That all-important DO

We know that fish and other organisms can because of oxygen dissolved in the water. But how does that oxygen get into and stay in the water?

First of all, oxygen doesn’t “dissolve” in water in the same manner as, say, salt or sugar. Salt, for example, is a compound of one sodium and one chlorine atom. In water, it ceases to be a solid, splitting into its two parts – a positively charged sodium ion and a negatively charged chlorine ion. Oxygen gas, on the other hand, consists of two oxygen atoms, and it remains in that form when mixed with water. So instead of referring to dissolved oxygen, should we really speak of suspended oxygen? Or entrained oxygen? (OK, now we are officially splitting hairs.)

Dissolved oxygen (DO) is single most important component of water quality because, quite simply, without adequate DO, next to nothing lives in the water. How does oxygen get into your lake? Mainly from the atmosphere (which contains about 20 percent oxygen) and from photosynthesis – the process by which plants turn sunlight into food. The amount of oxygen in water is tiny compared to the amount in the air. A healthy lake or stream will contain 6 to 8 parts per million (ppm) of oxygen – or 0.0006 to 0.0008 percent. If DO is less than 2 or 3 ppm, most fish will become stressed or die.

So does more of the oxygen in your lake come from plants or from the atmosphere? Well, besides the plants you recognize, like cabbage weed, coontail and water lily, your lake’s water is full of microscopic algae – phytoplankton. The plants and all these tiny algae constantly photosynthesize and produce oxygen while the sun shines. But if that’s the case, then why isn’t a lake with heavy weed growth or in the middle of an algae bloom rich in oxygen? Why in fact can an algae bloom lead to oxygen depletion and fish kills?

Well, remember that photosynthesis requires sunlight – so it doesn’t happen at night or on days with heavy cloud cover. During those times, the plants and algae are consuming food and using up oxygen, just like most other living things. Furthermore, the algae can die off en masse, at which point oxygen-using (aerobic) bacteria break the cells down. That can cause DO to plummet to levels unsafe for fish. Most periods of oxygen stress happen in summer, because warmer water holds less oxygen than colder water, and because oxygen-consuming processes, like decay of dead matter, happen faster at higher temperatures.

So, while algae and plant life contribute to dissolved oxygen, they are a bit of a mixed bag. The main source of life-giving oxygen in your lake is the atmosphere. And the most oxygen mixes in when a healthy wind kicks up waves. So next time you see a wind raising whitecaps out on the water, imagine that your lake has just filled its lungs with a big, deep breath of fresh air. And now the fish can, too.