Difficulty: Introductory | Prerequisites: Basic understanding of photosynthesis and cellular respiration.
This material sits at the intersection of ecology and environmental science. It covers how carbon and nitrogen move through ecosystems, what happens when human activity disrupts those cycles, and how feedback loops either stabilise or accelerate change. If you can explain what photosynthesis does with CO₂ and what decomposers do with dead organic matter, you have enough background to follow everything here. These topics tend to appear together on the exam because they form a connected story: nutrients cycle, energy flows, and human interference throws both off balance.
Carbon and nitrogen cycle through living and non-living systems, and human activities (burning fossil fuels, fertiliser runoff) disrupt both. Excess nutrients cause dead zones by triggering algal blooms that consume all available oxygen. Feedback loops can either amplify a disruption (positive feedback) or dampen it (negative feedback), and knowing which is which matters for predicting how ecosystems respond to change.
Carbon sink
A reservoir that absorbs and stores more carbon than it releases. On land, the main sinks are long-lived plants and trees. In water, dissolved organic matter and petroleum deposits beneath the ocean floor serve the same role.
In simple terms, a carbon sink is anything that locks carbon away so it stays out of the atmosphere.
Net primary productivity (NPP)
The total amount of energy (or biomass) that primary producers fix through photosynthesis, minus what they use for their own respiration.
Think of it as the "leftover" energy available to feed the rest of the food web.
Dead zone
An area of water (typically coastal) where dissolved oxygen drops so low that most marine life cannot survive. Examples include the Gulf of Mexico and Hood Canal.
In simple terms, a dead zone is a patch of ocean or lake that has been suffocated by algal decay.
Positive feedback loop
A cycle where 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, gathering more snow.
Negative feedback loop
A cycle where the result of an initial change counteracts the original change, pushing the system back toward equilibrium.
Think of it as a thermostat: when the room gets too warm, the heater switches off.
Eutrophication
The process by which excess nutrients (especially nitrogen and phosphorus) enter a body of water, triggering excessive plant and algae growth, eventual die-off, and oxygen depletion.
In simple terms, too many nutrients cause an algal bloom, and when the algae die, decomposers use up all the oxygen.
Nitrogen fixation
The conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds that organisms can use. Carried out by certain bacteria, some of which live in mutualistic relationships with plant roots.
In simple terms, it is the process that converts unusable nitrogen gas into a form plants can absorb.
Carbon moves between the atmosphere, living organisms, the ocean, and geological deposits. The key exam-relevant points sit in two categories: where carbon is stored, and how human activity shifts the balance.
Terrestrial sinks
Trees and long-lived plants are the dominant land-based sinks
They pull CO₂ from the atmosphere through photosynthesis and lock it into wood, roots, and soil organic matter
Deforestation and burning release that stored carbon back into the atmosphere
Aquatic and geological sinks
Dissolved organic matter in the ocean absorbs atmospheric CO₂
Petroleum products (oil, natural gas) trapped under the ocean floor and in rock formations are long-term carbon stores
Burning fossil fuels moves carbon from geological storage into the atmosphere on a timescale far faster than it was deposited
The carbon balance
In an undisturbed system, carbon uptake (photosynthesis, ocean absorption) roughly balances carbon release (respiration, decomposition)
Human combustion of fossil fuels and large-scale deforestation have tipped this balance, increasing atmospheric CO₂
Nitrogen is essential for amino acids and nucleic acids, but atmospheric N₂ is unusable by most organisms. The cycle depends on bacteria that convert nitrogen between forms.
Nitrogen fixation converts N₂ into ammonia (NH₃), carried out by nitrogen-fixing bacteria
Some of these bacteria live in root nodules of legumes, in a mutualistic relationship with the plant
The plant provides sugars; the bacteria provide usable nitrogen
This mutualism is conditional: plants only invest in root nodule structures when soil nitrogen is scarce
Human impacts on the nitrogen cycle
Synthetic fertilisers add massive quantities of reactive nitrogen to ecosystems
Sewage discharge introduces nitrogen compounds into waterways
Both contribute to nutrient loading in aquatic systems
Dead zones form through a specific chain of events driven by excess nutrients. The sequence matters for the exam.
Nutrient input increases (fertiliser runoff, sewage)
Primary producers (algae) proliferate, NPP spikes
Algae eventually die off in large quantities
Decomposers (bacteria) break down the dead algae
Decomposition consumes enormous amounts of dissolved oxygen
Oxygen levels crash, creating a hypoxic zone where most marine organisms cannot survive
Named examples: Gulf of Mexico, Hood Canal (Puget Sound region)
Root cause: human-driven nutrient enrichment, not natural processes
Feedback loops describe how a system responds to a change. The critical distinction is direction: does the response push the system further from its starting point, or pull it back?
The result of the initial change reinforces that change, driving the system further in the same direction.
Example: forest fires and climate change
Increased atmospheric CO₂ raises global temperatures
Warmer, drier conditions increase the frequency and severity of forest fires
Burning forests release stored carbon back into the atmosphere as CO₂
More atmospheric CO₂ leads to further warming
The cycle accelerates: warming causes fires, fires cause more warming
Positive feedback loops are inherently destabilising. They do not self-correct.
The result of the initial change opposes that change, pushing the system back toward its original state.
Example: CO₂ and plant growth
Human activity increases atmospheric CO₂
Higher CO₂ levels stimulate additional plant growth (CO₂ is a raw material for photosynthesis)
More plants absorb more CO₂ from the atmosphere
Atmospheric CO₂ levels are partially reduced
The system partially self-corrects
Negative feedback loops are stabilising. They keep systems within a functional range.
Trace the chain from the initial change back to the variable that changed. If the chain brings the variable back toward where it started, the feedback is negative. If it pushes the variable further from where it started, the feedback is positive. Drawing the loop as a simple cycle diagram helps.
The dead zone in the Gulf of Mexico is a direct consequence of fertiliser runoff from Midwest agriculture travelling down the Mississippi River. It is one of the largest hypoxic zones in the world, and its size fluctuates with seasonal agricultural activity. Understanding the nutrient loading sequence from lecture explains why simply reducing one source of nitrogen is rarely sufficient.
The positive feedback loop between warming and forest fires is visible in the increasing severity of wildfire seasons in the western United States and parts of Australia. Each fire season releases carbon that contributes to the conditions that make the next season worse.
Students often think "positive feedback" means something good and "negative feedback" means something bad. The terms refer to direction (amplifying vs. dampening), not desirability. A positive feedback loop can be catastrophic.
Students sometimes believe dead zones are caused by pollution toxins directly killing organisms. The actual mechanism is oxygen depletion: excess nutrients fuel algal growth, and when the algae die, decomposition consumes the dissolved oxygen.
Students confuse carbon sinks with carbon sources. A sink absorbs more carbon than it releases. A forest that is burning is a carbon source in that moment, even though a standing forest is a sink.
Students assume nitrogen fixation is done by plants. The fixation itself is performed by bacteria. Plants in mutualistic relationships with nitrogen-fixing bacteria benefit from the process but do not carry it out themselves.
⚠️ Be able to trace the full sequence from nutrient input to dead zone formation, step by step
⚠️ Know the difference between positive and negative feedback and be prepared to identify which type a given scenario describes
⚠️ Be able to name specific carbon sinks on land (trees, long-lived plants) and in water (dissolved organic matter, petroleum deposits)
⚠️ Understand the mutualism between nitrogen-fixing bacteria and plants, including why the relationship is conditional (plants only invest when nitrogen is scarce)
⚠️ Be ready to explain how human activity disrupts both the carbon and nitrogen cycles
True or false: A positive feedback loop always returns a system to equilibrium.
Fill in the blank: The major carbon sinks on land are ______.
True or false: Dead zones are caused by a decrease in nutrients in aquatic systems.
Fill in the blank: Nitrogen fixation is performed by ______, not by plants directly.
True or false: In a negative feedback loop, the response to a change opposes the initial change.
Answers: 1. False (positive feedback amplifies, it does not return to equilibrium). 2. Trees and long-lived plants. 3. False (dead zones are caused by an increase in nutrients). 4. Bacteria. 5. True.
Q: Describe the sequence of events that leads to dead zone formation, starting from nutrient input.
A: Excess nutrients (from fertiliser or sewage) enter a body of water. This triggers rapid growth of primary producers, especially algae, increasing NPP. The algae eventually die in large quantities. Decomposing bacteria break down the dead algae, consuming dissolved oxygen in the process. Oxygen levels drop so low that most marine organisms cannot survive, creating a dead zone.
Q: A wildfire releases CO₂ into the atmosphere, raising temperatures, which in turn dries out forests and increases fire risk. Is this positive or negative feedback? Explain.
A: This is positive feedback. The initial change (more CO₂ and higher temperatures) leads to more fires, which release more CO₂, which raises temperatures further. The response amplifies the original change rather than counteracting it.
Q: Why do legumes form root nodules, and under what conditions would you expect a legume to reduce investment in these structures?
A: Legumes form root nodules to house nitrogen-fixing bacteria in a mutualistic relationship. The bacteria convert atmospheric N₂ into usable forms, and the plant provides sugars in return. If soil nitrogen is abundant, the plant would reduce investment in nodule formation because the cost of maintaining the mutualism is no longer justified.
Q: Identify two terrestrial carbon sinks and two aquatic or geological carbon sinks.
A: Terrestrial: trees and long-lived plants, soil organic matter. Aquatic/geological: dissolved organic matter in the ocean, petroleum deposits trapped beneath the ocean floor and in rock formations.
Q: Rising CO₂ levels cause increased plant growth, which absorbs more CO₂ from the atmosphere. What type of feedback is this, and why does it not fully offset human CO₂ emissions?
A: This is negative feedback, because the response (increased plant growth absorbing CO₂) opposes the initial change (rising CO₂). It does not fully offset emissions because the rate of fossil fuel combustion far exceeds the rate at which additional plant growth can absorb the extra CO₂.
This material connects directly to biodiversity and species interactions (covered in the companion study notes). Dead zones reduce biodiversity by eliminating habitat for marine organisms, which is a concrete link between nutrient cycling and conservation biology. Feedback loops also appear in population ecology: predator-prey cycles are a form of negative feedback, and understanding the concept here will make those dynamics easier to parse.
The mutualism between legumes and nitrogen-fixing bacteria ties into the broader topic of species interactions, where mutualism is conditional and cost-benefit driven.
Carbon cycle, carbon sink, carbon source, CO₂, greenhouse gas, fossil fuels, deforestation, nitrogen cycle, nitrogen fixation, nitrification, denitrification, eutrophication, dead zone, hypoxia, algal bloom, NPP, net primary productivity, positive feedback, negative feedback, feedback loop, nutrient cycling, Gulf of Mexico dead zone, Hood Canal, fertiliser runoff, mutualism, nitrogen-fixing bacteria, legumes, root nodules, BIOL180, Exam 4, Introduction to Biology, UW Biology