The Chemistry That Keeps Ecosystems in Balance

A lake in January

Picture a lake in northern Ontario in January. The air is −15 °C and the lake is covered in thick ice. Yet below the ice, fish are swimming, breathing oxygen from the water. Why doesn't the whole lake freeze solid? Where does the fish's oxygen come from, and where does the carbon dioxide they breathe out go?

The answers are all chemistry: properties, changes and interactions of matter that keep ecosystems working. Remember from Grade 7: you learned that water, carbon and nitrogen cycle through ecosystems. This lesson looks at the chemical properties and reactions behind those cycles.

Dynamic equilibrium

A healthy ecosystem is in dynamic equilibrium. "Dynamic" means things are always changing; "equilibrium" means balance. Matter is constantly moving in and out of living things, but the amounts stay roughly steady because opposite processes happen at about the same rate.

Think of a bathtub with the tap on and the drain open. If water flows in as fast as it drains out, the level stays the same, even though the water is always moving. If you turn the tap up (or plug the drain), the level changes and a new balance must form.

Two reactions that balance each other

Photosynthesis (in plants, algae and some bacteria) uses light energy:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

carbon dioxide + water → glucose + oxygen

Cellular respiration (in almost all living things, including plants) releases the energy stored in glucose:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

The two reactions are opposites. The products of one are the reactants of the other, so carbon and oxygen keep cycling. In a balanced ecosystem, the amount of carbon dioxide taken in by photosynthesis is about equal to the amount released by respiration and decomposition.

Water's unusual physical properties

Two physical properties of water make life in Canadian lakes possible.

1. Ice floats. Most substances are denser as solids than as liquids, but water is different. When water freezes, its molecules lock into an open pattern, so ice is less dense than liquid water (about 0.92 g/cm³ compared with 1.00 g/cm³). Ice forms on top and insulates the water below.

2. Water is densest at about 4 °C. In fall, the surface water cools and sinks until the whole lake is near 4 °C. Colder water (below 4 °C) then stays on top and freezes. The bottom stays at about 4 °C all winter, where fish can survive.

Gases dissolve in water, but less when it's warm

Fish don't breathe the oxygen in water molecules (H₂O). They breathe dissolved oxygen gas (O₂), which mixes into water from the air and from photosynthesis by water plants and algae.

Gases are more soluble in cold water than in warm water. So warm water holds less oxygen. When lakes and rivers warm up in a heat wave, or when warm water is released from factories, fish such as trout can struggle to get enough oxygen.

When human activity tips the balance

Extra carbon dioxide. Burning fossil fuels releases carbon that was stored underground for millions of years. Photosynthesis can't keep up, so the CO₂ in the air has risen from about 280 parts per million before the industrial era to over 420 today. Part of this extra CO₂ dissolves in the oceans and reacts with water to form carbonic acid:

CO₂ + H₂O → H₂CO₃

The ocean's surface pH has fallen from about 8.2 to 8.1. That sounds tiny, but remember that each pH step is a factor of 10. This ocean acidification makes it harder for mussels, oysters, corals and some plankton to build their calcium carbonate shells.

Acid precipitation. Burning coal and smelting metal ores release sulfur dioxide (SO₂) and nitrogen oxides, which react with water in clouds to form acids. Many lakes on the Canadian Shield sit on granite, which cannot buffer (neutralize) acid, so they became too acidic for many fish. Where the ground contains limestone (calcium carbonate), the limestone neutralizes the acid.

Fertilizer runoff. Extra nitrogen and phosphorus from fertilizers and sewage wash into lakes, causing huge algal blooms, as in parts of Lake Erie. When the algae die, decomposers use up the dissolved oxygen, which can create low-oxygen "dead zones".

Many ways of knowing about water

Many First Nations, Métis and Inuit communities have deep knowledge of the lakes and rivers on their lands, built up over many generations of living with them. Today, many Indigenous communities in Ontario run water monitoring programs that combine this knowledge with scientific testing of pH, temperature and dissolved oxygen. Anishinaabe teachings describe water as alive and deserving of respect and care. The Anishinaabe grandmother Josephine Mandamin walked around all five Great Lakes, beginning in 2003, to raise awareness about protecting water, and others continue her Water Walks today.