Eutrophication and Thermal Pollution

Dissolved oxygen: the number that tells the story

Fish don't breathe air. They take in oxygen gas that has dissolved in the water. This is dissolved oxygen (DO), measured in milligrams of oxygen per liter of water (mg/L).

Water gets its oxygen from two sources: the atmosphere (mixing at the surface, rapids, waves) and photosynthesis by algae and aquatic plants. It loses oxygen when organisms respire, especially decomposers breaking down dead matter.

Different organisms need different amounts:

Temperature matters a lot. Gas dissolves better in cold water, just like a cold soda stays fizzy while a warm one goes flat. Water at 5 °C can hold about 12.8 mg/L of oxygen at saturation, but water at 30 °C holds only about 7.6 mg/L.

Eutrophication: too much of a good thing

Algae and aquatic plants need nutrients, mainly nitrogen (as nitrate, $\text{NO}_3^-$) and phosphorus (as phosphate, $\text{PO}_4^{3-}$). In most natural waters one of them is scarce, which limits growth. Add a lot of either one and the system changes.

Where do the extra nutrients come from? Fertilizer runoff from farms and lawns, animal manure from feedlots, and sewage and detergents from cities. Fertilizer runoff is a nonpoint source, because it enters from a wide area. A sewage pipe is a point source, because it comes from one identifiable spot.

The chain of events

The key idea: the algae don't use up the oxygen directly. The decomposers do. A bloom actually raises surface DO during the day, because algae photosynthesize. The damage comes when the huge mass of algae dies and sinks. Bacteria decompose it and respire, removing oxygen from the deep water. The amount of oxygen that decomposers need is called biochemical oxygen demand (BOD). High BOD means a lot of decomposable organic matter and, soon, low DO.

Some blooms also release toxins and block sunlight, so submerged plants die, and the habitat they provided disappears.

Reducing eutrophication

• Farms: apply only the fertilizer crops need, time it away from heavy rain, plant cover crops, and leave buffer strips of vegetation along streams.

• Cities: upgrade wastewater treatment to remove nitrogen and phosphorus, use phosphate-free detergents, and reduce lawn fertilizer.

• Restoring wetlands between fields and rivers, because they filter and absorb nutrients.

Reading water-quality data

On exams you will have to read tables like the ones students collect in lab: nitrate, DO, algae counts. The skill is to look for a relationship between variables and then explain it with a mechanism.

How the data are collected

In a lab or field investigation, you measure DO with a probe or a titration kit (mg/L). You measure nitrate with a test kit or probe that compares color or voltage to a standard (mg/L). You measure chloride with a kit or probe too. High chloride from road salt or wastewater tells you that runoff or sewage is entering the water. To compare sites fairly, sample at the same depth and time of day, and repeat each measurement.

Now look at a full summer in a lake. Two different layers behave differently, which is why a table with surface and bottom DO is so informative.

Thermal pollution

Thermal pollution is a change in water temperature caused by human activity, usually warming. The biggest source is cooling water from power plants and factories. A power plant that burns coal or natural gas, or uses nuclear fission, makes steam to spin a turbine. The steam must be cooled, so the plant pulls in river or lake water, uses it as coolant, and discharges it warmer. Removing shade trees along a stream and urban runoff from hot pavement also warm water.

Warm water harms life in three linked ways:

1. Less oxygen is dissolved. Gas solubility drops as temperature rises (see the graph in the first section).

2. Metabolism speeds up. Fish are ectotherms, so their body temperature follows the water. Warmer water means a higher metabolic rate, so they need more oxygen just when there is less.

3. Stress and loss of habitat. Many cold-water species such as trout and salmon can't tolerate a rise of a few degrees. Their eggs may fail to develop, they are more vulnerable to disease, and they are replaced by warm-water species. Sudden shutdowns of a plant can also cause thermal shock: a rapid temperature drop that kills fish that had adapted to warm discharge.

Reducing thermal pollution

• Cooling towers let heated water cool by evaporation before it returns to the river.

• Cooling ponds hold the water until it reaches near-ambient temperature.

• Closed-loop cooling reuses the same water over and over rather than discharging it.

• Reusing the waste heat to warm buildings or greenhouses.

• Planting shade trees along streambanks.

Back to the beginning: fertilizer on a cornfield feeds algae in the Gulf, and when the algae die, decomposers use up the bottom oxygen. A power plant's warm water holds less oxygen and speeds up fish metabolism. Both end with low DO.

Summary