What is marine biology?
Marine biology is the study of living things in the sea and how they interact with each other and with sea water.
The ocean covers about 71% of Earth's surface. Sea water is salty (about 35 g of salt in every litre). Life here must cope with salt, pressure, cold and darkness.
Ocean zones by depth
- Sunlit zone (0–200 m): enough light for photosynthesis. Most marine life lives here.
- Twilight zone (200–1000 m): dim blue light, too little for plants. Many animals glow (bioluminescence).
- Dark zone (below 1000 m): no sunlight, near 2–4 °C, huge pressure. Food sinks down from above as "marine snow".
Pressure grows by about 1 atmosphere for every 10 m of depth.
Plankton, nekton and benthos
Marine organisms are grouped by how they live and move, not by what family they belong to.
- Plankton drift with currents; they cannot swim against them. Phytoplankton (tiny algae, like diatoms) make food by photosynthesis and produce a large share of the oxygen we breathe. Zooplankton (tiny animals, fish eggs and larvae, krill) eat them.
- Nekton swim actively and choose where to go: fish, squid, turtles, seals, dolphins, whales.
- Benthos live on or in the sea floor: seaweeds attached to rocks, corals, sponges, starfish, crabs, clams, worms.
A jellyfish is big but still plankton, because it mostly drifts. A young crab is plankton, and later becomes benthos.
Marine ecosystems and food chains
In the sea, phytoplankton are the main producers. A typical chain: phytoplankton → zooplankton → small fish (sardine) → big fish (tuna) → shark. Only about 10% of the energy passes to each next level, so top predators are few.
Key marine ecosystems
- Coral reefs: built by tiny coral animals that live with algae inside them. They hold a huge variety of species in warm, clear, shallow water.
- Mangroves: salt-tolerant trees on tropical coasts. Their roots are nurseries for young fish and shield the coast from waves.
- Seagrass meadows and kelp forests: underwater "grasslands" and "forests" that feed and shelter many animals.
- Open ocean: huge, with plankton, fish shoals, whales.
- Deep sea and hydrothermal vents: bacteria use chemicals from hot vents to make food (chemosynthesis), supporting tube worms and crabs without sunlight.
Adaptations
Fish take oxygen from water with gills; streamlined bodies cut drag; marine fish get rid of extra salt; deep-sea animals make their own light; whales store fat (blubber) against cold.
Ocean resources and desalination
Bio-resources
- Food: fish, prawns, crabs, molluscs; seaweed as food.
- Aquaculture (mariculture): farming fish, prawns, oysters and seaweed in the sea.
- Materials: agar and alginate from seaweed are used in food, medicines and labs.
- Medicines: some cancer and pain drugs come from sponges, cone snails and other sea life.
Desalination
Desalination removes salt from sea water to make fresh water.
- Distillation: heat sea water, collect and cool the steam. Uses a lot of energy.
- Reverse osmosis: pump sea water at high pressure through a membrane that lets water through but stops salt. Most new plants use this.
Problems: high energy use, and the very salty leftover water (brine) can harm sea life if dumped carelessly.
Threats and protection
- Overfishing: fish are caught faster than they can breed.
- Pollution: plastics, oil spills, sewage and farm fertiliser (which can cause low-oxygen "dead zones").
- Warming: hot water makes corals push out their algae and turn white (coral bleaching).
- Acidification: extra carbon dioxide makes sea water more acidic, so shells and corals form less easily.
Solutions: marine protected areas, fishing limits and closed seasons, nets that let small fish escape, less single-use plastic, and protecting mangroves and reefs.
Try it: build a food web
On paper, draw boxes for: phytoplankton, zooplankton, sardine, mackerel, tuna, shark, turtle, jellyfish, seagrass. Draw arrows from food to eater. Then remove the sardine. Which animals lose food? Then open the last 3D step and dive from 0 m to 4000 m: at what depth does light reach zero?
Key formulas and definitions
- Plankton = drift; Nekton = swim; Benthos = on the sea floor
- Sunlit 0–200 m; twilight 200–1000 m; dark below 1000 m
- Pressure ≈ 1 atm + 1 atm for every 10 m of depth
- About 10% of energy passes to the next level of a food chain
- Desalination: distillation or reverse osmosis
Worked examples
1. Classify: shark, diatom, starfish, jellyfish, octopus on a reef floor.
Shark: nekton. Diatom: phytoplankton. Starfish: benthos. Jellyfish: plankton (it mostly drifts). Octopus on the floor: benthos (it can also swim, but lives on the bottom).
2. What is the pressure at 500 m depth?
1 atm at the surface + 500 ÷ 10 = 50 atm from water = about 51 atm.
3. Phytoplankton in an area store 10 000 kJ of energy. About how much reaches small fish that eat zooplankton?
Zooplankton get about 10%: 1000 kJ. Small fish get about 10% of that: 100 kJ.
4. Why do almost no green algae live at 1500 m?
There is no sunlight at that depth, so photosynthesis is impossible.
5. How can deep-sea vent animals live without sunlight?
Bacteria at the vent use chemicals like hydrogen sulfide to make food (chemosynthesis). Tube worms and crabs feed on or live with these bacteria.
Common mistakes
- Thinking plankton means only tiny things. Jellyfish are large but still plankton, because they drift.
- Saying the ocean has no producers. Phytoplankton are the main producers.
- Thinking all ocean life needs sunlight. Vent life uses chemosynthesis.
- Mixing up desalination methods: distillation boils, reverse osmosis pushes water through a membrane.