Dissolved Oxygen
Oxygen spells life!
In 2010, we added something new to our water sampling: dissolved oxygen measurements. These data can tell us a lot about the health of our ponds. Oxygen is crucial for life. If our body is deprived of it, we die. We continuously have to breathe air, a mixture of a handful of gases, the second most abundant component of which is oxygen, at about 20% of the mixture. Our lungs are able to extract oxygen from this mix, and attach it to our blood, which distributes it throughout the body.
If we take a look at water, we find something really curious. Chemists tell us that water is a mixture of about 90% oxygen and 10% hydrogen. That’s a far bigger percentage of oxygen than in air. Nevertheless, if we fall into a pond and can’t swim, we drown; we die of a lack of oxygen. This is because the oxygen in water is so tightly bound to the hydrogen – it has formed a new chemical: water - that our body can’t break it up and use the oxygen.
However, there usually is some oxygen dissolved in water, just like salt can be dissolved in water. This is different from combining to make a new chemical. At room temperature, a quart of water can absorb about 1/4000 of an ounce of oxygen. Add any more oxygen, and it will form a gas bubble, rise to the surface and evaporate. Fish, shellfish, etc. have gills that allow them to access and live on this minute amount of oxygen. Their life is precarious, though, and can be made impossible by just small changes in the temperature or pressure of water which may result in decreasing this life sustaining gas. Or else the oxygen can be used up by another process, for example by the decay of organic matter, as we will see farther below.
The Data
The Town of Plymouth has provided us with an instrument to measure the amount of dissolved oxygen in water: a probe suspended from a long cable. This probe measures not only the amount of dissolved oxygen but also water temperature. We routinely take it to two of the deepest locations on the pond, one (called by us GH-10) about 45 feet deep, the other (called GH-11) 35 ft, and take data in 3 foot steps all the way from the water surface to the lake bottom. You can find these locations on one of our sampling maps. The first thing to recognize is that aquatic plants and algae consume oxygen at night, but produce oxygen during the day, especially in bright sun light. So there will be a change depending on the time of day. We would also expect stark differences, certainly in temperature, depending on the season, and, of course, that’s what we find. The same is true for the amount of oxygen dissolved in the water. In Spring, and again in Fall, water temperature and oxygen levels are more or less the same everywhere, a bit warmer, and richer in oxygen, near the surface, a bit colder, with less oxygen at the higher water pressure near the bottom. However, the oxygen level is near 100% saturation everywhere. This is an indication of a healthy lake in which the frequent strong Spring and Fall storms churn the water all the way to the bottom, mixing top and bottom water thoroughly. Fish and aquatic plants thrive under these conditions.
Something startling happens in our ponds in the summer. Near the surface, the water is warm, as expected, and stays that way down to about a depth of 20 ft. But then, at that depth, the temperature very abruptly changes within a couple of feet and becomes about 5 degrees centigrade colder. Going down even farther, it stays at about that temperature all the way to the bottom. So there are two regions in which there is water mixing, separated by a thin layer not crossed by the water. The oxygen measurements show a very similar pattern. Near the surface, during the day, there is often an excess of oxygen above 100% saturation, attributed to large amounts of algae releasing extra oxygen into the water. This bloom of algae near the pond surface is stimulated by too much nutrients – mostly phosphorus. The abrupt change of temperature near a depth of 20 ft is mirrored by an abrupt change in the oxygen level at the same pond depth, except that below this depth the water becomes devoid of oxygen, all the way to the bottom - it is a dead zone. An explanation can be found in the life cycle of algae, which is no more than a couple of weeks. Once they die, they start decaying, a process that consumes oxygen, and slowly sink to the bottom. If there is an abundance of them, their decay consumes all the available oxygen in the water near the bottom.
The Conclusion
The depletion of oxygen near the pond bottom in the Summer is caused by heavy algae growth in the upper, sunlit reaches of the pond. This indicates an unhealthy pond condition and may lead to fish and shellfish kills, odor problems (rotten egg smell), allergic reactions when swimming (swimmers’ rash). The trigger is an excess of nutrients, predominately phosphorus. Actually, there is a vicious cycle at work during the Summer. In Spring and Fall, oxygen dissolved in the water prevents the phosphorus in the muck, accumulated at the pond bottom over the years, to change into a form that is released into the water. Once the water has lost its oxygen, the chemistry changes and the muck begins releasing phosphorus into the pond, adding to the nutrient problem. So the key to a healthy pond begins and ends with reducing nutrients, something every resident can help accomplish, by use of low or zero phosphate household and other chemicals and fertilizer, and by properly disposing of animal waste.
