The bottom of the ocean has a great diversity of life. Pretty much like trees on land, corals, seagrasses and algae form the basis for a wide variety of marine ecosystems, which we call marine forests. Kelp forests, seagrass meadows or coral gardens are biodiversity-rich ecosystems that are home to specific communities of fish and invertebrates – ultimately a source of food for humans.
Coral reefs harbour an incredible variety of organisms and are considered the most biodiverse of all marine ecosystems. However, not all corals form exuberant and colourful shallow-water reefs in tropical coasts. Deep- or cold-water corals, for example, can extend to the deeper and darker parts of the ocean, down to 3000 m deep and can thrive in water temperatures as cold as 4°C. In these deeper and darker parts of the ocean, some stony corals can also form deep-water coral reefs (e.g. at around 200-400 m off Norway) or mounds (up to 350 m in height off Ireland). Some soft corals, such as gorgonians or black corals, can also form dense assemblages in temperate to tropical waters, so-called gorgonian gardens and black coral forests, and thus are also responsible for engineering their surrounding habitats.
These biodiversity hotspots provide substrate, food and shelter for many organisms and contribute to fish stocks. However, all these coral habitats are severely threatened by destructive fishing methods, such as bottom trawling.
Seagrass meadows are shallow marine habitats dominated by rooted and flowering plants that colonize soft marine sediment in the presence of low or moderate wave energy. They are able to live and complete their life cycle under haline and submerged conditions.
Besides providing a home for many economically important fish and shellfish species, these submerged forests protect the coast from erosion by buffering the impact of waves with their canopies. In past times, they have been used as garden fertilizers, as roof covering or mattress filling, in house insulation and in traditional medicine. Other relevant ecosystem services provided by seagrasses to human populations are the removal of excess nutrients and other pollutants from runoff inputs, increase of water transparency, stabilization of sediments, and carbon sequestration. Among the largest carbon sinks in the ocean, together with mangroves and salt marshes, seagrass meadows store more than half of the carbon sequestered by the ocean, despite covering only 0.5% of the seabed.
The occupation and unsustainable exploitation of coastal areas over the last 100 years have led to the loss and degradation of habitats suitable for seagrass occupation. To this add other factors related to anthropogenic activities, such eutrophication, overfishing, dredging and pollution. Global climate change is severely threatening many seagrass species, mainly due to the effects of sea surface temperature, sea level rise, ocean acidification and increases in the frequency and intensity of extreme natural events such as abnormally high temperatures, storms, precipitation extremes, drought and desiccation. Sea level rise, in particular, causes changes in currents, tides and turbidity, as well as salinity intrusion to low salinity areas. These threats, predicted to increase in frequency and intensity as a result of global climate change, are known to affect the physiology, abundance and distribution of seagrass communities, therefore affecting the ecological services they provide, such as coastal protection and fishing resources, and will have flow-on effects to other marine ecosystems adjacent to seagrass meadows. Worldwide, it is believed that the extent of seagrasses has suffered a reduction of at least 30% over the last 100 years.
Seaweed banks are functionally similar to seagrass meadows. They are often present in rocky substrates. This is the case of Sargassum, for example, but certain seaweed groups also have the capacity to colonize and stabilize sediment areas, like Caulerpa species in sandy areas. This is particular relevant because it allows the colonization of sediment bottoms, once thought to be marine deserts, by a myriad functional groups of organisms that depend on the presence of seaweed. The bottom of the sea functions like a submerged forest, even in sandy areas.
Due to their very complex structure and high primary production, seaweeds, just like plants on land, offer food and nursery habitats for many invertebrate and fish species. Being at the bottom of the marine food web, seaweeds establish important trophic relations with several species of herbivores. They also provide forage for many endangered species, such as seahorses, sharks, sea turtles, dugongs and manatees. Kelp forests, for example, occur worldwide throughout temperate coasts and are recognized as some of the most productive marine ecosystems.