Difficulty: Introductory | Prerequisites: Basic understanding of photosynthesis and cellular respiration.
This material sits at the end of BIOL 180 and ties together how energy and matter move through ecosystems, how humans disrupt those flows, and how ecosystems respond. If you are comfortable with what photosynthesis and cellular respiration do with carbon and oxygen, you have everything you need. The topics here (carbon sinks, nitrogen cycling, dead zones, feedback loops) all connect: nutrients cycle, humans alter the inputs, and the system either amplifies or dampens the change. Understanding these patterns is the thread that runs through the entire exam.
Carbon and nitrogen cycle through ecosystems, and human activity (burning fossil fuels, adding fertiliser) pushes extra nutrients into those cycles. Excess nutrients can trigger algal blooms that create oxygen-depleted dead zones. Ecosystems respond through feedback loops: positive feedback amplifies a change, negative feedback dampens it.
Carbon sink
A reservoir that absorbs and stores more carbon than it releases. On land, the major sinks are long-lived plants and trees. In water, dissolved organic matter, petroleum products beneath the ocean floor, and carbon locked in rocks serve the same role.
In simple terms, a carbon sink is anything that soaks up CO2 faster than it lets it go.
Net primary productivity (NPP)
The total amount of energy that primary producers (plants, algae) fix through photosynthesis, minus what they use for their own respiration. Higher NPP means more energy available to the rest of the food web.
Think of it as the "profit" left over after plants pay their own energy bills.
Dead zone
An area in a body of water where dissolved oxygen has dropped so low that most marine life cannot survive. Triggered by nutrient overload (eutrophication), typically from agricultural runoff or sewage.
In simple terms, too many nutrients feed too much algae, the algae die and decompose, decomposers use up the oxygen, and everything else suffocates.
Positive feedback
A feedback loop in which the result of an initial change amplifies that same change, pushing the system further in the original direction.
Think of it as a snowball rolling downhill, getting bigger as it goes.
Negative feedback
A feedback loop in which the result of an initial change counteracts the original change, pushing the system back towards equilibrium.
Think of it as a thermostat: the room gets too warm, the heater switches off.
Eutrophication
The process by which excess nutrients (especially nitrogen and phosphorus) enter a water body, spurring rapid algal growth and, ultimately, oxygen depletion.
In simple terms, fertiliser washes into the water, algae bloom like mad, then crash, and the water runs out of oxygen.
Carbon moves between the atmosphere, living organisms, the ocean, and the Earth's crust. Photosynthesis pulls CO2 out of the air; respiration and decomposition put it back. The balance between these processes determines whether a system is a net source or a net sink of carbon.
Land-based sinks: trees and other long-lived plants. The longer a plant lives, the more carbon it locks away in wood and roots.
Ocean-based sinks: dissolved organic matter in seawater, petroleum deposits beneath the ocean floor, and carbon incorporated into sedimentary rocks.
Why it matters for the exam: burning fossil fuels moves carbon from geological sinks (where it was locked away for millions of years) into the atmosphere very quickly, faster than natural sinks can absorb it. This is the core mechanism behind rising atmospheric CO2.
A dead zone forms when dissolved oxygen in a water body drops to levels that cannot support most aquatic life. The sequence runs like this:
Nutrient input increases. Human sources, typically agricultural fertiliser and sewage, add excess nitrogen and phosphorus to waterways.
Primary producers boom. The extra nutrients fuel rapid growth of algae and other primary producers, raising NPP.
Algae die off. The bloom cannot sustain itself; large quantities of algae die.
Decomposers consume oxygen. Bacteria break down the dead organic matter, using up dissolved O2 in the process.
Oxygen crashes. The water becomes hypoxic. Fish, invertebrates, and other aerobic organisms either flee or die.
Key examples: the Gulf of Mexico (fed by agricultural runoff from the Mississippi River basin) and Hood Canal in Washington State.
Connection to the nitrogen cycle: excess nitrogen from fertiliser is one of the primary nutrients driving eutrophication. This is why the nitrogen cycle and dead zones are tested together.
Feedback loops describe how a system responds to a change. The distinction between positive and negative feedback is one of the most commonly tested concepts in this unit.
The output of a process reinforces the original change, pushing the system further from its starting point.
Example: forest fires and climate change. Increased atmospheric CO2 warms the climate. Warmer, drier conditions increase the frequency and intensity of forest fires. Burning forests release stored carbon back into the atmosphere as CO2. More CO2 means more warming, which means more fires, and the cycle continues.
The key idea: each pass through the loop makes the next pass stronger. Positive feedback loops tend to be destabilising.
The output of a process counteracts the original change, pushing the system back towards its starting point.
Example: CO2 and plant growth. Human activity raises atmospheric CO2. Higher CO2 levels stimulate additional plant growth (because photosynthesis uses CO2 as an input). More plants pull more CO2 out of the atmosphere, partially offsetting the original increase.
The key idea: the system self-corrects. Negative feedback loops tend to be stabilising.
Ask: does the result push the system in the same direction as the original change (positive) or the opposite direction (negative)? "Positive" does not mean "good" and "negative" does not mean "bad." The terms describe direction, not value.
Nitrogen is essential for building amino acids and nucleic acids, but atmospheric nitrogen (N2) is inert. Only certain organisms can convert it to usable forms.
Nitrogen fixation: specialised bacteria (including those in root nodules of legumes) convert atmospheric N2 into ammonia (NH3), a form plants can use. This is the entry point for nitrogen into biological systems.
Nitrogen-fixing plants and mutualism: legumes form a mutualistic relationship with nitrogen-fixing bacteria. The plant provides sugars; the bacteria provide usable nitrogen. This mutualism is conditional: the plant will not build root nodule structures unless nitrogen is scarce.
Human impacts on the nitrogen cycle: manufacturing synthetic fertiliser (the Haber-Bosch process) massively increases the amount of reactive nitrogen entering ecosystems. Agricultural runoff carries this excess nitrogen into waterways, fuelling eutrophication and dead zones.
Connection to dead zones: excess nitrogen from fertiliser and sewage is one of the primary drivers. The sequence is the same: nutrients in, algal bloom, die-off, oxygen crash.
Students often think "positive feedback" means something beneficial. It does not. Positive feedback simply means the change is amplified. A positive feedback loop in climate change accelerates warming, which is not beneficial at all.
Students sometimes confuse carbon sources with carbon sinks. A source releases more carbon than it absorbs (e.g. burning fossil fuels). A sink absorbs more than it releases (e.g. a growing forest). A system can flip from sink to source if conditions change.
Dead zones are sometimes thought of as permanent features. They are often seasonal and can shrink if nutrient inputs are reduced.
Students conflate the nitrogen cycle with the carbon cycle. While both involve nutrient cycling and human disruption, the mechanisms differ. Nitrogen enters biology through bacterial fixation; carbon enters through photosynthesis.
⚠️ Be able to trace the full dead zone sequence: nutrient input, algal bloom, die-off, decomposition, oxygen depletion. Expect a question that asks you to identify which step is missing or to predict what happens if one variable changes.
⚠️ Know the difference between positive and negative feedback and be prepared to classify a novel example. The exam may give you a scenario you have not seen and ask which type of feedback it represents.
⚠️ Understand why nitrogen-fixing bacteria and legumes are a mutualistic relationship, and why the plant only invests in root nodule structures when nitrogen is limiting.
⚠️ Be ready to identify carbon sinks on land versus in water. A question might ask "which of the following is a carbon sink?" and mix sources with sinks.
True or false: a positive feedback loop always produces a beneficial outcome. (False. "Positive" refers to amplification, not benefit.)
Fill in the blank: on land, the major carbon sinks are ________. (Trees and long-lived plants.)
True or false: dead zones are caused by a decrease in nutrients entering a water body. (False. They are caused by an increase in nutrients.)
Fill in the blank: in a negative feedback loop, the result of the initial change pushes the system in the ________ direction. (Opposite.)
True or false: nitrogen-fixing bacteria convert atmospheric N2 into a form plants can use. (True.)
Q: Describe the sequence of events that leads to the formation of a dead zone.
A: Excess nutrients (nitrogen, phosphorus) enter a water body from human sources such as fertiliser runoff or sewage. These nutrients fuel rapid algal growth (an algal bloom). When the algae die, decomposing bacteria break down the organic matter and consume large amounts of dissolved oxygen. Oxygen levels drop so low that most aerobic organisms cannot survive, creating a dead zone.
Q: A wildfire burns a large forest, releasing CO2 into the atmosphere. The increased CO2 warms the climate, leading to drier conditions and more fires. Is this positive or negative feedback? Explain.
A: This is positive feedback. The initial change (more CO2, warmer climate) leads to more fires, which release more CO2, which warms the climate further. Each cycle amplifies the original change.
Q: Why do legumes form root nodules to house nitrogen-fixing bacteria, and under what conditions might they not?
A: Legumes benefit from the mutualism because the bacteria convert atmospheric N2 into ammonia, a form the plant can use. Building and maintaining root nodules costs energy, so the plant will not invest in them if nitrogen is already abundant in the soil. The mutualism is conditional.
Q: Give one example of a carbon sink on land and one in the ocean.
A: On land: trees (or any long-lived plants). In the ocean: dissolved organic matter, or petroleum and carbon locked in rocks beneath the ocean floor.
Q: Explain how increased atmospheric CO2 can trigger a negative feedback loop involving plants.
A: Higher CO2 levels stimulate plant growth because photosynthesis uses CO2 as a substrate. The additional plant growth removes CO2 from the atmosphere, partially counteracting the original increase. The result opposes the initial change, so it is negative feedback.
The carbon cycle connects directly to photosynthesis and cellular respiration (earlier in the course): photosynthesis fixes carbon, respiration releases it. Understanding the energy balance from those earlier units makes the ecosystem-level carbon budget much easier to follow.
Dead zones and eutrophication link to the species interactions material in the companion study notes. When oxygen crashes, the community structure of the affected area changes dramatically, which ties into concepts like trophic levels and food webs.
Feedback loops appear again in population ecology (e.g. density-dependent regulation is a form of negative feedback). Recognising the pattern here will help you spot it in other contexts.
Carbon cycle, carbon sink, carbon source, carbon sequestration, CO2, greenhouse gases, photosynthesis, cellular respiration, dead zone, hypoxia, eutrophication, algal bloom, nutrient loading, nitrogen cycle, nitrogen fixation, nitrogen-fixing bacteria, legumes, root nodules, mutualism, Haber-Bosch process, fertiliser runoff, Gulf of Mexico dead zone, Hood Canal, positive feedback, negative feedback, feedback loop, amplifying feedback, dampening feedback, NPP, net primary productivity, primary producers, decomposers, dissolved oxygen, BIOL 180, ecology, ecosystem ecology