Difficulty: Introductory to Intermediate | Prerequisites: Basic understanding of cell biology and the concept of energy in living systems.
Big picture: This chapter sits at the foundation of marine ecology. Before you can understand food webs, population dynamics, or why certain ocean regions teem with life while others are relative deserts, you need to understand where energy comes from and how it moves through marine ecosystems. The two main entry points for energy are photosynthesis (sunlight-driven) and chemosynthesis (chemical-driven). From there, energy flows upward through trophic levels, losing a large share at each step. If you missed earlier material on basic cell biology or what autotrophs and heterotrophs are, review that first.
Marine ecosystems get their energy from two sources: photosynthesis (sunlight-driven, in the photic zone) and chemosynthesis (chemical-driven, at places like hydrothermal vents). Energy flows upward through trophic levels, but only about 10% transfers from one level to the next, with the rest lost to heat, respiration, and waste. High productivity at the base of a food web supports more trophic levels and greater biodiversity.
Photosynthesis
The process by which autotrophs (primary producers) use sunlight, CO2, and water to produce organic molecules such as glucose. Occurs in the photic zone.
In simple terms, this is how plants and algae turn sunlight into food, both for themselves and for everything that eats them.
Chemosynthesis
The process by which certain bacteria use chemical energy (e.g. hydrogen sulfide) from dissolved minerals to synthesise glucose, without any need for sunlight.
Think of it as the deep-sea alternative to photosynthesis: bacteria at hydrothermal vents make energy from chemicals instead of light.
Autotroph
An organism that produces its own food from inorganic substances, using either light energy (photoautotroph) or chemical energy (chemoautotroph).
In simple terms, autotrophs are the self-feeders, the base of every food chain.
Productivity (primary production)
The rate of production of biomass by primary producers. Measured as energy capture per unit area (or per unit volume in aquatic systems) per year.
Think of it as how fast the ecosystem's "engine" is running. High productivity means more food entering the system at the bottom.
Trophic level
A feeding position in a food chain or food web. Producers sit at the first trophic level, primary consumers at the second, secondary consumers at the third, and so on.
In simple terms, it is where an organism sits on the who-eats-whom ladder.
Photic zone
The upper layer of the ocean where enough sunlight penetrates to support photosynthesis.
Think of it as the sunlit layer, typically the top 200 metres or so.
Energy pyramid (pyramid of energy)
A diagram showing how energy decreases at each successive trophic level in a food chain. Widest at the producer base, narrowest at the top predator.
In simple terms, it is a visual that shows most energy is at the bottom and very little makes it to the top.
Ten percent rule
The general guideline that roughly 10% of the energy at one trophic level is transferred to the next. The remaining 90% is lost to respiration, heat, excretion, and egestion.
Think of it as: for every 100 kJ a plant captures, only about 10 kJ ends up in the herbivore that eats it.
Performed by primary producers (autotrophs): phytoplankton, algae, seagrasses, mangroves
Uses sunlight + CO2 + water to produce glucose (organic energy molecules)
Takes place in the photic zone, the sunlit upper layer of the ocean
The glucose produced fuels the producer's own metabolism and becomes the energy source for every consumer up the food chain
General equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
Performed by specialised bacteria, not by plants or algae
Uses chemical energy from dissolved minerals (commonly hydrogen sulfide, H2S) instead of sunlight
Occurs where sunlight cannot reach, most notably at hydrothermal vents on the deep ocean floor
Bacteria capture the chemical energy of dissolved minerals and convert it into glucose
Supports entire deep-sea ecosystems that are completely independent of solar energy
Both are forms of autotrophic production: the organism makes its own food
Both produce glucose as the primary energy-storage molecule
Both serve as the energy entry point for their respective food chains
Energy source: photosynthesis uses sunlight; chemosynthesis uses chemical compounds
Location: photosynthesis is restricted to the photic zone; chemosynthesis occurs in deep, dark environments
Organisms involved: photosynthesis is carried out by plants, algae, and cyanobacteria; chemosynthesis is carried out by specific bacteria and archaea
Scale: photosynthesis drives the vast majority of marine productivity; chemosynthesis supports localised but ecologically important communities
Productivity is the rate at which biomass is produced by primary producers
Measured as energy capture per unit area (terrestrial) or per unit volume (aquatic) per year
In most ecosystems, green plants or phytoplankton are the primary producers, so "productivity" almost always refers to primary production
Every consumer in the food chain depends, directly or indirectly, on the energy captured by producers
Higher productivity at the base means more energy is available to support herbivores, predators, and top predators
Ecosystems with high productivity tend to support more trophic levels and greater biodiversity
Tropical rainforests (on land)
Coral reefs, algal beds, and estuaries (in the ocean)
Upwelling zones where nutrient-rich deep water rises to the surface
Conditions favouring high productivity: abundant sunlight, warm temperatures, good nutrient supply
Open-ocean "deserts" far from coasts and upwelling zones
Deep ocean below the photic zone (no light for photosynthesis)
Polar regions during winter months (limited sunlight)
Energy enters the food chain through producers (trophic level 1)
It passes to primary consumers (herbivores, trophic level 2), then secondary consumers (trophic level 3), then tertiary consumers (trophic level 4)
At each transfer, a large proportion of energy is lost
Energy does not transfer perfectly from one level to the next. The losses come from several sources:
Respiration: organisms burn glucose for movement, growth, and maintenance, releasing energy as heat
Excretion and egestion: not all food consumed is absorbed; waste products carry away energy
Incomplete consumption: not all of an organism's body is eaten by the next consumer (bones, shells, etc.)
The net result: only about 10% of the energy at one trophic level reaches the next
Diatoms → Copepods → Arctic cod → Ringed seal → Polar bear
This chain has five trophic levels. With each step, the available energy drops sharply.
Much solar energy is reflected off the water surface
Light only penetrates the upper ocean; it does not reach deep water
Photosynthesis is not 100% efficient as a chemical process
Not all wavelengths of visible light are absorbed by photosynthetic pigments
Producer efficiency (how much incoming solar energy producers capture):
(10,000 / 900,000) × 100% = 1.11%
Interpretation: producers capture just over 1% of the sun's energy that hits the surface
Primary producers to secondary consumers:
(47 / 10,000) × 100% = 0.47%
Secondary consumer to tertiary consumer:
(2 / 47) × 100% = 4.25%
These very low percentages illustrate why food chains rarely exceed four or five trophic levels. There simply is not enough energy left at the top to sustain another level.
Efficiency (%) = (Energy at higher trophic level / Energy at lower trophic level) × 100%
This single formula is used for all the efficiency calculations in this chapter. Plug in the energy values for any two adjacent levels.
A pyramid of energy for the example food chain would look like this (widest at the base, narrowest at the top):
Level 5 (Tertiary consumer): Polar bear, 2 kJ
Level 4 (Secondary consumer): Ringed seal, 47 kJ
Level 3 (Primary consumer): Arctic cod
Level 2 (Primary consumer): Copepods
Level 1 (Producer): Diatoms, 10,000 kJ
Solar input: 900,000 kJ
Note: the source material gives values at selected levels. The pyramid narrows dramatically at each step, reinforcing the ten percent rule.
The efficiency of energy transfer between trophic levels is the reason that fishing lower on the food chain (e.g. harvesting anchovies or sardines rather than tuna) yields far more food per unit of ocean productivity. This underpins debates around sustainable fisheries management.
Chemosynthesis at hydrothermal vents has reshaped how astrobiologists think about life on other planets. If life can thrive without sunlight in Earth's deep ocean, similar ecosystems might exist beneath the ice of Europa or Enceladus.
"All marine life depends on sunlight." Not true. Chemosynthetic communities at hydrothermal vents thrive entirely without solar energy. Exam answers that claim all ocean life depends on photosynthesis will lose marks.
"Energy is created at each trophic level." Energy is never created; it is transferred and transformed. At each level, most is lost as heat through respiration. The total decreases as you move up.
"The ten percent rule means exactly 10% transfers every time." It is a rough average, not a fixed law. The worked examples in this chapter show actual transfer efficiencies ranging from about 1% to about 4%, well below 10%.
"Producers use all the sunlight that hits the ocean surface." Most sunlight is reflected, absorbed as heat, or is the wrong wavelength. Producer efficiency in the example is only 1.11%.
⚠️ Be able to calculate efficiency between any two trophic levels using the formula. This is a near-certain exam question. Show your working clearly.
⚠️ Know the differences between photosynthesis and chemosynthesis (energy source, location, organisms). A Venn diagram format is commonly requested.
⚠️ Be ready to explain why energy decreases at each trophic level (respiration, heat loss, excretion, egestion, incomplete consumption).
⚠️ Understand why high productivity supports greater biodiversity and more complex food webs.
⚠️ Be able to explain why not all sunlight is captured by producers (reflection, depth limitation, wavelength mismatch, photosynthetic inefficiency).
1. True or False: Chemosynthesis requires sunlight.
False. Chemosynthesis uses chemical energy (e.g. hydrogen sulfide), not light.
2. Fill in the blank: Productivity is measured as energy capture per unit ______ per year.
Area (or volume, for aquatic systems).
3. True or False: Approximately 90% of energy is transferred from one trophic level to the next.
False. Only about 10% is transferred; the other 90% is lost.
4. Fill in the blank: The efficiency formula is (Energy at ______ level / Energy at ______ level) × 100%.
Higher trophic level / Lower trophic level.
5. True or False: Photosynthesis and chemosynthesis both produce glucose.
True.
Q: Describe one key similarity and one key difference between photosynthesis and chemosynthesis.
A: Both produce glucose as an energy source for the food chain (similarity). Photosynthesis uses sunlight as its energy source, while chemosynthesis uses chemical energy from dissolved minerals such as hydrogen sulfide (difference).
Q: If 900,000 kJ of solar energy reaches the ocean surface and producers capture 10,000 kJ, what is the efficiency of the producers? Show your working.
A: (10,000 / 900,000) × 100% = 1.11%. Producers capture roughly 1% of incoming solar energy.
Q: Explain why food chains rarely have more than four or five trophic levels.
A: Energy is lost at each trophic level through respiration (heat), excretion, egestion, and incomplete consumption. Only about 10% of energy passes from one level to the next, so by the fourth or fifth level there is too little energy remaining to support another population of consumers.
Q: Give two reasons why not all sunlight that reaches the ocean surface is used by primary producers.
A: (1) Much of the light is reflected off the water surface. (2) Photosynthetic pigments do not absorb all wavelengths of visible light. (Other valid answers: light does not penetrate to depth; photosynthesis is inherently inefficient as a chemical reaction.)
Q: A secondary consumer contains 47 kJ of energy and the tertiary consumer above it contains 2 kJ. Calculate the transfer efficiency.
A: (2 / 47) × 100% = 4.25%.
This connects to ocean zones and habitats (Chapter 1/2 material) because the photic zone boundary determines where photosynthetic production can occur, which in turn shapes where life concentrates in the ocean.
Energy flow through trophic levels connects directly to food webs and ecosystem structure covered later in the course. Understanding why energy decreases at each level explains why top predators are rare and why disrupting one level can cascade through the whole web.
Chemosynthesis links to extremophiles and deep-sea biology, a topic that overlaps with astrobiology and the search for life beyond Earth.
Photosynthesis, chemosynthesis, autotroph, primary producer, primary production, productivity, biomass, trophic level, food chain, food web, energy pyramid, pyramid of energy, ten percent rule, 10% rule, energy transfer efficiency, photic zone, euphotic zone, hydrothermal vent, hydrogen sulfide, glucose, respiration, heat loss, excretion, egestion, diatoms, copepods, Arctic cod, ringed seal, polar bear, marine ecology, marine biology, FIU, Florida International University, intro to marine biology, energetics of marine ecosystems, Chapter 3