Mother Earth is indeed a blue planet when viewed from space. Its surface is almost three-quarters covered by ocean which has a mean depth of about 3 700 m, and which ranges from shallow coastal waters to trenches as deep as 11 000 m. The ocean is by far the biggest habitable space on this planet although most of it is never reached by sunlight.

In fact, almost 60% of the Earth's surface is completely dark, which has not prevented life, except for plants, from colonising it. Because of the vastness of the oceans and the difficulty in seeing or registering what is below the surface it is the least investigated and poorest-known biome.

Without a living ocean, life on land would be impossible. Even from the narrow perspective of mankind it has always been of utmost importance and this has been accentuated in modern times.

To take on the task of giving a comprehensive natural history of the world's oceans is therefore a major and difficult undertaking. The BBC has done this and presents it on three DVD discs with extraordinarily good footage of various processes and events from different parts of the world's oceans.

Disc one deals with a number of important processes, like ocean-atmosphere interaction, the role of waves, ocean currents and upwelling of deep water - all of which are important for distribution of nutrients and thus also for the primary (microscopic algae) production. Also the influences that the moon and sun have upon tides, production in general in the sea and the behaviour of animals in relation to daily, monthly or seasonal cycles are aptly shown by several good and spectacular examples. These include vertical migration of plankton, migrating sardines along the eastern seaboard of South Africa, spawning herring, reproducing squids in the Northeast Pacific Ocean and the annual event of turtles coming ashore in Costa Rica to lay eggs. All are spectacular events which emphasise the abundance of eggs and larvae produced, but also demonstrate the risk of exposure to predators of both young and adult animals. Sometimes sequences showing the predation by hunting fish, sharks, dolphins and birds become too dominating and the viewer is left with a feeling that it is surprising that anything can survive in the sea under natural conditions.

Although processes are emphasised, the contents list comprise four episodes: 'The blue planet'; 'The deep'; 'Open ocean' and 'Frozen seas'.

It is obvious that only a few examples can be chosen and shown and that there are other processes in the oceans which are equally important but less easy to demonstrate. The only thing I really miss is the importance of downwelling in the North Atlantic which is fundamental for the transport of warm water into this part of the ocean and which is responsible for the mild climate in much of Europe.

The second disc deals with four different parts of the ocean: Seasonal seas, Coral seas, Tidal seas and Coasts. Again this is a spectacular show of some outstanding events and features of these seas and, even if you know better, you receive the impression that everywhere things are terribly dramatic. The marine ecosystems are never in a calm equilibrium, but picking the raisins from the cake tends to bring out the dramas. The seasonal or temperate seas are the most abundant in fish (not species abundance) well demonstrated by the salmon in the Northeast Pacific and the herring in the North Atlantic. The importance of fishing is not mentioned here, and the decline in cod populations or the huge fluctuations in various clupeid populations (herring, anchovies, anchovettas, sardines) could have been demonstrated to give a little balance to the picture. (The threats to marine life are, however, brought in on the third DVD disc.)

The richness in species of coral seas and the spectacular spawning event of the corals are beautifully pictured as are some examples of the role of tides for marine life in coastal areas.

The third disc, finally, emphasises the threat to marine life due to ever more efficient fishing methods. More could have been made of other detrimental human influences, particularly in coastal areas, e.g., effects of communal, industrial or agricultural waste effluences. Fishing alone is indeed a major danger particularly to schooling fish species, but also to deep living or long-lived and late spawning species (like some rockfishes). The third disc is a healthy reminder of the vulnerability of marine ecosystems and the need to take precautionary actions.

The third disc also includes a section on the difficulties that the BBC photographic teams had to get the sometimes extraordinarily good footage that is presented. This is also a good educational tool for the public in general. The majority of us cannot expect to see any of the extraordinary events shown. They might be happening when we are out at sea but we may have to be a trained diver to see some of them.

Yet, for the general vacationer there is a lot to see and experience in shallow shore areas although it will not always be as colourful and dramatic.

The narration by David Attenborough is generally precise and well timed. There are a few mistakes or slips, which are caused by the need to generalise to present an overview.

There is a lot of information both in words and pictures, and it is well worth playing the discs several times. You will find new details every time.

It is good to have all this material on DVD discs, which allow you to view separately whichever section you want. For the teacher who wants to demonstrate particular processes or features this is ideal.

Jarl-Ove Strömberg
Prof. emeritus
Royal Swedish Academy of Sciences
Kristineberg Marine Research Station
450 34 Fiskebäckskil, Sweden

Related Web sites
BBC Blue Planet Web site
http://www.bbc.co.uk/nature/blueplanet