Chemical compounds help identify discrete ocean regions

Concentrations of chemical compounds in specific ocean layers may contribute to the biodiversity of species found in the darkest depths of the planet. 

Weeks after a checklist of animals that dwell at the bottom of the Clarion-Clipperton Zone (CCZ) in the northwest Pacific Ocean was published, a British National Oceanography Centre-led collaboration with 13 other deep-sea research groups has found evidence for a biogeographic boundary that divides groups of organisms.

The CCZ spans from about 3,500 metres below the water’s surface to the seafloor, around 6,000m deep. Now, this international research collaboration has broken the region into shallow and deep abyssal areas thanks to a transitional boundary at depths of 4,300–4,800m.

Here, the chemical composition of the ocean water column fluctuates, with concentrations of calcium carbonate higher in the shallow abyss. Saturation levels of calcium carbonate in this upper layer support molluscs, cnidarian and echinoderms with harder structures.

For example, while more structured cnidarian species like Alcyonacea corals dominated in shallow depths, they virtually disappeared in the lower CCZ region. In contrast, soft anemones became the dominant cnidarian group below 4,300m. Similarly, brittle stars – a type of echinoderm – were abundant in the shallow abyss, but declined in number below 4,800m, and were overtaken by their sea cucumber cousins. Molluscs like sea snails, mussels and clams appeared above the transition boundary, while below that niche was filled by deep-sea octopuses.

“We were surprised to find a deep province so clearly dominated by soft anemones and sea cucumbers and a shallow abyssal where suddenly soft corals and brittle stars were everywhere,” says Erik Simon-Lledó, a marine biologist at the British National Oceanography Centre, who led the study.

But while the composition of animal life shifted across this marine boundary, the relative diversity of species did not. Previously, it was thought biodiversity at the farthest depths of the ocean would be limited by low nutrient supplies combined with the extreme conditions of low light, cold temperature and high pressure.

Quantifying biodiversity “vital” for seabed mining negotiations

These assumptions have changed in recent years with the exploration of regions like the CCZ.

This assessment found species biodiversity was similar between the shallow and deep abyss, though the abundance that could be supported did reduce at greater depths, consistent with the researchers’ assumptions.

“We have known for some time that the abyssal plains are relatively high in biodiversity,” says study co-author Dr Adrian Glover, principal scientist at the Natural History Museum in London.

“What has been missing is knowledge of how that diversity is distributed and how it changes across broad spatial scales. These new data revolutionise our understanding of abyssal Pacific biogeography.”

Glover says the research will be “vital to inform urgent policy decisions on potential deep-sea mining”.

The CCZ seabed is one region of interest to mining companies and some supportive nations for access to polymetallic nodules – hand-sized aggregations of rare metals that proponents say could be used to build clean energy technology, such as batteries.  Others have called for a moratorium on mining until the environmental impacts of seabed extraction can be properly quantified.

Right now, signatory nations to the UN Convention on the Law of the Sea are discussing such mining proposals at the 28^th session of the International Seabed Authority in Jamaica. The ISA is a United Nations body created to administer the ocean floor.

The final stage of the session – a meeting of all 167 ISA member nations – concludes Friday local time.