In some of the deepest, darkest parts of the ocean volcanic vents spew gas into the water. Not too long ago, scientists discovered methane-breathing life way down here.
These organisms are the exception. Every other living thing on earth requires oxygen for cellular respiration. And yes, that does include plants.
Plant cells, like animals’, need oxygen to convert sugars into usable energy for normal processes. However, during the daylight hours, plants also photosynthesize (make food from sunlight) which creates more oxygen than their cells require.
Aquatic (water living) animals also need oxygen, but for it to be usable, the oxygen needs to be dissolved in the water. The colder the water, the more oxygen it can hold. Thus trout – who require high oxygen levels – will be found in the colder waters of northeast Iowa but not in our warmer rivers and streams here.
Water can gain oxygen through a few different processes. Aquatic plants can give it off during photosynthesis. And the wind can stir up the surface water, mixing in more oxygen.
Depending upon certain characteristics, some ponds might be prone to a summer fish kill. Fish, in general, have higher oxygen requirements than other underwater life and thus are usually first affected by low oxygen levels. By the hot days of August, the amount of oxygen the water can hold is usually limited. Add a series of days without wind, and less oxygen is being mixed back in to replace that used by the aquatic life. If such weather continues, fish will begin to die of suffocation.
Other ponds, though, might be more sensitive to a winter kill. While the cold water holds more oxygen, the pond ecosystem has less opportunity to recharge in winter. Ice cover creates an effective barrier between the water and air. Depending upon the color of the ice and whether snow also covers the pond, water plants, already limited by low sunlight and temperature, may fail as well to add enough oxygen back. If the ice is on long enough, the oxygen is again used up and a winter fish kill results.
Human factors may also cause fish kills. The most noticeable would be a spill that directly kills the fish through poison. An algae bloom would be an indirect way, causing oxygen deprivation.
Fertilizer, in any form, can enter a water system through spills or run-off from homes, farms, gold courses, etc. High enough amounts will create an algae bloom. Essentially, the fertilizer, as well as other wastes, feeds algae, a type of plant, and it grows like crazy (fertilizer, after all, encourages plant growth). Unfortunately, the algae eventually begin to die off and decompose.
The decomposition process requires oxygen. The unnaturally high level of dying algae calls for a similar high level of oxygen. Again, the oxygen is used up faster than it is replenished and a fish kill results.
This same process also occurs in woodland ponds as well. Only in this case, it is the decomposition of all the leaves that fall into the pond that use up the oxygen.
Algae blooms are not limited to Iowa. In fact, not even all Iowa-caused algae blooms are limited to here. Like all other natural processes, the water cycle does not acknowledge borders. Water from our land washes off into streams and rivers and eventually the Mississippi River. The Mississippi River carries it all downriver to the Gulf of Mexico.
Beginning in 1985, scientists have been recording and studying a “dead zone” or an area of extreme low oxygen within the Gulf. That means no fish, no shrimp, or other commercial marine life. The actual size of it changes each year, larger in summer and smaller in winter (when there is less run-off and fertilizer use). In 1993, after a summer of floods in the Midwest, this dead zone doubled in size. In 2010, it was the size of New Jersey. After the drought of this past summer, it is at its fourth lowest size since ’85.
Attempts to slow the overall growth of the zone, particularly in the use of fertilizers and other causes within the Mississippi River drainage area, have met with failure and resistance. I wonder, though, what we would say when the commercial Gulf fishermen begin to rightly blame us for the decline of their livelihood. It is a bit unfortunate when we don’t want to acknowledge our actions have results that ripple across our personal borders. Seems kind of selfish to me.