Anthropogenic impact in the deep sea: what is at risk?

Litter recovered from the deep seafloor of the west Mediterranean (Eva Ramirez-Llodra, ICM-CSIS, Barcelona)

The Census of Marine life documented many new species and habitats in the deep ocean and we need to minimise the threat to them in the future.

An international study conducted during the Census of Marine Life project SYNDEEP has concluded that human activities are increasingly affecting deep-sea habitats, resulting in the potential for biodiversity loss and, with this, the loss of many goods and services provided by deep-sea ecosystems.

The new findings, published in the scientific journal PLoS One, are the result of work done by over 20 deep-sea experts, including Professor Paul Tyler and Dr Maria Baker of the University of Southampton’s School of Ocean and Earth Science based at the National Oceanography Centre, Southampton. The study was led by Dr Eva Ramirez-Llodra of the Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, Barcelona, Spain.

The oceans cover 71% of our planet, with over half with a depth greater than 3000 metres. Although our knowledge is still very limited, we know that the deep ocean contains a diversity of habitats and ecosystems, supports high biodiversity, and harbours important biological and mineral resources.

The researchers have conducted a semi-quantitative analysis of the most important anthropogenic impacts that affect deep-sea habitats at the global scale in the past, present and future scenarios. They grouped impacts in three major categories: waste and litter dumping, resource exploitation, and climate change. They conclude by identifying which deep-sea habitats are at highest risk in the short- and mid-term, as well as what will be the main anthropogenic impacts affecting these areas.

During the Census of Marine Life programme, which lasted 10 years, researchers from the five deep-sea projects sampled and studied different deep-sea habitats around the globe. The new analysis is based on the results of the Census of Marine Life projects, synthesized during SYNDEEP, and also from data published previously in the scientific literature.

In the past, the main human impact affecting deep-sea ecosystems was the dumping or disposal of litter into the oceans. These activities were banned in 1972, but their consequences are still present today, together with the continuing illegal disposal of litter from ships and the arrival of litter and contaminants from coastal areas and river discharges.

In particular, the accumulation of plastics on the deep seafloor, which degrade into microplastics, called ‘mermaid tears’, that can be ingested by the fauna, has consequences still unknown but predicted to be important. Moreover, there is increasing evidence of the accumulation of chemical pollutants of industrial origin, such as mercury, lead and persistent organic pollutants (e.g., dioxins, PCBs) in the sediment and fauna, including in species of commercial interest.

Currently, and because of the reduction of resources on land and in shallow waters, the largest direct impacts come from the exploitation of deep-sea resources and, in particular, from fisheries. In the future, however, the researchers predict that the most pervasive impacts may come from ocean acidification and climate change, which act at the global scale and can have important effects from surface waters to the abyssal seafloor. Some of these effects include an increase in water temperature that can cause important changes in water column mixing, accumulation of nutrients, and oceanic water circulation with corresponding alteration of oxygen levels and ecological communities.

“The combined effects of climate change and activities such as resource exploitation may be particularly damaging to deep-sea ecosystems,” says Professor Tyler.

According to the experts, seamounts, cold-water corals, upper margin slopes and submarine canyons are the habitats most likely to be affected by anthropogenic impacts in the short and mid time scale. The activities that may be of highest impact are deep-water fishing together with climate change and ocean acidification, as well as the accumulation of marine litter and chemical pollutants.

The researchers also highlight other activities that may put at risk deep-sea communities in the near future: mineral extraction at hydrothermal vents and possibly on abyssal plains. These extractive activities will target important deposits of copper, nickel and cobalt in the manganese nodules of the Pacific abyssal plain, deposits of manganese, iron, cobalt, cupper and platinum in the ferromanganese crust of seamounts, and large amounts of exploitable metals (gold, zinc, copper, lead, cadmium and silver) in the massive sulphide deposits of hydrothermal vents.

It is in these hydrothermal vents where the impact of mineral exploitation is likely to happen first. This exploitation appears to be commercially viable and detailed pilot projects and environmental impact assessments have been conducted in areas of Papua New Guinea, where extraction should start in the near future. Although researchers, industry and policy makers have worked together since the start to evaluate potential impacts and minimise its effects, the real impact of such industrial activity is unknown as our knowledge of the biodiversity and functioning of hydrothermal vents, both active and extinct, is still limited.

Furthermore, a host of new and unpredicted industries are taking aim at the deep sea, such as phophorites off Namibia and rare earth elements in deep-sea muds in the Pacific. Rare earth elements are a crucial resource for novel electronic equipment and green-energy technologies, and their exploitation would have unknown consequences on the diversity and sustainability of the deep sea.

The deep sea is considered to expand from the end of the continental shelf at approximately 200–250 m depth to the great abyssal depths between 3000 and 6000 m, which may reach down to 11 kilometres in areas such as the Mariana Trench. The deep seafloor itself covers 73% of the oceans with an estimated area of 326 million square kilometers.

“Because only a tiny fraction of the deep seafloor has been sampled biologically it is hard to estimate the true impact of climate change and humans activities on deep-sea habitats,” explains Professor Tyler. “The Census of Marine life documented many new species and habitats in the deep ocean and we need to minimise the threat to them in the future,” added Dr Baker.

The researchers are Eva Ramirez-Llodra (Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, Barcelona, Spain), Paul Tyler and Maria Baker (School of Ocean and Earth Science, University of Southampton, National Oceanography Centre Southampton, Southampton, UK), Odd Aksel Bergstad (Institute of Marine Research, Flødevigen, Norway), Malcolm Clark and Ashley Rowden (National Institute of Water and Atmospheric Research, Wellington, New Zealand), Elva Escobar (Universidad Nacional Autónoma de México, Instituto de Ciencias del Mar y Limnología, México), Lisa Levin6 (Scripps Institution of Oceanography, USA), Lenaick Menot(Ifremer, France), Craig Smith(University of Hawaii, USA) and Cindy Van Dover (Duke University, USA).

Ramirez-Llodra, et al. (2011) Man and the Last Great Wilderness: Human Impact on the Deep Sea. PLoS One 6(8): e22588. doi:10.1371/journal.pone.0022588