As the global emissions of carbon dioxide (CO2) continues to increase, and the resulting effects of climate change become increasingly evident, international organisations across the globe continue to search for ways to encourage countries and industrial corporations to limit and reduce their carbon output. One such strategy is carbon offsetting and the utilisation of carbon credits. This involves setting up financial incentives to those who reduce CO2 emissions, or remove CO2 from the atmosphere. Forests have become a major source of carbon credit in many less economically developed countries, using the amazing ability of trees to sequester CO2. There have also been increasing interests in developing the usage of ‘blue carbon’.
Blue carbon includes carbon captured within the oceans and coastal ecosystems. Initiatives such as the Blue Carbon Initiative encourage individuals, events and organisations to recognise marine and coastal ecosystems as important carbon sinks, since studies have shown that the ocean alone has limited anthropogenic CO2 emissions by 20-35 percent. However, carbon stored within organisms in the deep oceans has largely been ignored, and it seems we may be withdrawing from this abyssal bank without realising the costs.
What’s in the Deep?
People don’t usually associate the deep oceans as much more than a dark and mysterious place with few inhabitants apart from the odd angler fish (figure 1). However, a study in 2014 has suggested that the bentho-pelagic ecosystem may play an important role in the ocean carbon cycle, with carbon being transferred here and the Deep acting as an important carbon sink.
How did it get there?
To understand the existence of an abyssal bank, we must first understand how the carbon reached here. A small proportion of the carbon supplied to demersal fish is via the passive settling of particles sinking into the deep. Large food-falls, such as whale carcasses, are also important sources of food for those living in the deep. However, it seems that a biological process known as diel vertical migration (DVM) actually provides the majority of carbon demersal fish need.
The Ocean’s Great Migration
DVM is a daily migration of many species of zooplankton, larger marine vertebrates and fish, and has been said to be the largest migration on earth. Every day, as the sun sets, the participants of this great migration move towards surface waters to feed on phytoplankton, avoiding potential predators. These animals stay near the surface all night, and only returns to the deep in the morning (figure 2). By transporting carbon into the depths daily, DVM acts as an essential process which drives the deep ocean’s ecosystem, enabling demersal fish to have a constant supply of food.
As demersal fish feed on the migrants of the DVM, the carbon which they incorporate into their bodies become sequestered, and stored in the depths. There, the carbon is most likely to remain, as there are very few natural processes bar deep ocean currents, which have a transit time of around 1000 years, transporting nutrients back towards the surface. As a result, the depths of the oceans become an abyssal bank where carbon is sequestered for long periods, and effectively removed from the atmosphere. However…
Withdrawing from the Abyssal Bank
Bottom trawling is a method of fishing which targets organisms living at the bottom of the ocean. It involves using fishing net to reach the seafloor and dragging the net, catching anything and everything along the way (figure 3). There have been many environmental implications when bottom is trawling is concerned, as it can also be destructive to the benthic community, physically destroying the ecosystem. However, when we consider the deep water community as an important means of carbon storage, the problems associated with the deep-water trawl fishery become deeper than physical destruction. When fish and large invertebrates are extracted from the deep ocean, we will also be effectively withdrawing from the abyssal bank, removing stored carbon and releasing them again at the surface.
The amount of carbon captured by demersal fishes on the UK continental slope has been found to capture around 1million tonnes of CO2 annually, which is valued at about £10million. This is actually far higher than the net economic gain from the UK deep-water trawl fishery. However, since the carbon sequestration role of these fish is not considered, we do not realise that we are exploiting an ecosystem which is providing us with an important service.
Our continued harvesting of deep sea fishes taking stored carbon and releasing it may have deeper implications than we previously realised. In addition to possible damage to biodiversity caused by over-exploitation, we may be disrupting a major ecosystem service provided by these species. A possible solution to this would be to include deep-sea fishing into carbon crediting schemes, thus making deep-sea fisheries much less profitable than they currently are.
Exploiting the unknown
There are still many things we don’t know about the deepest parts of our oceans, leading to the popular saying that we know less about our oceans than we do of space. This brings up an important ethical argument of whether we should really be exploiting an ecosystem we know so little about. The deep oceans have typically been protected from exploitation due to its dangerous environment to humans, but as technology advances, the depths are no longer safe from us, as we not only have deep-sea fisheries which target the organisms living there, but there are also increasing amounts of deep-sea mining taking place, targeting the minerals which have previously been hidden away from humankind. By continuing our exploitations without thought, we might well cause the destruction of a unique environment before really understanding it, as have happened many times before in our history.
- Khatiwala S, Primeau F, Hall T. Reconstruction of the history of anthropogenic CO(2) concentrations in the ocean. Nature. 2009;462: 346–349.
- Trueman CN, Johnston G, O’Hea B, MacKenzie KM. Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes. Proc Biol Sci. 2014;281.
- Hays GC. A review of the adaptive significance and ecosystem consequences of zooplankton diel vertical migrations. Migrations and Dispersal of Marine Organisms. Springer Netherlands; 2003. pp. 163–170.
- England MH. The Age of Water and Ventilation Timescales in a Global Ocean Model. J Phys Oceanogr. 1995;25: 2756–2777.
- Trueman C. Carbon capture and storage roles of fish ecosystems on the continental slope. The conservation value of European deep-sea habitats; 2015 Oct 13; Zoological Society of London.