Cambridge IGCSE Marine Science 5: Marine ecology

Topic 5 applies ecological ideas to the open ocean, rocky and sedimentary shores, mangrove forests and tropical coral reefs. Cambridge 0697 specifies named zones, organisms and adaptations. Sampling and shore-measurement procedures belong to the separate practical hub; this note owns the concepts and interpretation.

Ecological organisation

A species is a group whose members can reproduce to produce fertile offspring. A population contains organisms of one species in one area at one time. A community contains all populations of different species in an area. A habitat is where an organism lives and interacts. The environment includes biotic and abiotic surroundings. An ecosystem contains a community and its environment interacting as a unit.

Food supply, disease, competition and predation affect population growth. Explain the mechanism: low food can reduce survival and reproduction; disease raises mortality; competition restricts access to resources; predation removes individuals. Their effects depend on density and context.

Marine ecosystems include open ocean and coastal wetlands, coral reefs, sandy, muddy and rocky shores, kelp forests, seagrass beds and mangrove forests. Coastal systems are more exposed to direct human activity and often more productive because run-off brings nutrients.

Open-ocean zones

The pelagic zone is the whole open-water column; the benthic zone is the seabed. Depth divides the water column into:

• sunlight zone, 0 to 200 m;
• twilight zone, 200 to 1000 m;
• midnight zone, below 1000 m.

The sunlight zone has available light, relatively high dissolved oxygen and biomass, low pressure, variable temperature and photosynthetic organisms. Phytoplankton are microscopic producers such as diatoms, dinoflagellates and cyanobacteria. Zooplankton are consumers and include fish or invertebrate larvae and larger drifting animals such as jellyfish. Photosynthesis supports the large biomass.

The twilight zone has little light, some oxygen, more stable temperature and higher pressure. Sunlight and twilight species may migrate, exploit varied foods, swim rapidly where shelter is scarce and use countershading. A darker upper surface blends with deep water from above; a lighter underside blends with surface light from below.

The midnight zone has no light, little oxygen, stable low temperature and very high pressure. Bioluminescence can attract mates or prey or illuminate predators. Low temperature and oxygen favour slow movement and long life. Dark colouring reduces visibility in biological light, gelatinous bodies tolerate pressure, and backward-facing teeth retain rare prey.

Benthic organisms may camouflage or burrow. Some invertebrates use legs or tube feet ventrally and have a protective spiny upper surface. Flatfish lie on one side and develop both eyes on the upward side. Skates and rays are dorsally flattened with expanded fins around the body margin.