A deadly combination

A deadly combination

Summertime means hours at a pool, running through a sprinkler, catching fireflies and dissolved oxygen.

Dissolved, what?

Dissolved Oxygen is an important parameter for water quality monitoring and in determining fish survival. With water levels dropping in the Brazos River under the Texas summer sun, people are not the only creatures trying to beat the heat. The death of some of our aquatic life results from a combination of summertime events.

Dissolved oxygen is the amount of pure oxygen gas present in a water body, including a reservoir or river. Dissolved oxygen is necessary for aquatic species like fish and insects that rely on water for their intake of oxygen, absorbing it through their gills or bodies. 

Dissolved oxygen, or DO, also is an important parameter used in water quality monitoring. The concentration of DO in water is dependent on the temperature of the water. Warm water holds less pure oxygen gas than cold water because water molecules expand as they heat up, leaving less room for oxygen molecules to dissolve.

Dissolved oxygen in water follows daily and seasonal patterns based on biological processes and water temperature. DO is highest when the sun is at its peak in the afternoon as aquatic plants are photosynthesizing, producing food by taking up carbon dioxide and releasing pure oxygen gas into the water. The rate is lowest in the morning after bacteria have been respiring or breathing, which is when they take up pure oxygen gas from the water and release carbon dioxide during the night. 

Biological functions such as photosynthesis and respiration are major factors in the daily cycle of DO, whereas ambient air and water temperatures factor more into the seasonal cycle. In the winter months when the water is colder, more pure oxygen gas can dissolve into the water. As the water heats during the summer, sometimes very quickly when the water is shallow or pooled, less oxygen dissolves into the water. The reduced oxygen holding capability of the water and high temperatures can result in the mortality or death of aquatic animals when experienced for a prolonged period.  

Most mass fish kills are directly caused by a depletion in dissolved oxygen. Fish species, such as striped bass, are intolerant to low dissolved oxygen and are not adapted to warm water, which is why they are abundant and thrive in a reservoir but have difficulty surviving in the river.


Research associated with conditions related to fish summer mortality determined striped bass populations in Keystone Reservoir, Oklahoma stopped feeding and died when water temperatures were sustained at higher than 80.6°F and dissolved oxygen concentrations were low. The stripers in this study were trapped between two layers of water, a warm upper layer with high dissolved oxygen concentrations due to exchange with the atmosphere and a cool, anoxic or low oxygen layer on the bottom. The fish were unable to escape once the water warmed beyond their tolerance.

For several months, heavy spring rainfall prompted the Brazos River Authority to pass water from Possum Kingdom Lake downstream through Lake Granbury to Lake Whitney’s flood control pool.  This release moved thousands of reservoir-dwelling striped bass and other fish out of their comfortable reservoir homes and into a less hospitable river.  

As releases from the reservoirs slowed, the stripers began to move upstream to areas just below the dam, attracted to the increased feeding opportunities due to the unusually high rains. As the BRA lowered the release rate at the dam when inflows into the reservoir slowed, the river below the dam receded, trapping fish in the shallow pools. The temperature of the water increased and dissolved oxygen decreased beyond the fish’s ability to survive, causing a fish kill. 

In both the case of Keystone Reservoir and the Lake Granbury fish kills, local authorities increased the rate release to lower the water temperatures and increase oxygen to save what fish they could.  Some fish survived, many did not.

But what led up to those events?

Water is constantly interacting with the ambient air above it. Rivers and streams are mixed regularly as long as they are flowing and not pooled. Flowing water assists in transporting oxygen and nutrients. 

That interaction comes to a standstill in the summer months as Texas faces little to no rain and drought conditions worsen across the Brazos River basin. 

Unlike flowing rivers, reservoirs become stratified during summer months, with a layer of warm water (epilimnion) resting on top of a layer of cool water (hypolimnion). In deep reservoirs, the hypolimnion is lacking oxygen because sunlight cannot reach the bottom; therefore, no photosynthesis takes place in this layer, bacteria are continually respiring, and no mixing is occurring. Only the epilimnion or this top layer is mixing with the atmosphere and exchanging gases, which provides a habitable area for fish like stripers.  

Strong winds and precipitation can cause partial mixing in deep lakes or total mixing in shallow lakes between the two water layers, which can distribute due to deeper depths.  In the fall, the surface water in the epilimnion cools and becomes denser, causing it to sink and mix with the hypolimnion, a process called lake turnover.    


Reduced oxygen concentration and even total depletion are caused by several factors:

  • increase in water temperature over time
  • algal blooms
  • pollution
  • drought
  • reduced flows

Shallow water is more susceptible to radical changes in temperature because it takes less sunlight to heat a small volume of water. Think of trying to boil water on the stove: the less water you have in the pot, the shorter time it takes to bring it to a boil.  

Rivers and streams in Texas experience reduced flows during the summer because more water is leaving the system through evaporation, regular downstream flow, and water use than is being added to the system through rainfall.  During this time, rivers and streams can become a series of pools and shallow riffles or can stop flowing altogether, limiting the passage of large-bodied fish seeking to escape to deeper, cooler pools upstream or downstream. 

When these pools and shallow areas form, the water is susceptible to rapid heating, reducing the amount of oxygen available to fish and other aquatic organisms.  

The summer is also the time when plants are the most productive, and harmful algal blooms can occur when pollution introduces an excess of nutrients useful to plants for photosynthesis. A major increase in plant production contributes more oxygen to the water, but more plants are dying, decaying, and being consumed by bacteria that take up the available oxygen.  This phenomenon is known as eutrophication and is devastating to fish and resident aquatic animal populations.  

The water is a great retreat for people and animals during the hot Texas summers. For some, though, it can be fatal. 

Article by: Brazos River Authority Aquatic Scientist, Cory Scanes