Why the deep sea matters

The deep sea is the largest biome on Earth. Defined as the ocean and seafloor region that lies 200m below the sea surface, it dwarfs the planet’s continents, covering an immense area of over 360 million km², with a mean depth of 3.7 km. Conditions are almost alien to us land-dwellers – a combination of darkness, low temperatures and high pressure that make the environment challenging to access and explore – and the deep sea is often referred to as ‘interspace’, or Earth’s final frontier.

Yet this vast realm likely contains the greatest number of animal species and the greatest living organism biomass in the world. In addition, the deep sea provides regulating, provisioning and cultural services upon which we, and all life on Earth, depend, including the regulation of the Earth’s climate by acting as a carbon and heat sink. Despite their remote nature, deepsea ecosystems face similar threats to those found in terrestrial and coastal ecosystems, including climate shifts, ocean acidification, overfishing and the extraction of deep mineral deposits. As demand from human industry, technology and population has expanded, the hidden depths have become an increasingly targeted area for fisheries and mineral prospection.

Deep water research is not a new field, but it has been a prohibitively expensive one. Many deep water dredging expeditions from the early 1900s provided the first holotypes of deep sea critters, breaking the paradigms about the deep sea being void of abundant complex lifeforms. However, it is only during the past two decades that the development and broader use of submersibles, remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), together with novel imaging and molecular genetic tools, have sped the collection and analysis of deep sea data, leading to revelations of new species and communities across the remotest habitats. The study of deep sea environments is perhaps entering a ‘golden age’, which has the potential to greatly enhance our appreciation of life on Earth, providing a new impetus to responsibly use and protect the last remaining, truly untouched ocean ecosystems.

Much of the seafloor region consists of flat abyssal plains made up of sediment and ooze (planktonic shell debris) habitats. However, these plains are punctuated by mid-oceanic ridges and seamounts, providing a hard substrate for sessile animals such as corals and sponges to colonise and forming complex deep sea reefs. These, in turn, attract many other smaller and larger species seeking food or shelter. It’s along these steep slopes that the highest concentration of valued minerals are found. The Eastern Tropical Pacific, which lies across various plate boundaries and spreading centres, is furrowed by deep sea ridges and seamounts between shifting continental plates. The Galapagos Archipelago has a particularly high density of seamounts, steep island slopes, which combine with diverse and productive oceanography to make this region a prime candidate for hosting complex and diverse communities. Owing to its remoteness and the establishment of the Galapagos Marine Reserve, banning industrial resource extraction since 1999, the Archipelago’s deep water habitat is a fascinating reference point for a near-pristine state ecosystem.

Thanks to various international research cruises that have taken place in Galapagos since the 2000s, with submersibles and ROVs, scientists have confirmed that the steep slopes and seamounts here do indeed host a variety of complex communities, including coral and sponge ‘forests’ at depths of between 200m and 3000m. Myriad invertebrate and fish communities have been observed, with most of those sampled being new species to the region and often new to science.

As research members of the Galapagos Deep-sea Systems Research Project at the Charles Darwin Research Station, we have had the chance to witness spectacular biodiversity hotspots that bloom into life, illuminated along rocky walls against the pitch darkness of deep water. By bringing new knowledge about the hidden marine biodiversity that exists in the GMR to us surface dwellers, our goal is to strengthen the improved stewardship, management and protection of these poorly understood and under-represented ecosystems.