The deep sea: a rich but fragile environment

Life without light

At depths of more than around 200 meters, there is no longer enough sunlight penetrating the ocean for photosynthesis to occur. There are thus no plants in deep-sea ecosystems: no seaweed, seagrass or phytoplankton. The life which develops in the deep sea depends on the organic matter produced at the surface, which reaches the ocean depths in very small quantities. Apart from very specific, so-called “chemosynthetic” sites (such as hydrothermal springs or sepages of cold methane), the deep-sea ecosystem is characterized by the abscence of primary production in situ, by low biomasses and by a majority of predatory or scavenger fish.

This food shortage imposes structural restrictions on life in the deep sea: a “slow motion” lifestyle , slow growth and metabolisms, extremely high longevity, and low biomasses. The deep ocean environment also suffers from little disturbence apart from human actions and the mortality rates are very low; these stark conditions lead to a very low resilience of wildlife and the environment to outside disturbances.

The deep sea: unexplored and poorly understood

Less than 1% of the oceans have been explored. This explains why most scientific questions about deep-sea environments remain unanswered.

The estimated number of scientific samples taken since deep-sea exploration began (in the last quarter of the 19th century) is around 3 000. On average, this is not even one sample per 100 000km2. Similarly, the estimated number of seamounts in the oceans is 50 000, but the number of these that have been explored barely exceeds 200 and only around thirty have been sampled. These figures are a result of the fact that very few scientific submersibles (less a dozen in the world) are capable of exploring depths greater than 1 000 meters.

Our knowledge of the deep sea has more holes in it than a piece of cheese and only a few fragments (the most spectacular, but not necessarily the most representative, for example chemosynthetic processes) are known to scientists. Boundary zones as productive and heavily exploited as the continental margins are thus little-studied, in spite of their key role in the global oceanic system and of the growing dangers they face. A publication by American researchers Lisa Levin and Paul Dayton expresses a fear common to the whole of the scientific community concerned with the deep sea: that habitats and biodiversity are disappearing before we have even had the chance to discover them, let alone understand them.

An area of exceptionally rich biodiversity

The low biomass which characteristises the deep sea has long masked the exceptional diversity of its species from observers.

In 1992, Grassle & Maciolek’s famous study, consisting of fine sampling covering 40m2 at a depth of 2 000 meters to the North-East of the United States, allowed for an estimation of deep-sea biodiversity to be made. Out of 558 silt samples, some 58% of the 1 597 species identified were new. Extrapolated to all deep-sea sediments, their calculation gives an estimated biodiversity of 10 million species.

The biology of the ocean environment is at its richest at depths of between 200 and 1 000 meters: the exact same depths at which deep-sea trawlers do most of their fishing.

Sediment: the oceans’ DNA bank

Today, it is estimated there are 1 to 10 million species (mostly small) which remain undiscovered, and 90% of this extraordinary marine biodiversity is found… in sediment! Contrary to popular conceptions about the deep-sea biodiversity reservoir, a rich variety of species is not necessarily found in spectacular structures such as deep-sea coral reefs or hydrothermal oases, but in the sediment, among populations of small species: meiofauna and macrofauna. To really understand the variety of creatures, we need to think smaller.

The abundance and diversity of meiofauna (0.1 to 1mm in size) and macrofauna (1 to 10mm) is phenomenal: on an area equivalent to an A4 page, over 10 000 individuals representing 1 000 different species can be found! Each sampling identifies dozens of new species. Moreover, there is no genetic relationship between deep-sea species in basins that are far enough apart: if two oceanic provinces are compared, 50 to 70% of species will be different. This demonstrates that the deep-sea has a high level of endemism.

From this, it is clear that sediment should be seen as the oceans’ DNA bank, and as a record of the evolutionary history of life on Earth.


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