Water Quality: Dissolved Oxygen

Summer means warm temperatures, including warm water temperatures. While that may be the kind of water you want to swim in, it’s not always the kind of water our Chesapeake Bay animals want to live in. Warmer water holds less oxygen and many animals have gills, which they use to breathe oxygen out of the water.

This time of year can bring issues with low dissolved oxygen levels in the water, which is called hypoxia. Normal levels of dissolved oxygen are above 4 parts per million (ppm), hypoxia starts when dissolved oxygen drops below 3 ppm. When dissolved oxygen levels reach 0.5 ppm and below, we call this anoxia. Anoxia means there is not enough oxygen to support life. When this happens, dead zones occur. If animals can swim fast enough, they may escape a deadzone. However, slower moving or non-moving animals, cannot escape and die, and these animals include crabs and oysters.

Just about any body of water can experience a dead zone under the right conditions. Every summer, the Chesapeake Bay experiences a dead zone, though in recent years, it has been smaller than average. Often, a dead zone is caused by eutrophication. We talked about how excess nitrogen can cause this back in April, but a quick refresher is that too many nutrients in the water cause plants like phytoplankton to grow like crazy. Eventually, the phytoplankton run out of food and the population crashes. The decomposition of the dead phytoplankton uses up a lot of oxygen, causing hypoxia and even anoxia.

The Chesapeake Bay dead zone typically occurs in the summer due to the excess nutrients that enter the Bay during spring rains and the lack of rain, lack of strong winds, and warm water temperatures in the summer. Basically, we see a spring boom for phytoplankton populations due to nutrient runoff and by the time summer rolls around, many of those phytoplankton have died. When they die, they fall to the bottom where decomposers break them down in a process that uses oxygen. Without more rain or strong winds, there isn’t much mixing of surface and bottom waters so the bottom water rapidly loses oxygen.

When thinking about dissolved oxygen, it’s important to consider where the oxygen in the water is coming from. Some of it comes from plants as they undergo photosynthesis, turning carbon dioxide and sunlight into sugar and oxygen. Some of it comes from the air itself, so we tend to see higher levels of dissolved oxygen at the surface of the water just due to diffusion of oxygen molecules from the air to the water. When we have water that is well mixed, we tend to see a more even distribution of dissolved oxygen from the surface to the bottom. However, when we have water that is not well mixed, we might see depletion of the oxygen from the bottom waters as there is no mechanism to replace that oxygen once you drop below the depth at which plants are growing and photosynthesizing.

The Chesapeake Bay Watershed Agreement has the goal that Watershed Implementation Plans, which help individual jurisdictions set water quality goals and create pathways to achieve those goals, will be in place and achieving those goals by 2025. The process of achieving those goals should allow for dissolved oxygen standards to be met. However, the pollution reduction goals that will ultimately influence dissolved oxygen levels, are not on track to be met by the 2025 deadline. It is important to note that the reductions we have seen, along with favorable weather conditions, have produced smaller dead zones in the last several years compared to previous years.

Ultimately, the goal of dissolved oxygen levels is to sustain life and support a healthy ecosystem. The table above shows the dissolved oxygen goals for the Chesapeake Bay based on habitat and use. You can see that dissolved oxygen levels above 1 mg/mL (or ppm) is the target for deep water and channels, while open water is above 4 mg/mL, and in the spring when the Bay is used by migratory fish for spawning and as a nursery for juveniles, the target is above 5 mg/mL. The other thing to note from this table is that dissolved oxygen levels are variable, so we might use a single reading or we might use averages over a period of time, and the goal for those numbers might be slightly different. Again, if water temperatures are high (such as in the middle of the day), we might get a lower oxygen reading than if we were testing in the morning.

When it comes to actually measuring dissolved oxygen, there are a couple different ways to get this data. One is to use a YSI sonde, which we’ve talked about already. Another method is to use a color based change on a reactive reagent. 

Once again, the colorimetric method tends to be cheaper and less accurate but is good for getting a general idea of dissolved oxygen levels, whereas purchasing a YSI is a large initial cost and requires regular maintenance and calibration but provides a more precise measurement. 

Here at Phillips Wharf, if we are collecting dissolved oxygen data for scientific or research purposes, we use a YSI. When working with students, we use colorimetric methods so that students understand the measurement and it’s more hands on than seeing a screen spit out a number.

Leave a Reply