Comment on 2014 DO results: We did not find the division into three zones, oxygen rich water near the top, a narrow transition layer, then a low or zero oxygen layer near the bottom in any of our data, all year long. Instead we always found oxygen-rich water all the way to the bottom.

We are not sure why this happened. Was the weather stormy enough to churn it all the way to the bottom? Another possible explanation is that the pond has become clean enough to restrict algae growth and the associated DO-depleting decay.  Whatever the reason, along with the fish and shellfish, we were delighted and hope it continues.

Dissolved Oxygen and Temperature

Oxygen spells life
In 2010, we added dissolved oxygen (DO) measurements to our sampling. The amount of DO in the water can tell us a lot about the health of our ponds. Oxygen is crucial for our lives as well as pond life. Every molecule of water is made of two atoms of hydrogen and one atom of oxygen, but the oxygen in water is so tightly bound to the hydrogen 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. Algae and rooted aquatic plants produce oxygen during the day using dissolved nutrients and the energy of sunlight through photosynthesis, and infuse it into surrounding water. Oxygen can also dissolve into water from the atmosphere, especially with agitation/aeration for example from wind or waterfalls. 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. The plants use DO at night in respiration, and it is also used by bacteria during the decay of organic matter.

The Data
The Town of Plymouth provided HPWA with an instrument to measure the amount of dissolved oxygen in water: a probe suspended from a long cable, with a controller/display attached at the other end (see the picture to the right). 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 one meter (about 3.3-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.

In Summer water near the surface is heated by sunlight and forms a layer less dense which "floats" above and inhibits mixing with colder water below. Near the surface, during the day, there is often an excess of oxygen above 100% saturation, attributed to large amounts of algae releasing oxygen as a by-product of photosynthesis into the water. "Blooms" of algae near the pond surface are triggered by sunshine, but also by too much nutrients – mostly phosphorus dissolved in the water. 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
Depletion of oxygen near a pond bottom in Summer is caused by heavy algae growth in the upper, sunlit reaches of a pond. This is 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 input from overland runoff and/or released from previous inputs that accumulate in pond sediments. So the key to a healthy pond begins and ends with reducing nutrients, something every resident can help accomplish, by maintaining shoreline buffers to prevent runoff, and use of low or zero phosphate household and chemicals and lawn fertilizer, maintaining the septic system, and properly disposing of animal waste.

YSI ProODO meter