The concentration of oxygen in water is a key component in the assessment of water quality in rivers, streams, and lakes.

The fact that we and many other organisms specifically require oxygen in the air to stay alive was not always known. Instead, the air was thought to possess some vital force necessary to sustain or give rise to life. Joseph Priestley (1733 -1804) is credited with the discovery of oxygen, noting its importance to life, and that photosynthesis produced this important gas. He carried out a number of experiments including one with mice and mint plants within sealed glass bell jars.

A bell Jar

As expected, a mouse alone in the bell jar perished but remained alive in the presence of a mint plant. The plant produced oxygen through the absorption of the carbon dioxide given off by the mouse. Water and sunlight were the other required ingredients. Oddly enough, the same type of experiment was done with one man placed in a sealed chamber with plants1. Yes, a sample size of one. It might have been difficult finding volunteers! However, not all organisms require oxygen. Some bacteria are capable of surviving without oxygen and play an essential role in the cycling of phosphorus, iron, sulfur, and magnesium, especially in lakes but also rivers.

Via Queen Mary University of London

Oxygen compared to carbon dioxide diffuses very slowly in water. If not for the natural turbulence in streams and rivers and wind action on lakes, very little oxygen from the atmosphere would make its way into these habitats. Hence, in lakes, photosynthesis can play a very important role indeed.

The amount of oxygen within the Petitcodiac River is also influenced by temperature and the partial pressure of oxygen in the air. As temperature increases, oxygen becomes less soluble in water and the excess is released back into the atmosphere. Likewise, a decrease in the partial pressure of oxygen with an increase in altitude or a low-pressure system also results in less oxygen being dissolved in water. The instruments we use to measure oxygen in the Petitcodiac River correct for the difference in the solubility of oxygen with temperature and pressure. Oxygen is measured in mg/L and can also be reported as percent saturation. There are charts and diagrams that allow you to determine for a given temperature what the concentration of oxygen would be in mg/L if the water was 100% saturated or less. Why is this important?

Use the chart below for nomagrams for calculating oxygen saturation

Example: Let’s say you find that the concentration of oxygen in your water sample is 8.5 ppm (mg/L) and the water temperature is 15 °C. What is the percent saturation? Print out a copy of the diagram to not damage your computer screen or imagine the following instructions. Place a ruler so that the straight edge lines up with the 15 on the temperature scale and 8.5 on the oxygen scale. This gives you a percent oxygen saturation of approximately 85%. High photosynthetic activity can also lead to supersaturation where values are above 100%. Hence, the % saturation scale extends beyond 100%.

Water temperatures are rising with global warming. Fish are cold-blooded, meaning their body temperature follows that of the water. As water temperatures go up, so does the body temperature of the fish, which increases their demand for oxygen. Water contains less oxygen at higher temperatures, so fish are really in a bad way. They not only need more oxygen to stay alive but there is less of it available. To compensate for less oxygen in the water, the heart rate of the fish goes up to deliver a large volume of blood to the tissues. The end result when pushed too far or for too long is cardiac failure, quite simply put a heart attack.2 Hence, the importance of oxygen to life in freshwater cannot be discussed without including temperature.

Not all fish show the same sensitivity to higher water temperatures. Salmonids such as brook trout, Atlantic salmon, smelt, and gaspereau are the most sensitive. Juvenile salmon die when exposed for seven days to temperatures that reach or exceed 27.8±0.2 °C 3. This underscores the importance of maintaining vegetation along the river that provides shade as well as protecting areas of inflowing cold water such as springs that seep into the river. An example of a more tolerant species would be the American eel which has an upper lethal limit of 38 °C 4. However, this is only one species, and the loss of all salmonids in the Petitcodiac River would be a terrible blow to its biodiversity. I have skipped over the impacts that will occur to lots of other species in the river with increasing water temperatures. The macroinvertebrates, animals visible to the naked eye and with no backbone will also be affected. Many are sensitive to even small changes in temperature and oxygen concentration. Even if not directly killed by less oxygen and higher temperatures, insects might hatch earlier in the season. Fish are highly dependent on insect prey early in their life history and if they hatch at a different time than the usual peak availability of their prey, their survival will suffer. This offset in timing of predator and prey is referred to as the uncoupling of ecosystems 5.

We will come back to oxygen in a later issue of this blog when we look at some important nutrients essential to life in the Petitcodiac River. However, these same nutrients can present serious problems when in excess. Before we look at the latter, in the next issue we will look at another water quality criterion, pH (acidity).

1Martin, D., Thompson, A., Stewart, I., Gilbert, E., Hope, K., Kawai, G., & Griffiths, A. (2012). A paradigm of fragile Earth in Priestley’s bell jar. Extreme Physiology & Medicine, 1(1), 1-5.

2 Heart-Stopping Science: The Importance of Fish Physiology. August 2015. Retrieved May 3, 2023.

3 Elliott, J. M. (1991). Tolerance and resistance to thermal stress in juvenile Atlantic salmon, Salmo salar. Freshwater Biology, 25(1), 61-70.

4 Sadler, K. (1979). Effects of temperature on the growth and survival of the European eel, Anguilla anguilla L. Journal of fish biology, 15(4), 499-507.

5Winder, M., & Schindler, D. E. (2004). Climate change uncouples trophic interactions in an aquatic ecosystem. Ecology, 85(8), 2100-2106.