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1 Living with the oceans. – A report on the state of the world's oceans

Biodiversity

Marine biodiversity – a vital resource

> For a long time the significance of biological diversity in the world’s oceans was unclear. It is now known to play a vital role in maintaining the functionality and productivity of ecosystems. It also makes habitats more resilient to environmental change. But the well-balanced species communities are becoming increasingly unstable.

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The rapid disappearance of species

Biological diversity in the oceans has decreased dramatically since industrialization began in the 19th century. The primary causes for the losses include the destruction of habitats by trawler fishing, pollution and eutrophication of the seas, as well as the steady progress of climate change. Biological diversity is probably declining more rapidly than ever before in the history of the Earth. But at the same time, only a small fraction of the species in the deep sea and polar oceans have so far been identified, making the loss of species in the oceans much more difficult to record and evaluate than on land.

Why is marine biodiversity important?

Every ecosystem performs certain functions that are critically important for organisms. One of the most important functions of marine ecosystems is the production of plant biomass from sunlight and nutrients (primary productivity), which represents the basic food source for all life in the ocean, and ultimately also for humans. Around half of the worldwide primary productivity is achieved by microscopically small plants, the phytoplankton, which grow and divide in the ocean. Another function performed by ecosystems is the creation of habitats, or structures, in coastal ecosystems. For example, macroalgae, seagrass and corals form large undersea forests, meadows or reefs that provide habitats for many other species such as molluscs, crustaceans and fish. Kelp forests and seagrass meadows in the Baltic Sea are vital habitats for the fry and juvenile fish that grow up here before swimming into the open ocean as adults. Gastropods and small crustaceans likewise feed on microalgae growing on the kelp or seagrass. They thereby ensure that the structure-forming plants are not smothered, and are allowed to grow – that is their contribution to the ecosystem. The molluscs and crustaceans that feed on microalgae are the basic food source for larger predatory crustaceans and fish.

Seagrass and kelp itself have relatively long life spans because they are poor food sources for grazing crustaceans and molluscs. They store nutrients in their biomass for a long time, including nitrogen and phosphorous compounds transported by rivers from agricultural areas to the sea. Seagrass and macroalgae thus function as a kind of biological purification system in coastal ecosystems.

Scientists have addressed the question of whether the dramatic decline in biological diversity has consequences for the stable functioning of ecosystems. After 10 years of intensive study, the answer is clear – yes, it does. Experiments in coastal ecosystems, particularly seagrass meadows and kelp forests, have shown that biological diversity in the oceans is essential for maintaining the ecosystem functions described above. Species diversity was decreased in various ways during these experiments in order to compare the ecosystem functions of species-rich with species-poor areas. In one field experiment, for example, the number of seaweed species was artificially reduced by removing some at the beginning of the growth period. The total algal biomass in this species-poor area did, in fact, decrease, thereby resulting in a decline in the food for consumers as well as the number of available habitats. In another experiment, the number of grazing species that feed on the microalgae growing on seagrass was reduced. It was found that the species-poor grazer communities consumed fewer microalgae than species-rich communities. The shortage of grazing species resulted in a slower growth of seagrass because the increased growth of microalgae repressed photosynthesis in the seagrass.
These two experiments indicate that a decrease in biological diversity has a negative impact on the structure of the habitat, regardless of whether the number of species of producers (macroalgae) or consumers (grazers) is reduced. >

5.11 > Hundreds of fish species live in kelp forests like this one off California. These include the yellowtail rockfish or “greenie” Sebastes flavidus.
5.11 > In Kelpwäldern, wie diesem vor Kalifornien, leben Hunderte Fischarten, unter anderem der gelbliche Greenie, Sebastes flavidus. © David Wrobel/SeaPics.com
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