Ocean ecology: sunlit surface waters

The first 200m or so of the open ocean is called the epipelagic zone. Enough sunlight enters these waters to allow photosynthesis - harnessing of the sun’s energy to produce food.

In the open waters, photosynthesis is performed by phytoplankton: microscopic floating algae. Like plants on land, phytoplankton need sunlight and nutrients to grow.

The sunlight is no problem, but in general the epipelagic zone is low in nutrients because organic debris (such as dead animals) sinks to much greater depths. Thus much of these waters are a marine 'desert', with very little life.

However, in some areas nutrients are brought up from the ocean depths by upwellings, storms, and ocean currents. In these areas, phytoplankton grow rapidly - and can become so numerous that the water turns green from their chlorophyll, the same pigment that gives land plants their colour. These areas are some of the most productive on the planet, supporting billions of tonnes of life.

Zooplankton: one step up the food web
Phytoplankton are eaten by zooplankton - small animals which, like phytoplankton, drift in the ocean currents. The most abundant zooplankton species are copepods and krill: tiny crustaceans that are the most numerous animals on Earth. Other types of zooplankton include jelly fish and the larvae of fish, marine worms, starfish, and other marine organisms.

From zooplankton to larger animals
The zooplankton are eaten by a huge variety of other animals. These range from small fish like sardines to giant manta rays, whale sharks, and even the largest animal on Earth, the blue whale. Some seabirds also feed exclusively on zooplankton.

The small fish are in turn eaten by bigger animals - squid, tuna, marlin, sharks, seabirds, dolphins, and toothed whales, to name a few.

Incredible voyagers
The open ocean is vast, and large areas are devoid of life. Thus animals higher up the food chain need to travel huge distances to find food and places to breed.

Some fish predators are large, powerful, and fast, allowing them to cross the oceans in search of food. Sail fish are the fastest fish in the ocean, reaching speeds of 120km/h. Bluefin tuna are almost as fast, and can accelerate faster than a Porsche. In addition, unlike most fish bluefin tuna are warm-blooded, allowing them to survive in cooler waters.

Many whale species migrate thousands of kilometers each year between their warm breeding grounds and their rich Arctic and Antarctic feeding grounds. Marine turtles too make epic voyages across the oceans between their nesting beaches and feeding grounds.

Seabirds also travel huge distances. Albatross, for example, fly for thousands of kilometres without landing, scouring the oceans for fish, krill, and other food.

Some shelter
There is some shelter at the ocean surface. Driftwood, pieces of kelp, and other natural debris floating on the water provide valuable shelter where small fish can hide from predators. Man-made debris can similarly be used.

Islands of life
There is also some land in the open ocean: underwater mountains known as seamounts, which can come within metres of the ocean surface.

Formed by volcanoes or sinking islands, seamounts rise steeply from the ocean floor. Deep-water currents are forced up their sides, bringing nutrient-rich water to the surface around shallow seamounts. This allows plankton to thrive - supporting complex food chains and making these areas islands of life in the open ocean.

Visitors also drop by. Some fish species periodically gather at seamounts to feed and spawn, while marine turtles and whales stop at shallow seamounts for food and shelter during their long migrations.

Find out about deep-sea life on seamounts...