Ecosystems and Energy Flow

Autotrophs, Heterotrophs and Energy Capture

Part 1 of 7 — Autotrophs, Heterotrophs and Energy Capture

Every living thing needs a steady supply of free energy to stay organized, grow and reproduce. Ecology asks a simple question about that energy: where does it come from, and where does it go? The answer organizes this whole topic (CED 8.2, Energy Flow Through Ecosystems).

The central contrast: energy flows through an ecosystem in one direction. It enters mostly as sunlight, is stored for a while in the chemical bonds of organic molecules, and leaves as heat. Matter, such as carbon, nitrogen, phosphorus and water, cycles: the same atoms are used again and again.

Because heat cannot be turned back into chemical energy by organisms, an ecosystem needs a constant input of new energy. A forest cut off from sunlight would run down in weeks; its carbon atoms would still be there.

Roadmap for this topic

Autotrophs: the Energy Gatekeepers

An autotroph ("self-feeder") builds its own organic molecules from inorganic carbon, usually $\text{CO}_2$. Autotrophs are the producers of an ecosystem. They differ in the energy source they use to power that carbon fixation.

Photosynthesis captures light energy and stores it in sugar:

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Chemosynthesis captures energy released when an organism oxidizes small inorganic molecules from its surroundings, such as hydrogen sulfide ($\text{H}_2\text{S}$), ammonium ($\text{NH}_4^+$) or ferrous iron ($\text{Fe}^{2+}$). That energy is then used to fix $\text{CO}_2$ into sugar. The CED stresses one point: chemosynthesis can occur in the absence of oxygen. Some chemoautotrophs use $\text{O}_2$ to accept electrons, but others do not, so chemosynthetic life can thrive in anoxic sediments and around hydrothermal vents.

Case: life without sunlight

In 1977, researchers exploring hydrothermal vents on the Pacific seafloor found dense communities of giant tubeworms, clams and crabs more than 2,000 meters below the surface, where no light reaches. The base of that food web is chemoautotrophic bacteria and archaea that oxidize vent chemicals. Many of these microbes live inside the tubeworms, which have no mouth or gut.

Misconception check: "Every ecosystem runs on sunlight" is false. Most do, but vent and cave communities show that chemical energy can support an entire food web. A second trap: "Plants make food, so they do not do cellular respiration." Autotrophs do respire; they break down their own sugars to make ATP, day and night.

Checkpoint — Energy Capture

Heterotrophs: Living on Captured Energy

A heterotroph ("other-feeder") cannot fix carbon. It obtains both energy and carbon from organic molecules made by other organisms. Animals, fungi and most bacteria are heterotrophs.

The CED's key statement: heterotrophs capture energy present in carbon compounds produced by other organisms, and they may metabolize carbohydrates, lipids and proteins by hydrolysis as sources of free energy.

Fats store more than twice as much energy per gram as carbohydrates or proteins, which is why animals that face long fasts, such as migrating birds and hibernating mammals, store energy as fat.

Heterotrophs include several roles you will meet again in Part 3:

• Consumers eat living organisms (herbivores, carnivores, omnivores).
• Detritivores eat dead organic matter (earthworms, millipedes).
• Decomposers, mainly bacteria and fungi, secrete enzymes that hydrolyze dead material and absorb the products. Decomposers release the atoms locked in dead bodies back to the soil, water and air, which is why they are central to nutrient cycling.

At every step, cellular respiration converts some of the chemical energy into ATP and the rest into heat. The heat warms the organism and its surroundings, then radiates away. That is the one-way exit for ecosystem energy.

Gross and Net Primary Production

Primary production is the amount of light or chemical energy that autotrophs convert to chemical energy in organic compounds during a given time. Ecologists separate two measures:

• Gross primary production (GPP): the total energy fixed by producers.
• Net primary production (NPP): what remains after producers use some of that energy in their own cellular respiration ($R$).

$\text{NPP} = \text{GPP} - R$

NPP is the energy stored as new producer biomass. It is the only energy available to herbivores, decomposers and every level above them, so NPP sets the energy budget for the rest of the ecosystem.

Worked example

A temperate forest's producers fix 2,400 kcal/m²/yr (GPP) and use 1,300 kcal/m²/yr in respiration.

1. $\text{NPP} = 2{,}400 - 1{,}300 = 1{,}100$ kcal/m²/yr
2. Fraction of GPP stored: $\dfrac{1{,}100}{2{,}400} \approx 0.46$, or about 46%
3. Interpretation: just under half of the energy the trees capture becomes new leaves, wood and roots that consumers can eat.

Where is NPP highest?

Per square meter, tropical rain forests, coral reefs and estuaries are among the most productive ecosystems because they combine abundant light, warmth, water and nutrients. Per square meter, the open ocean and deserts are low. Yet the open ocean contributes a large share of Earth's total NPP because it covers so much of the planet. Always check whether a question asks about productivity per unit area or in total.

AP Connection: Free-response questions often give GPP and respiration in a table and ask for NPP, or give NPP and respiration and ask for GPP. Write the equation first, substitute, and keep the units.

Practice — Calculating NPP

The producers in a grassland have a GPP of 6,000 kJ/m²/yr and use 3,600 kJ/m²/yr in cellular respiration.

1. What is the NPP, in kJ/m²/yr?

2. What percent of the GPP is stored as NPP? (Enter a number only.)

Checkpoint — Primary Production

Organism Energetics

Part 2 of 7 — Organism Energetics

Part 1 followed energy into an ecosystem. This part zooms in on a single organism and asks how it spends the energy it takes in. The CED's starting point: organisms use free energy to maintain organization, grow and reproduce.

The individual energy budget

Think of an animal's food as income. Every joule is accounted for:

Respiration releases most of the absorbed energy as heat. Only the growth, storage and reproduction categories add new biomass, and only that biomass is available to the next trophic level (Part 3).

Maintenance comes first. An organism that cannot cover its maintenance costs has nothing left for growth or reproduction.

Endotherms and Ectotherms

Organisms use different strategies to regulate body temperature and metabolism.

Endotherms use thermal energy generated by metabolism to keep a homeostatic body temperature. This lets them stay active in the cold and at night, but it is expensive. A shrew must eat almost constantly.

Ectotherms lack efficient internal heat-producing mechanisms, but many still regulate their temperature through behavior. A desert lizard that shuttles between sun and shade can keep its body near 35 °C for much of the day. Because ectotherms do not burn fuel for heat, a snake can survive for weeks on a single meal.

Misconception check: "Ectotherm" does not mean "cold." A basking lizard can be warmer than a mammal. The terms describe the source of body heat, not its temperature.

Body Size and Metabolic Rate

Metabolic rate is the amount of energy an organism uses per unit time. The CED states a relationship you must know:

Metabolic rate per unit body mass is inversely related to body size. Generally, the smaller the organism, the higher its mass-specific metabolic rate.

Two different measures are easy to confuse:

• Total metabolic rate (kcal/day) rises with body size. An elephant burns far more energy per day than a mouse.
• Mass-specific metabolic rate (kcal per kg per day) falls with body size. Each gram of mouse burns energy much faster than each gram of elephant.

Why small animals run hot

A major reason is the surface area to volume ratio. A small animal has much more body surface for every gram of tissue, so it loses heat to the environment faster and must produce more heat per gram to keep its temperature steady. Small endotherms also breathe faster and have faster heart rates.

Worked example (illustrative values)

The human uses 500 times more energy per day in total, but each kilogram of mouse uses $150 \div 25 = 6$ times more energy than each kilogram of human. This is why a mouse eats a much larger fraction of its body mass each day.

Checkpoint — Temperature and Size

Net Gain, Net Loss and Reproduction

Compare energy absorbed with energy spent over some period:

Animals manage this balance in predictable ways:

• Storing before scarcity. Migratory birds fatten before long flights; bears and ground squirrels build fat before winter.
• Cutting expenditure. Hibernation and daily torpor lower body temperature and metabolic rate, stretching stored energy through cold months when food is scarce.
• Timing reproduction. Reproduction and raising offspring require free energy beyond what is used for maintenance and growth. Many songbirds time egg-laying so that nestlings hatch when caterpillars are most abundant. In poor food years, many animals produce fewer young or skip breeding altogether.

Different organisms use different reproductive strategies in response to energy availability. Some invest a little energy in each of many offspring; others invest heavily in a few. (The population-level consequences of these strategies are covered in Population and Community Ecology.)

AP Connection: An FRQ may describe an animal losing mass during a drought and ask you to explain it. Name the imbalance (energy spent exceeds energy absorbed), name the source of the missing energy (stored molecules being broken down by respiration), and state the consequence (less energy for growth and reproduction).

Practice — Mass-Specific Metabolic Rate

A 0.5-kg squirrel uses 120 kcal/day. A 50-kg deer uses 2,400 kcal/day. (Illustrative values.)

1. What is the squirrel's mass-specific metabolic rate, in kcal/kg/day?

2. What is the deer's mass-specific metabolic rate, in kcal/kg/day?

3. How many times greater is the squirrel's rate per kilogram than the deer's?

Checkpoint — Energy Balance

Food Webs, Trophic Efficiency and Pyramids

Part 3 of 7 — Food Webs, Trophic Efficiency and Pyramids

Parts 1 and 2 showed how energy enters producers and how individuals spend it. This part follows energy between organisms: who eats whom, how much energy each transfer passes on, and why that limits the shape of every ecosystem.

Trophic levels

A trophic level is an organism's position in the sequence of energy transfers.

Detritivores and decomposers feed on dead material and wastes from every level. They are often drawn off to the side because they do not occupy a single level.

Food chains and food webs

A food chain is a single path of energy transfer. A food web is the set of linked food chains in a community. Food webs are more realistic because most consumers eat more than one kind of food, and omnivores feed at more than one trophic level. A bear that eats berries is a primary consumer; the same bear eating salmon is a secondary or higher consumer.

Arrows in food webs point from the food to the eater, in the direction energy flows. Reversing them is one of the most common errors on the exam.

Trophic Efficiency: Why So Little Is Passed On

Trophic efficiency is the percentage of production at one trophic level that appears as production at the next level:

$\text{trophic efficiency} = \frac{\text{production at level } n+1}{\text{production at level } n} \times 100$

On average, about 10% of the energy at one level becomes biomass at the next. This "10% rule" is only an approximate average: measured efficiencies typically range from about 5% to 20%, depending on the organisms and the ecosystem.

Where does the other ~90% go?

1. Not eaten. Much plant and animal biomass is never consumed; it dies and goes to decomposers.
2. Eaten but not assimilated. Indigestible material such as cellulose, bone and fur leaves in feces, which also feeds decomposers.
3. Assimilated but respired. Cellular respiration releases most assimilated energy as heat, which leaves the ecosystem.

Case: one caterpillar's budget (illustrative)

A caterpillar eats leaf tissue containing 200 J of energy.

Only the 33 J in new tissue, about one-sixth of what the caterpillar ate, is available to a bird that eats the caterpillar. Notice that energy is never destroyed: all 200 J are accounted for. It is converted to forms the next level cannot use.

Checkpoint — Trophic Efficiency

Worked Calculations

1. Energy remaining after several levels

If each transfer is about 10% efficient, the energy available at trophic level $n$ is

$E_n = E_1 \times (0.10)^{\,n-1}$

where $E_1$ is the NPP of the producers.

Example: Producers in a meadow store 20,000 kcal/m²/yr as NPP.

Only $\frac{20}{20{,}000} = 0.1\%$ of the producers' energy reaches the tertiary consumers.

2. Calculating efficiency from data

Zooplankton in a lake produce 1,500 kJ/m²/yr of new biomass; the small fish that eat them produce 120 kJ/m²/yr.

$\text{efficiency} = \frac{120}{1{,}500} \times 100 = 8\%$

3. Working backward

How much NPP is needed to support 5 kcal/m²/yr of production in a top predator at the fourth trophic level? Each step back multiplies by 10: $5 \times 10 \times 10 \times 10 = 5{,}000$ kcal/m²/yr.

What the numbers explain

• Food chains are short, usually four or five levels, because too little energy is left to support another population.
• Top predators are rare and need large territories.
• Eating lower on the food chain lets the same cropland feed more people, since grain fed to livestock loses about 90% of its energy before it becomes meat.

Practice — Energy Transfer

1. Producers in a pond store 80,000 kJ/m²/yr as NPP. Assuming 10% transfer at each step, how much energy (kJ/m²/yr) reaches the tertiary consumers?

2. Grasshoppers in a field produce 4,000 kJ/m²/yr of biomass, and the spiders that eat them produce 300 kJ/m²/yr. What is the trophic efficiency from grasshoppers to spiders, in percent?

Ecological Pyramids

An ecological pyramid stacks one bar per trophic level, with producers at the bottom.

Why can a biomass pyramid flip?

Phytoplankton reproduce and are eaten so quickly that a small standing crop can still have a high rate of production. A snapshot of biomass misses that turnover. The energy pyramid, which measures production over time, stays upright.

When Energy Availability Changes

The CED emphasizes that a change in energy availability ripples upward through an ecosystem:

• A change at the producer level changes the number and size of the trophic levels above it. More NPP can support larger consumer populations and sometimes an extra level; less NPP shrinks them.
• A change in an energy source, such as sunlight reaching the ground, does the same thing by changing NPP. Haze from a large volcanic eruption that dims sunlight for a season lowers plant growth, and herbivores and predators decline afterward.

These bottom-up effects start at the base of the food web. Top-down effects, in which predators control the levels below them (keystone species and trophic cascades), are covered in Population and Community Ecology.

Checkpoint — Pyramids and Energy Availability

Carbon and Water Cycles

Part 4 of 7 — Carbon and Water Cycles

Energy passes through an ecosystem once. Matter does not: the atoms in your cells were once in the air, in rocks, in other organisms. A biogeochemical cycle traces how an element moves between living (biotic) and non-living (abiotic) parts of Earth.

Every cycle is described with two kinds of terms:

• Reservoirs (pools): places where the element is stored, such as the atmosphere, the ocean, living biomass, soil and rock.
• Fluxes (processes): the transfers that move it between reservoirs, such as photosynthesis, respiration, decomposition and burial.

Why organisms need carbon

Carbon is the backbone of biological molecules. Organisms use carbon to build carbohydrates, lipids, proteins and nucleic acids, and to make storage compounds and new cells. Producers take carbon in as $\text{CO}_2$; consumers get it by eating organic molecules. Every carbon atom in a hawk's feathers was fixed by a producer at some point.

The Carbon Cycle

Reservoirs

The ocean holds far more carbon than the atmosphere, and sedimentary rocks hold the most of all, but rock carbon cycles very slowly.

Fluxes

Photosynthesis and respiration are nearly balanced over a year in an undisturbed ecosystem. Notice that respiration appears for producers, consumers and decomposers. Plants are a source of $\text{CO}_2$ at night and a sink during the day.

Checkpoint — Carbon Fluxes

How Human Activity Alters the Carbon Cycle

Two human activities add carbon to the atmosphere faster than natural sinks remove it:

1. Fossil fuel combustion. Coal, oil and natural gas are the remains of organisms buried for millions of years. Burning them moves that long-stored carbon into the atmosphere as $\text{CO}_2$ in decades, far faster than burial and rock formation can return it.
2. Deforestation. Burning or decomposing cleared trees releases their carbon, and the land then fixes less carbon each year because there is less photosynthesis.

The data

Air bubbles trapped in ice cores show that atmospheric $\text{CO}_2$ was about 280 ppm before the Industrial Revolution. Direct measurements at Mauna Loa, Hawaii, which began in 1958, show it passing 420 ppm in the 2020s. The record (the Keeling curve) has two features:

• A long-term rise, caused mainly by fossil fuel combustion.
• A yearly zigzag: $\text{CO}_2$ falls during the Northern Hemisphere growing season, when photosynthesis outpaces respiration, and rises in fall and winter, when respiration and decomposition dominate.

Worked example: percent change

$\text{percent change} = \frac{\text{new} - \text{old}}{\text{old}} \times 100 = \frac{420 - 280}{280} \times 100 = 50\%$

About a quarter of the extra $\text{CO}_2$ dissolves into the ocean, which slows the atmospheric rise but makes seawater more acidic. The effects of rising $\text{CO}_2$ on ecosystems, such as warming, range shifts and coral bleaching, are covered in Ecology and Ecosystems. Here, focus on the mechanism: carbon moves from a slow reservoir (fossil fuels) to a fast one (the atmosphere).

Practice — Carbon Budgets

1. Atmospheric $\text{CO}_2$ measured at Mauna Loa rose from about 350 ppm in the late 1980s to about 420 ppm in the 2020s. What is the percent increase?

2. Over one year, the plants in a forest plot fix 120 g C/m² by photosynthesis. Respiration by the plants, animals and decomposers releases 110 g C/m². How many g C/m² does the plot store that year?

The Water Cycle

Water cycles through the biosphere driven by solar energy (which evaporates it) and gravity (which brings it back down and moves it downhill).

The oceans hold about 97% of Earth's water. Most of the rest is in ice and groundwater; only a small fraction is in lakes, rivers and the atmosphere at any time.

Water and living things

• Water is the solvent for cellular chemistry and the medium for transport in organisms.
• It is a reactant in photosynthesis (split to release $\text{O}_2$) and in hydrolysis, and a product of cellular respiration and dehydration synthesis.
• Over land, transpiration returns a large share of the water that falls as rain to the atmosphere, so forests help generate their own rainfall.

Case: clearing a forest. Removing trees cuts transpiration, so less water vapor returns to the air and rainfall downwind can decline. Without roots and leaf litter to slow it, more rain becomes runoff instead of infiltrating, increasing erosion and lowering groundwater recharge.

Checkpoint — Carbon and Water

Nitrogen and Phosphorus Cycles

Part 5 of 7 — Nitrogen and Phosphorus Cycles

Carbon builds the skeleton of every biological molecule (Part 4), but two other elements are just as essential and are often in much shorter supply.

Because organisms need these elements in quantity and the environment often supplies them slowly, nitrogen or phosphorus frequently sets the limit on how much producers can grow.

The nitrogen paradox

About 78% of the atmosphere is nitrogen gas ($\text{N}_2$), yet plants and animals cannot use it. The two nitrogen atoms are held by a very strong triple bond. Only certain prokaryotes can break it, so almost all nitrogen enters living things through microbes.

The Nitrogen Cycle, Step by Step

Connections worth noticing

• Rhizobium and legumes are mutualists. The plant supplies sugars to bacteria living in its root nodules; the bacteria supply fixed nitrogen. Farmers rotate legumes such as soybeans, clover or alfalfa with other crops to enrich nitrogen-poor soil.
• Nitrifying bacteria link back to Part 1. They oxidize ammonium for energy and fix carbon with it, so they are chemoautotrophs.
• Denitrification needs low oxygen. It speeds up in waterlogged soils and marsh sediments, which is why flooded fields can lose nitrate to the air.
• Lightning and industry also fix nitrogen. Human activity, mainly industrial fertilizer production, has roughly doubled the amount of fixed nitrogen entering land ecosystems each year.

Common mix-up: nitrogen fixation turns $\text{N}_2$ gas into ammonia; nitrification turns ammonium into nitrate. Neither one returns nitrogen to the air; only denitrification does.

Checkpoint — The Nitrogen Cycle

The Phosphorus Cycle

Phosphorus has no significant gaseous phase. It cycles through rocks, soil, water, sediments and organisms, mostly as phosphate ($\text{PO}_4^{3-}$).

1. Weathering of phosphate-bearing rock slowly releases phosphate into soil and water.
2. Assimilation: plants and algae absorb phosphate and build it into DNA, RNA, ATP and phospholipids. Consumers obtain it by eating.
3. Decomposition returns phosphate from wastes and dead organisms to the soil.
4. Runoff carries phosphate to rivers, lakes and the ocean.
5. Sedimentation: phosphate settles and becomes part of new rock. Geological uplift may expose that rock to weathering again, over millions of years.

Because there is no atmospheric shortcut, phosphorus cycles locally and slowly. Phosphate also binds tightly to soil particles, so little is freely available to roots at any moment.

Limiting Nutrients

A limiting nutrient is the element in shortest supply relative to what producers need. Adding it increases primary production; adding other nutrients does not.

• Nitrogen commonly limits production in many terrestrial ecosystems and in much of the ocean.
• Phosphorus commonly limits production in freshwater lakes.

Case: Lake 226

In 1973, scientists in Ontario's Experimental Lakes Area divided Lake 226 with a plastic curtain. One basin received phosphorus, nitrogen and carbon; the other received only nitrogen and carbon. Within months the phosphorus basin turned green with algae; the other did not. Because the only difference between basins was phosphorus, the experiment identified phosphorus as the limiting nutrient in that lake.

AP Connection: In a nutrient-addition experiment, the limiting nutrient is the one whose addition (alone or with others) produces the growth response, compared with a control that received none.

Eutrophication and Dead Zones

Eutrophication is the over-enrichment of a body of water with nutrients, especially nitrogen and phosphorus. Human sources include fertilizer runoff from farms and lawns, animal waste and sewage.

The causal chain

1. Fertilizer runoff delivers extra nitrate and phosphate to a lake or coastal zone.
2. Limiting nutrients are suddenly abundant, so algae and cyanobacteria grow explosively in an algal bloom.
3. The bloom shades the water, killing submerged plants.
4. The short-lived algae die and sink.
5. Decomposers break down the dead algae by aerobic respiration, using up dissolved oxygen.
6. Deep water becomes hypoxic (low in oxygen). Fish and other animals that cannot leave suffocate, creating a dead zone.

The algae themselves release oxygen during the day, so the problem is not the living bloom but the decomposition that follows its death.

Example: nitrogen and phosphorus carried down the Mississippi River feed a hypoxic dead zone in the Gulf of Mexico that forms each summer and often covers thousands of square miles.

Key reasoning step: the link between nutrients and fish death runs through decomposers' oxygen use. Answers that skip this step or blame the nutrients as directly toxic will not earn credit.

Practice — Build the Eutrophication Chain

Choose the correct entry for each step.

Checkpoint — Limiting Nutrients and Eutrophication

AP Exam Practice: Energy and Matter Data

Part 6 of 7 — AP Exam Practice: Energy and Matter Data

The AP Biology exam rarely asks you to recite the steps of a cycle. It hands you data and asks you to calculate, interpret, predict and justify. This part works through three data sets of the kind you will see in multiple-choice sets and free-response questions.

Skills checklist for this topic

Strategy: before calculating, write the relationship in words ("NPP is what is left after producers respire"). Most errors on these items come from subtracting in the wrong order, not from arithmetic.

Data Set 1: The Light-Bottle, Dark-Bottle Method

Aquatic ecologists measure primary production with dissolved oxygen ($\text{O}_2$). Pond water is sealed in two bottles. The light bottle is clear, so its algae both photosynthesize and respire. The dark bottle is covered, so its algae only respire. Both are left in the pond for 24 hours.

Worked solution

1. Respiration (R): the dark bottle lost oxygen only to respiration. $R = 8.0 - 6.8 = 1.2$ mg/L per day.
2. Net primary production (NPP): the light bottle's change is photosynthesis minus respiration. $\text{NPP} = 9.5 - 8.0 = 1.5$ mg/L per day.
3. Gross primary production (GPP): add back what respiration used. $\text{GPP} = \text{NPP} + R = 1.5 + 1.2 = 2.7$ mg/L per day. A shortcut: $\text{GPP} = \text{light} - \text{dark} = 9.5 - 6.8 = 2.7$.

Why oxygen works as a measure: photosynthesis releases $\text{O}_2$ and respiration consumes it, in amounts tied to the carbon fixed and the carbon respired. Oxygen is a proxy for the energy captured.

Limitation to mention in an FRQ: the bottles contain bacteria and small animals as well as algae, so "respiration" includes the whole community, not just the producers.

Practice — Light and Dark Bottles

A second pond gives these results after 24 hours: initial dissolved $\text{O}_2$ = 7.0 mg/L, light bottle = 8.6 mg/L, dark bottle = 6.1 mg/L.

1. Respiration (mg/L per day)

2. Net primary production (mg/L per day)

3. Gross primary production (mg/L per day)

Data Set 2: Production in a Stream Food Web

Ecologists measured the energy stored as new biomass at each trophic level of a stream.

How to read it

• Efficiency from producers to primary consumers: $\frac{1{,}500}{12{,}000} \times 100 = 12.5\%$.
• Efficiency from secondary to tertiary consumers: $\frac{12}{120} \times 100 = 10\%$.
• Every efficiency falls inside the typical 5–20% range, and each level stores far less energy than the one below, so an energy pyramid for the stream is upright.
• The trout's 12 kJ/m²/yr is only $0.1\%$ of the producers' 12,000, which is why a stream supports few large predators.

Try the remaining efficiency yourself in the checkpoint below.

Checkpoint — Stream Data

Data Set 3: A Nutrient-Enrichment Experiment (FRQ-style)

Students filled 12 identical tanks with lake water containing algae. Three tanks received each treatment. After two weeks they measured algal biomass as chlorophyll concentration.

Model answers

Identify the variables. Independent variable: the nutrient added. Dependent variable: chlorophyll concentration (a measure of algal biomass). Controlled variables include tank size, water source, light and temperature. Three tanks per treatment are replicates.

Claim: phosphorus is the primary limiting nutrient for these algae. Evidence: adding P alone raised chlorophyll from 5 to 21 µg/L, while adding N alone barely changed it (5 to 6 µg/L). Reasoning: a limiting nutrient is the one whose addition increases primary production. Algae need phosphorus for nucleic acids, ATP and phospholipids, so without enough P they cannot build new cells.

Extend the reasoning. The N + P tanks (48 µg/L) grew much more than P alone. Once phosphorus was supplied, nitrogen became the next limiting nutrient. This pattern is called co-limitation.

Predict. If these tanks were left in the dark for a week after the bloom, the N + P tanks would show the largest drop in dissolved oxygen, because decomposers respiring the dead algae would consume oxygen with no photosynthesis to replace it. That is eutrophication from Part 5, reproduced in a tank.

Checkpoint — Experiments and Predictions

Synthesis Review

Part 7 of 7 — Synthesis Review

This topic comes down to one contrast, applied over and over: energy flows, matter cycles. The table below is the master map. If you can explain every row, you can handle almost any question on CED 8.2 and the nutrient cycles.

Energy versus matter

Threads That Tie the Parts Together

1. Cellular respiration is everywhere. It is the heat-producing cost in an organism's energy budget (Part 2), the reason only about 10% of energy passes up a food chain (Part 3), a major flux returning $\text{CO}_2$ to the air (Part 4), and the oxygen-consuming step that turns an algal bloom into a dead zone (Part 5).

2. Chemoautotrophs show up twice. Vent bacteria that oxidize sulfide (Part 1) and nitrifying soil bacteria that oxidize ammonium (Part 5) use the same strategy: energy from inorganic chemicals, carbon from $\text{CO}_2$.

3. Thermoregulation affects food chains. Endotherms spend much of their assimilated energy on body heat, so they turn a smaller fraction of it into new tissue than ectotherms do (Part 2). A food chain with endothermic consumers passes less energy to the next level (Part 3).

4. Nutrients cap energy. If nitrogen or phosphorus is limiting, NPP is limited (Parts 1 and 5), and NPP sets how many consumers and trophic levels the ecosystem can support (Part 3). Adding the limiting nutrient raises NPP, which is the first link in eutrophication.

5. Decomposers serve both stories. As heterotrophs they use the last of the energy in dead matter, releasing it as heat. As recyclers they carry out ammonification and return $\text{CO}_2$ and phosphate, closing the matter cycles.

6. Humans shift matter between reservoirs. Fossil fuel combustion moves carbon from a slow reservoir to the atmosphere (Part 4); fertilizer manufacture and runoff move nitrogen and phosphorus into waterways (Part 5). The ecosystem effects of these shifts are covered in Ecology and Ecosystems.

Checkpoint — Connecting the Parts

Misconceptions to Retire

Practice — Complete the Master Map

Choose the correct ending for each statement.

FRQ-Style Synthesis: Nitrogen on a Grassland

Prompt. Ecologists add nitrogen fertilizer to plots of a grassland where nitrogen limits plant growth. Some of the fertilizer washes into a nearby pond.

(a) Predict the effect on NPP in the plots and justify. NPP increases. Nitrogen is the limiting nutrient, and plants need it to make amino acids, proteins and nucleic acids for new tissue.

(b) Predict the effect on herbivores and predators. Both can increase over time. More NPP means more energy at the base of the food web, and each higher level receives a fraction of it (a bottom-up effect).

(c) Calculate. Plot NPP rises from 3,000 to 4,500 kJ/m²/yr. With 10% efficiency per transfer, energy reaching secondary consumers rises from $3{,}000 \times 0.01 = 30$ to $4{,}500 \times 0.01 = 45$ kJ/m²/yr, a 50% increase.

(d) Explain the effect on fish in the pond. Runoff nitrogen fuels an algal bloom. When the algae die, decomposers break them down by aerobic respiration, consuming dissolved oxygen. The resulting hypoxia can kill fish.

(e) Identify a process that could return some of the added nitrogen to the atmosphere. Denitrification by bacteria in low-oxygen soils or pond sediments converts nitrate to $\text{N}_2$.

Scoring habits: name the process, state the direction of change, and connect it to energy or matter explicitly. "More nitrogen means more growth" earns less than "nitrogen is limiting, so adding it raises NPP and the energy available to consumers."

Final Checkpoint — Mixed Review