Energy flow & nutrient cycles

Energy flows, matter cycles

An ecosystem is a community of living things together with the non-living environment they interact with: air, water, soil and light. Two very different things move through every ecosystem: energy and matter.

Energy enters most ecosystems as sunlight, is captured by photosynthesis and stored in chemical bonds, and is passed from organism to organism as food. At every step, some of it is released as heat, which living things can't use again. So energy passes through an ecosystem in one direction, and it must be constantly topped up.

Matter, such as carbon, nitrogen, phosphorus and water, is different. The same atoms are used again and again, moving between living things, the air, water, soil and rock. These loops are called biogeochemical cycles.

Producers and productivity

Primary producers (plants, algae and cyanobacteria on land and in water) make their own food, so they form the first trophic level, or feeding level. Everything else depends on the energy they capture.

The total rate at which producers capture energy is gross primary productivity (GPP). But producers need energy too, and they use a large share of it in their own respiration. What's left over, stored in new plant tissue that consumers can eat, is net primary productivity (NPP): NPP = GPP − respiration by the producers.

In a classic study of the Silver Springs ecosystem, producers captured 20,810 kcal per square metre per year (GPP). They spent about 13,200 kcal of it on their own respiration, leaving an NPP of about 7,600 kcal/m²/yr, a bit over a third, for the rest of the ecosystem.

Up the food chain: the 10% rule

Above the producers are primary consumers (herbivores), secondary consumers (animals that eat herbivores), tertiary consumers, and so on. Decomposers such as bacteria and fungi feed on dead matter and waste from every level.

Only a small fraction of the energy at one trophic level ends up as new tissue at the next. This fraction is the trophic transfer efficiency. It's often around 10%, which gives the "10% rule", but it varies a lot between ecosystems and levels. Lindeman himself never called it a law, and reported efficiencies from under 1% to over 30%. In Silver Springs, the transfer from producers to herbivores was about 14%.

Where does the other 90% or so go? Three places:

• Not eaten. Most plant material, like wood and roots, is never eaten by herbivores. It goes to decomposers instead.
• Not absorbed. Some food passes through the gut and leaves as faeces, which also feed decomposers.
• Used for respiration. Most of the energy that is absorbed is used to power the animal's life, and ends up as heat. Only the rest is built into new body tissue that the next level can eat.

Because so much is lost at each step, energy runs out quickly. After four to six transfers there isn't enough left to support another level. That's why food chains are short, and why top predators are rare and need large territories.

Ecological pyramids

Stacking the trophic levels as bars, from producers at the bottom, gives an ecological pyramid. There are three kinds, and they don't always look the same:

• A pyramid of energy shows the energy flowing through each level per year. It's always upright, because energy is lost at every step.
• A pyramid of biomass shows the mass of living things at each level at one moment. It's usually upright, but not always.
• A pyramid of numbers shows how many individuals are at each level. It can look strange: one oak tree can support thousands of caterpillars.

In the English Channel, a snapshot found 4 g/m² of phytoplankton supporting 21 g/m² of zooplankton, an upside-down biomass pyramid. That's possible because phytoplankton reproduce so quickly. At any moment there's not much of them, but they're replaced so fast that, over a year, they produce far more energy than the zooplankton do.

Some pollutants move the other way. Chemicals that are stored in fat and not broken down, such as PCBs and mercury, become more concentrated at each trophic level, because each predator eats many prey in its lifetime. This is biomagnification, and it's why top predators are often the most contaminated.

The carbon cycle

Where does a tree's mass come from? In the 1600s, Jan Baptist van Helmont grew a willow in a weighed pot of soil for five years. The tree gained about 74 kg, but the soil lost only about 57 g. He concluded the mass came from water. He was partly right: water supplies hydrogen, but most of a plant's dry mass is carbon, which comes from CO₂ in the air.

1. 1Producers take in CO₂ and fix it into sugars by photosynthesis.
2. 2Consumers eat producers, passing the carbon along the food chain.
3. 3Every organism, producers included, releases CO₂ back into the air by cellular respiration. Decomposers release the carbon in dead matter the same way.
4. 4In the oceans, CO₂ dissolves and mostly becomes bicarbonate ions (HCO₃⁻). Some carbon is locked into shells and sediments.
5. 5Over millions of years, buried remains that don't fully decompose can become fossil fuels: coal, oil and gas. Burning them returns that ancient carbon to the air far faster than it was stored.

The nitrogen cycle

Nitrogen is needed for proteins and nucleic acids. The air is about 78% nitrogen gas (N₂), but plants and animals can't use it: the triple bond between the two atoms is too hard to break. Microbes do almost all the work in this cycle.

1. 1Nitrogen fixation: certain bacteria, including cyanobacteria, free-living Azotobacter and Rhizobium living in the root nodules of legumes, convert N₂ into ammonia (NH₃) or ammonium (NH₄⁺). Humans now also fix huge amounts industrially to make fertiliser.
2. 2Nitrification: bacteria such as Nitrosomonas oxidise ammonium to nitrite (NO₂⁻), and other bacteria oxidise nitrite to nitrate (NO₃⁻).
3. 3Assimilation: plants absorb ammonium or nitrate through their roots and build it into amino acids and nucleotides. Animals get nitrogen by eating.
4. 4Ammonification: when organisms die or excrete waste, decomposers convert organic nitrogen back into ammonium.
5. 5Denitrification: in waterlogged, low-oxygen soils, bacteria such as Pseudomonas convert nitrate back to N₂ gas, which returns to the air.

Phosphorus, limiting nutrients and eutrophication

Phosphorus, needed for DNA, ATP and phospholipids, has no significant gas form, so its cycle is slow. It enters ecosystems mainly as phosphate (PO₄³⁻) weathered out of rocks, and ends up in ocean sediments.

The nutrient that runs out first and holds back growth is the limiting nutrient. In many lakes it's phosphorus; in many oceans it's nitrogen. Adding the limiting nutrient can set off a chain reaction called eutrophication:

1. 1Fertiliser runoff or sewage adds nitrogen and phosphorus to a lake or coastal sea.
2. 2Algae and cyanobacteria grow explosively into a bloom.
3. 3The algae die, and decomposers break them down using huge amounts of dissolved oxygen for respiration.
4. 4Oxygen levels crash, and fish and other animals suffocate, creating a "dead zone" like the one that forms each year in the Gulf of Mexico.

Key terms

Trophic level

An organism's feeding position in a food chain: producer, primary consumer, secondary consumer and so on.

Gross / net primary productivity

The total rate at which producers capture energy / what's left after their own respiration.

Trophic transfer efficiency

The fraction of energy at one trophic level that becomes tissue at the next, often about 10%.

Decomposer

An organism, such as a bacterium or fungus, that breaks down dead matter and waste, releasing nutrients.

Biomagnification

The increasing concentration of a persistent pollutant at each higher trophic level.

Nitrogen fixation

The conversion of N₂ gas into ammonia or ammonium, by bacteria or by industry.

Nitrification / denitrification

Bacteria oxidising ammonium to nitrite and nitrate / bacteria converting nitrate back to N₂ gas.

Eutrophication

Nutrient enrichment of water that causes algal blooms and, as they decompose, oxygen loss.

Check yourself

Try answering in your head before you open each answer.

• 1.A country wants to feed more people from the same farmland. Using energy flow, explain why eating grain directly feeds more people than feeding the grain to cattle and eating beef.Show answerHide

  Each trophic step loses most of the energy, mainly as heat from respiration. Eating grain puts people at the second trophic level. Feeding grain to cattle adds a step, so only around a tenth of the grain's energy (often less, since cattle are endotherms) reaches people as beef.

• 2.In a lake, the zooplankton biomass is larger than the phytoplankton biomass on the day you sample. Does this break the rule that energy is lost at each step?Show answerHide

  No. Biomass is a snapshot of how much is there at one moment; energy flow is a rate over time. Phytoplankton reproduce and are eaten so fast that their small standing biomass produces more energy per year than the zooplankton do. The pyramid of energy is still upright.

• 3.A soil chemical kills nitrifying bacteria but nothing else. Predict what happens to ammonium and nitrate in the soil, and to plants that depend on nitrate.Show answerHide

  Ammonium builds up, because it's still being made by decomposers but is no longer oxidised. Nitrate falls, because nitrification was its source and denitrifiers and plants keep removing it. Plants that rely on nitrate would become nitrogen-deficient, though some plants can take up ammonium instead.