there, and even the way the river functions.  If DO levels get too low, certain species of fish can not survive.

Because of this, states have also established standards for the minimum level of acceptable DO.  In general, all fresh waters in Georgia have the same DO standard – only trout streams have higher standards.

The majority of the smaller rivers and practically all the streams in Georgia’s Coastal Plain have low DO concentrations during spring, summer, and fall which violate Georgia’s water quality standards.  By law, these rivers and streams must be managed to comply with water quality standards.  But, regulators and scientists do not yet completely understand the causes of low DO in the Coastal Plain.

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Ph.D. student Jason Todd retrieves water samples from an automated sampler installed within the Little River near Tifton. The samples are analyzed for a dye tracer used to understand how water flows through large in-stream swamps in the Coastal Plain.

The region is characterized by slow-moving, dark, tea-colored rivers – conditions ideal for naturally occurring low DO.  The Coastal Plain is also the most intensively farmed region in Georgia.  It is not clear if low DO conditions are a natural phenomenon or a result of human activity.  To better understand this problem, I lead a team of agricultural scientists and ecologists from the University of Georgia and the United States Department of Agriculture as well as several graduate students in a project designed to understand the complex processes that determine DO levels in the Coastal Plain.

We have come to understand that natural and human activities can affect the levels of DO in a river.  Natural conditions such as high temperatures, large amounts of leaves and woody debris that fall into the river from streamside forests and an absence of waterfalls or riffles to aerate the water can lower DO concentrations.  Nitrogen and phosphorus (N & P) added to rivers from point sources such as waste water treatment facilities or nonpoint sources such as runoff from agricultural or urban areas may enhance the growth of algae in streams and rivers.  As algae complete their life cycle and die, they become a food source for bacteria which consume oxygen as they decompose the algae.  Large populations of bacteria feeding on algae are able to consume all the oxygen available in water – thus leading to the death of other aquatic organisms, including fish, which depend on the DO.  This is often described as a fish kill in the popular media.

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Dissolved oxygen concentrations are consistently below the 4 mg/L standard during summer months in most of Georgia’s Coastal Plain rivers and streams as shown in these data from 4 different streams and rivers in the Willacoochee River watershed near Fitzgerald.

Ultimately, my team hopes to understand these complex processes enough so that they can simulate them using mathematical models.  These models would then allow scientists and decision makers to evaluate the causes of low DO in individual rivers and streams without spending years collecting data on each.  Anna Cathey, a recently graduated M.S. student in the Biological & Agricultural Engineering Department and a National Science Foundation Fellow applied one of these models to approximately 25 miles of the Little River which begins near Ashburn and flows into the Withlacoochee River near Valdosta.  Her work identified many of the individual processes which the team must better understand in order to meet their goals.

To fill in these information gaps, several studies are being conducted concurrently.  Richard Carey, an Ecology M.S. student working on the project recently found that several of the streams he studied contained levels of nutrients that could lead to excessive algal growth.

However, he also discovered that the forests which shade Coastal Plain streams exclude light and consequently limit the growth of algae.  To better understand the role of streamside forests, Andrew Mehring, a Ph.D. student in Ecology is studying tree leaf decomposition and its effects on DO levels.  When tree leaves and wood drop into the water from streamside forests, bacteria and fungi feed on them and consume oxygen just as they do when feeding on dead algae.  This natural source of oxygen demand can lower DO levels in streams and rivers.  M.S. Engineering and Ph.D. Ecology students Barb Crompton and Jason Todd have evaluated the importance of DO consumption by biological and chemical process that take place in the river bottom as well as the role played by in-stream swamps which dramatically reduce the flow rate of the water.

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M.S. student Richard Carey installs a passive diffusion periphytometer in a tributary of the Satilla river to determine if the stream contains enough nutrients to promote the nuisance growth of algae.

In the end, the team hopes that our results will assist environmental decision makers to establish dissolved oxygen standards that reflect the natural conditions of the Coastal Plain.  Without a good understanding of these complex processes, we will never be able to truly understand nor predict DO concentrations. And our regulatory agencies may continue investing taxpayer funds to solve a water quality problem that may not be a problem after all.


CSREES.logoThis work has been supported by a grant from the USDA-CSREES Integrated Research, Education, and Extension Competitive Grants Program – National Integrated Water Quality Program (Award No. 2004-5113002224), Hatch and State funds allocated to the Georgia Agricultural Experiment Stations, and USDA-ARS CRIS project funds.