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Marine Science GCSE

Topic 4: Nutrients and Energy

How energy flows and nutrients cycle through marine ecosystems.

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Topic Summary: This topic covers how energy enters and moves through marine ecosystems. You need to understand photosynthesis and chemosynthesis as energy sources, food chains and webs, trophic levels (with ~90% energy lost at each level), and how nutrients like nitrogen and phosphorus are recycled. Key concepts include primary production, biomagnification, and the biological pump.

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4.1 Photosynthesis and Primary Production

Photosynthesis is the process by which producers convert light energy into chemical energy (glucose). In the ocean, phytoplankton are the main producers.

Photosynthesis 6CO₂ + 6H₂O → (light + chlorophyll) → C₆H₁₂O₆ + 6O₂
Word Equation Carbon dioxide + Water → (light energy) → Glucose + Oxygen

Factors Affecting Photosynthesis Rate

FactorEffect
Light intensityMore light = more photosynthesis (up to a maximum)
CO₂ concentrationMore CO₂ = more photosynthesis (up to a maximum)
TemperatureIncreases rate up to an optimum; too hot denatures enzymes
Nutrient availabilityNitrogen and phosphorus needed for growth

Primary production is the rate at which producers convert energy into organic matter. Areas with high primary production (e.g., upwelling zones) support the most marine life.

Photosynthesis and respiration are opposite reactions. Both happen in producers — photosynthesis during the day, respiration all the time. Net production = photosynthesis - respiration.

4.2 Chemosynthesis

Chemosynthesis is the production of organic compounds using energy from chemical reactions rather than sunlight. Carried out by specialised bacteria at hydrothermal vents.

How It Works

  1. Hydrothermal vents release chemicals like hydrogen sulfide (H₂S)
  2. Chemosynthetic bacteria oxidise these chemicals
  3. The energy released is used to convert CO₂ and water into organic compounds
  4. These bacteria are the producers at the base of vent food chains
  • Chemosynthesis happens without light
  • Found at hydrothermal vents ~2,000-4,000m deep
  • The bacteria support giant tube worms, crabs, clams, and shrimp
  • Proves that life can exist without sunlight

4.3 Food Chains and Food Webs

A food chain shows the flow of energy from one organism to the next. A food web shows interconnected food chains in an ecosystem.

Example Marine Food Chain

Phytoplankton → Zooplankton (krill) → Small fish (herring) → Large fish (tuna) → Shark

Rules:

  • Arrows show the direction of energy flow (from eaten to eater)
  • All food chains begin with a producer
  • Food webs are more realistic — organisms usually eat more than one food source

Diagram: Simple Marine Food Web

Diagram: Simple Marine Food Web Simple Marine Food Web Sun Phytoplankton (Producer) Zooplankton (1st consumer) Small Fish (2nd consumer) Large Fish (3rd consumer) Shark (Top predator) Seaweed (Producer) Sea Urchin (1st consumer) Crab Decomposers (Bacteria) All dead matter Arrows show direction of energy flow

4.4 Trophic Levels and Energy Transfer

Trophic LevelNameMarine Example
1ProducersPhytoplankton, seaweed
2Primary consumers (herbivores)Zooplankton, krill, sea urchins
3Secondary consumersSmall fish (herring, anchovies)
4Tertiary consumers (top predators)Tuna, sharks, dolphins, orcas

Diagram: Pyramid of Energy

Diagram: Pyramid of Energy Producers (Phytoplankton) 10,000 kJ Primary Consumers (Zooplankton) 1,000 kJ Secondary (Small Fish) 100 kJ Top Predators 10 kJ ~90% energy lost as heat at each level Level 1 Level 2 Level 3 Level 4 Only ~10% of energy passes to the next level

The 10% Rule

Only about 10% of energy at each trophic level is passed to the next. The other ~90% is lost as:

  • Heat from respiration (~60-80%)
  • Waste products (faeces, urine)
  • Uneaten parts (bones, shells, scales)

Biomagnification

Biomagnification is the increasing concentration of toxins (e.g., mercury, DDT, microplastics) at higher trophic levels. Top predators accumulate the highest levels because they eat many contaminated organisms throughout their lives.

Don't confuse pyramids of ENERGY (always pyramid-shaped) with pyramids of BIOMASS (can be inverted in marine ecosystems because phytoplankton are tiny but reproduce very fast).

4.5 Nutrient Cycling

Nutrients are recycled through ecosystems (unlike energy, which flows in one direction). The key nutrients in marine ecosystems are nitrogen and phosphorus.

Why Nutrients Matter

  • Nitrogen (N): Needed for proteins and DNA. Often the limiting factor for phytoplankton growth
  • Phosphorus (P): Needed for DNA, cell membranes, and ATP (energy transfer)

The Biological Pump

The biological pump transfers carbon from the surface to the deep ocean. Phytoplankton absorb CO₂ for photosynthesis. When they die, their remains sink as marine snow, carrying carbon to the deep ocean. This is important for regulating atmospheric CO₂.

Eutrophication

Eutrophication occurs when excess nutrients enter water, causing: algal bloom → blocks light → plants die → bacteria decompose dead matter → oxygen depleted → fish die.

4.6 Decomposition and Detrital Pathways

Decomposers (bacteria and fungi) break down dead organic matter, releasing nutrients back into the water for producers to reuse.

Detritus and Marine Snow

Detritus (dead organic matter) sinks from the surface as "marine snow." This is the main food source for deep-sea organisms and provides nutrients when decomposed.

Aerobic Respiration

All organisms (including decomposers) release energy through aerobic respiration:

Aerobic Respiration C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Word Equation Glucose + Oxygen → Carbon dioxide + Water + Energy
  • Decomposition recycles nutrients — essential for new growth
  • Respiration releases energy and produces CO₂
  • Photosynthesis and respiration are opposite reactions
  • Energy flows through ecosystems; nutrients cycle

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