The deep sea: a key player to be protected for climate and ecosystems

The depth of the ocean is on average 4000 metres. In this area also known as “deep sea” there is no light, extremely high pressure, and temperatures that are much more stable than at the surface. Life exists under these conditions and every day we discover a little more about its diversity, weaknesses and the crucial services it provides to the entire planet.

The Deep Sea plays a major role in climate change mitigation. By storing a large part of the CO2 produced by human activities and by absorbing the heat accumulated by greenhouse effect, the Deep Sea slows down the warming of surface waters and land. Thanks to this immense mass of water, climate change is still “bearable” for most species on Earth.

In addition, Deep Sea ecosystems capture huge quantities of carbon. For instance, on the continental shelf, microorganisms play a major role in sustainable storage of carbon produced by phytoplankton, but are also filters for methane formed by this fossilized matter. By using methane as energy, these microorganisms transform this greenhouse gas, which is much more powerful than CO2, into minerals. This process prevents greenhouse gases from resurfacing and accelerating climate change.

Life and services provided by Deep Sea organisms depend very much on atmosphere and surface activity. As they are deprived of light, Deep Sea ecosystems are highly connected to food produced at the surface of the ocean, on which they depend greatly. Marine snow, a rain of organic matters, which drops from the surface, is often the base of the food web. Surface waters also provide oxygen for the deep abysses when they move down to the Deep Sea in the Pole Regions.

Consequently, modifications taking place at the surface, such as oxygen exhaustion or a decrease in phytoplankton, have impacts on life in the Deep Sea and can affect ecosystem functioning. According to observations, there are significant changes of food type and quantity originating from the surface and shifting to the abysses, several thousand metres deep. Are those “natural” events or the first signs of disturbances of the global water column from surface waters to the abyssal plains? 15 to 25 years observation series are still too short to come to a conclusion, but they confirm that Deep Sea biodiversity changes very quickly as soon as available resources are modified.

These ecosystems are dependent on many on-going changes. Concurrently with climate change, resource exploitation (minerals, hydrocarbons, fisheries) is spreading to the Deep Sea and brings its share of disruptions in fragile environments.

One point needs particular attention. When surface waters are warmer, they don’t mix as well with deep waters. By reducing Deep Sea “ventilation”, warming reduces the already low oxygen content, which naturally affects “intermediate” waters (several hundred meters deep) over wide regions of the tropical ocean. In some very productive regions, north of the Indian Ocean, the West Coast of the United States or even Peru, Chile or Namibia, observations show that less oxygenated waters or waters deprived of oxygen spread and greatly reduce habitat spaces for certain species, like Tuna or Marlin, in favour of other species capable of tolerating these conditions, such as calamari which can affect the entire ecosystem by proliferating.

Other more subtle modifications can have drastic impacts on ecosystems. The increase in water temperature, even by a 10th of a degree every 10 years in certain Polar Regions, enables some predator crabs to expand their territory and decimate species until then protected by extremely cold waters (-1,5°C).

In other regions, there are concerns over the acidification effect of waters, which have absorbed great quantities of CO2, and can cause the deterioration of deep coral reefs, while many fish and shellfish species depend on them. Lab studies show that a combination of this phenomenon with deoxygenation of waters, like in the Gulf of Mexico, where the warming of deep waters already unusually as warm, like in the Mediterranean, is particularly critical.

As deep biodiversity reacts rather rapidly to change, it is crucial to consider these risks in order to avoid jeopardizing the mitigation capacity of oceans toward climate disruption and other services provided by the Deep Sea’s biodiversity.

The first step is to explicitly acknowledge the role of the Deep Sea in climate change mitigation, the sensitivity specific to Deep Sea species, and their potential effects on the surface of the ocean or the atmosphere. The regulation of excess heat and CO2 can have irreversible short term consequences on Deep Sea organisms.

Shared effort is crucial to develop the knowledge and necessary tools for an efficient protection of most vulnerable ecosystems. Deep Sea ecology remains a young science. Less than 150 years ago, scientists factored out the possibility of marine life below 500 meters under the sea level. Since then, each exploration brings new proof of extraordinary diversity of life in the ocean and the capacity of species to live in exceptional conditions (temperature, oxygenation, acidity, pressure, etc.). The question is whether or not this adaptation capacity will resist to climate change-induced disruptions. However, it is important to acknowledge that for now there is very limited visibility of the destabilization these ecosystems suffer due to disturbances predicted by the greenhouse gas emission scenarios.

In this context, Marine Protected Areas are valuable, especially when they spread to the open sea and include remarkable Deep Sea ecosystems, such as those of canyons or underwater mountains which play an important role in the interaction with surface ecosystems. By protecting and enabling a long term follow up of these biodiversity “hotspots”, these Marine Protected Areas are also natural laboratories for a better understanding of climate change effects and impacts on these ecosystems.

Anticipating vulnerabilities and defining efficient protection measures is a crucial issue, while many climate emergencies tend to leave behind this remarkable heritage, invisible to most of us.