Life in the Sea

In February 1977 the research submersible Alvin dived to the Galapagos Rift, about 2,500 metres down, looking for hot water coming out of the seafloor. The geologists on board found it, and also something nobody had predicted: dense communities of giant tube worms, clams and crabs living in total darkness around the vents (check the details). There was no sunlight to feed them. The discovery overturned the assumption that all life depends on the Sun and showed that the deep sea holds whole ecosystems we had never imagined. This chapter surveys the living ocean, starting with its smallest and most important inhabitants, and ending with what humans take from it.

The Base of the Food Web: Plankton

Plankton are organisms that drift with the water rather than swimming against it. The word comes from Greek, meaning "wandering".

Passing Energy Up the Chain

Energy moves up the food web from plankton to small fish to larger fish, seabirds, seals and whales. At each step most of it is lost as heat or used for the animal's own life, and only a fraction, often taken as about 10 per cent as a rule of thumb, is stored in the tissues that the next animal eats (check). The practical result is that long food chains support far fewer animals at the top than at the bottom.

The shortest food chains are the most productive. The baleen whales, including the blue whale, the largest animal known, feed directly on krill and other small organisms near the base of the web, which is why a very large animal can live on a small step in the chain (check). The great Peruvian anchoveta fishery (Chapter 6) works for the same reason.

The Largest Migration on Earth

Every night, in all the oceans, a vast number of animals rise from the twilight zone (200 to 1,000 metres) toward the surface to feed in the safety of the dark, and sink back at dawn. This daily vertical migration of zooplankton, shrimp and small fish is the largest movement of animals on the planet by weight (check). Early naval sonar operators found it puzzling: they recorded a false seabed that rose and fell with the day, which was later called the deep scattering layer. The migration also moves carbon: animals feed at the surface, then excrete at depth, carrying carbon out of the atmosphere's reach (check).

Life Without Sunlight

Below about 1,000 metres there is no sunlight, and food comes from above. Much of it is "marine snow", a continual drift of dead plankton, faecal pellets and other debris, and the occasional larger fall: a dead whale can feed a community of specialised animals for decades (check).

• Hydrothermal vents. Along the mid-ocean ridges (Chapter 2) seawater seeps into the hot crust, picks up dissolved minerals and returns as hot fluid, sometimes above 350 degrees Celsius (check). Bacteria use the chemical energy of hydrogen sulphide in this fluid to make food, a process called chemosynthesis. The tube worms, clams and shrimp at vents depend on those bacteria, often living with them inside their tissues.
• Cold seeps. Slow leaks of methane and other chemicals from the seabed support similar communities.
• Animals of the dark. Many deep animals make their own light (bioluminescence), have huge eyes or none at all, and eat rarely. Bioluminescence is so common in the water column that most animals there are thought to produce it (check).

Coastal Habitats

Near the coast, the sea floor itself supports habitats that are small in area but rich in life.

• Coral reefs. They cover well under one per cent of the ocean floor yet are home to about a quarter of marine species (check). Warm-water corals live in partnership with algae inside their tissues, which makes them very sensitive to heat (Chapter 8): in a marine heatwave the corals expel their algae and turn white, a process called bleaching.
• Kelp forests. Large brown seaweeds in cool rocky seas, including those off Scotland, shelter fish, urchins and otters.
• Seagrass meadows and mangroves. Nursery areas for fish, and stores of carbon in their sediments.
• Salt marshes and estuaries. The Forth and the Wash are among the British estuaries that host huge numbers of migrating birds.

Fishing

Marine fisheries provide a significant share of the animal protein for billions of people. The United Nations Food and Agriculture Organization (FAO) classifies assessed stocks as underfished, fished at a maximum sustainable level, or overfished. In its recent reports about a third of assessed stocks were overfished, but because the fish that make up the largest catches are generally managed better than the rest, a larger share of the total catch comes from sustainably fished stocks (check the latest report).

Exercises