Habitats from reef to open water
0 of 3 hands-on
01 Introduction
A habitat is the place where an animal or plant lives, with the conditions it needs. The sea has many habitats, from rock pools to deep trenches, and they are often connected. Knowing which habitat you are in tells you what you are likely to see and how fragile it is.
Objectives
- Describe the main marine habitats and the life each supports
- Explain how habitats are linked, such as mangroves, seagrass and reef
- Identify the main threats to each habitat
02 Watch
A tour of habitats
5 min · Video in production
- 1
A journey from the shore to the open ocean: rock pool, mangrove, seagrass, reef and blue water.
- 2
A time-lapse of coral growth and a reef at night.
- 3
A kelp forest swaying in a surge, with otters and fish.
- 4
A wreck covered in life, a sandy slope with garden eels, and an open-water drop-off with pelagic fish.
03 Learn
| Habitat | What it is like | Typical life | Main threats |
|---|---|---|---|
| Coral reef | Warm, clear, shallow water with limestone built by corals | Corals, reef fish, turtles, sharks, invertebrates | Warming, bleaching, damage, pollution, overfishing |
| Kelp forest | Cool, nutrient-rich water with tall brown algae | Kelp, urchins, otters, fish, seals | Warming, urchin overgrazing, storms |
| Seagrass meadow | Flowering plants growing on sand in sheltered shallows | Turtles, dugongs, seahorses, juvenile fish | Dredging, runoff, anchors, trawling |
| Mangrove | Salt-tolerant trees at the edge of land and sea | Juvenile fish, crabs, birds, prawns | Clearance for farms and development |
| Rocky shore and intertidal | Zone between high and low tide: waves, drying and temperature swings | Barnacles, limpets, anemones, crabs | Trampling, collecting, pollution |
| Sand and mud | Seemingly bare, but full of burrowers | Rays, flatfish, garden eels, worms, shrimp | Trawling, dredging |
| Open water (pelagic) | No bottom. Life drifts or swims | Plankton, tuna, mantas, sharks, dolphins | Overfishing, ship strikes, noise |
| Deep sea | Dark, cold, high pressure | Specialised fish, squid, tubeworms, corals | Deep-sea trawling and mining |
| Polar waters | Very cold, with ice and strong seasons | Krill, seals, whales, penguins | Warming, ice loss, fishing |
| Wrecks and artificial reefs | Hard structures on a bare bottom that gather life | Corals, sponges, schools of fish, predators | Fragile structures and pollution |
Coral reefs
Coral reefs cover a tiny fraction of the sea floor but are home to a very large share of marine species. Reef-building corals depend on algae that live inside their tissue, which need light, so reefs grow in clear, shallow, warm water. When the water is too warm for too long, corals expel their algae and turn white. This is bleaching, and it can kill the coral if the heat persists.
Predict first
Why do reef-building corals grow best in shallow, clear water?
Nurseries: mangroves and seagrass
Many reef fish spend their early lives in mangrove roots and seagrass beds, where the shelter protects them from predators. When a mangrove or meadow is cleared, the effect can be felt on the reef offshore, and the reverse is also true. Seagrass and mangroves also trap carbon, buffer coasts from waves and filter runoff.
Kelp forests
Kelp is a large brown alga, not a plant, and it can grow tens of centimetres a day in cold, nutrient-rich seas. Kelp forests shelter many animals. If predators such as otters or fish are removed, urchins can multiply and graze the kelp away, leaving an 'urchin barren'.
Sand, wrecks and open water
A sandy bottom looks empty but supports burrowers, flatfish and rays that blend in. A wreck or an artificial reef provides hard surfaces that corals and sponges settle on, and gives shelter to fish. In open water there is nothing to hide behind, so animals depend on speed, schooling and camouflage.
Habitats are connected
- 1.Mangroves and seagrass shelter young fish.
- 2.The young fish move out to the reef as they grow.
- 3.The reef protects the coast and the seagrass from waves.
- 4.Mangroves and seagrass trap sediment, which keeps reef water clear.
- 5.The reef's fish feed predators in open water, which return nutrients.
Threats to the sea
- Climate change: warming water, bleaching and ocean acidification.
- Overfishing and destructive fishing, including trawling and blast fishing.
- Pollution: plastic, chemicals, sewage and fertiliser runoff.
- Coastal development and loss of mangroves and seagrass.
- Physical damage from anchors, careless divers and ship groundings.
- Invasive species, such as lionfish in the Atlantic.
04 Explore
Try it yourself
Match each habitat with its description.
Match each habitat with a description.
- Mangrove
- Kelp forest
- Seagrass meadow
- Coral reef
- Pelagic
05 Apply
Scenario
A dive centre plans to buy land beside a reef and clear a strip of mangrove for a beach, arguing that mangroves are ugly and have no value for divers.
What is the best response?
06 Check
07 Summary
Key points
- Main habitats include reef, kelp, seagrass, mangrove, shore, sand, open water, the deep, polar seas and wrecks.
- Habitats are connected: mangroves and seagrass shelter young fish that later live on the reef.
- Corals are fragile and slow growing, and bleach in warm water.
- Threats include warming, overfishing, pollution, development and physical damage.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.