The deep sea floor is one of the largest ecosystems, occupying 60% of the planet’s surface. Yet we know very little about it compared with other ecosystems. As easily accessible resources on land and the shelf seas are depleted and exhausted interest turns to the deep sea. Which contains an untapped wealth of resources because until now it has been either inaccessible due to the physical challenges poised or just not cost effective. Three direct future anthropogenic (caused or produced by humans) effects to the deep sea are fishing, mining and waste disposal.

Fishing Effects on Deep Sea Communities
Since the 1970’s marine fisheries have operated at increasing depths as the easily accessible shelf sea stocks have become overfished. Advances in fishing gear, winches, engines and boat design have facilitated ever deep trawls. As of 2002 40% of the world’s trawling grounds were in waters deeper than the continental shelf and while very few vessels routinely trawl at depths greater than 1000m, in the future this will become the norm.

Areas of high productivity and biological diversity, such as seamounts, canyon walls and cold water coral reefs, are intensively targeted by fisheries. The initial yield for such fisheries is typically greater than the same sized area found in shelf seas; the Hoplostethus atlanticus fishery southwest of New Zealand was reported yield a 60 tonne catch in only 20 minutes of trawling. However, the life histories of deep sea fish: long lifespan, low fecundity, slow growth and late maturation, make the industrial harvesting of such fish unsustainable. To the degree that such fisheries are typical exhausted at a far greater rate than shelf sea stocks. The H. atlanticus stock fell to below 20% of its pre-exploitation abundance within only a decade. Deep sea fish also physiologically differ from their shallow water counterparts and tend to be more fragile, so even if smaller individuals pass through the trawl net their injuries are usually fatal. Bycatch presents a major issues with deep sea fisheries, few species are valued with the majority of the catch being returned to the sea with a near 100% mortality rate. Trawling also has a physical effect on the environment, clearing large areas of once complex habitat. Fished seamounts have been found to have 95% bare rock coverage, whilst unfished seamounts had just 10%. Unfished seamounts were also found to be far more productive, with double the biomass and 46% more species. Deep sea fishing can be so destructive and unsustainable that it has been described as closer to mining than fishing; depletion is rapid and recovery unlikely.

The Orange Roughy (Hoplostethus atlanticus), one of the first deep sea fish to be commercially targeted. Source: http://www.scienceimage.csiro.au/image/2948

Alternative fishing methods such as: long lines, traps and gill nets have been proposed. Which would limit habitat damage and lower bycatch but ultimately fail to compensate for the life history strategy of deep sea fish. The high cost of fishing on the high seas also makes sustainable levels of fishing improbable as it would only be profitable if a strategy of serial depletion were pursued. As such deep sea fisheries have been labelled as non-renewable resources. As such contemporary methods of fisheries management will not work and the only option would be to create marine reserves around these productive deep sea habitats that prohibited fishing. The major issue with this is 50% of the planet is high sea, outside of national jurisdiction, making it difficult implement laws and police them.

Deep Sea Mining
As the easily accessible terrestrial sources of minerals are depleted, interest turns to the deep sea; which offers an untapped wealth of resources as the physical challenges of extracting them have prevented their exploitation up until now. Previous interest in deep sea-mining revolved around the concept of harvesting manganese nodules from the abyssal plain. This would have proved very damaging to benthic communities as the nodules are spread out over the seabed; mining would
have involved removing the top centimetres of sediment from the seabed over a large area. The environmental impacts would have been further compounded by the resuspension of sediment after filtration on the surface. This would smother any organism that had not been destroyed by the collection phase but also alter the microbial and chemical activity in the lower water column. The United Nations Convention on the Law of the Sea, ratified in 1994 has diminished industry interest in manganese nodule extraction by introducing environmental safeguards and financial burdens. Future deep-sea mining endeavours have shifted to the exploitation of Seafloor massive sulphide deposits.
Seafloor massive sulphide deposits surround hydrothermal vent sites and contain gold, silver, zinc and copper in densities far greater than terrestrial deposits. Methods of extraction include dredging and trench cutting. The risk to vent communities can be split into physical changes to the environment and biological responses. The physical risks include: loss of habitat, as it’s dug up to extract the ore; reducing the 3D complexity of the environment (flattening the environment). While the biological risks are: loss of local populations and smothering of organisms by sediment plumes. The light and noise of the machinery may also cause additional disturbances to mobile species, which may exit the area, reducing local biodiversity. However, hydrothermal vents are ephemeral and so environmental effects may be minimised restricting mining operations to extinct vent sites.

Plans for a potential mining operation at a Seafloor Massive Sulphide Deposit (Collins et al. 2013)

Waste Disposal
The coastal seas have been used as a dumping ground by man for thousands of years; but the physical challenges of deep sea has restricted its exploitation as a waste repository up until now. On first appearances the deep sea presents itself as an ideal repository of waste. Many miles away from land and population centres, the deep waters of this planet offer huge assimilative capacity for waste too hazardous or vast to bury on land. However, disposing of this waste may have catastrophic effects for deep sea communities that are still largely unknown to science. The life history strategy of deep sea species – low fecundity and slow growth, would make them especially susceptible to the side effects of dumping hazardous waste. Therefore such methods of waste disposal must be thoroughly investigated before any widespread adoption.

The ever expanding human population has led many governments to consider expanding their sewage waste disposal to the deep sea, as land based solutions become overburdened. Raw sewage is a potent pollutant, which can be contaminated with heavy metals, hydrocarbons and causes anoxic conditions. It can be severely disruptive to deep sea marine fauna by: burying organisms in sludge; clogging feeding apparatus; increasing the turbidity; contaminating the food chain and changing the community structure by nutrient enrichment. However, it has been argued that sewage sludge has a similar nutritional content to marine snow and could be in part beneficial to deep sea communities whom are entirely dependent on falling organic detritus as a source of energy.

A proposed method of disposing sewage in the deep ocean. A tanker ship sprays it onto the seabed through a long hose, where it forms a localised plume. (Gage & Tyler 1992)

In order to curb carbon emissions, governments and the energy industry have looked at capturing carbon dioxide from power plants and storing it in sinks, such as the deep sea, to avoid it being released into the atmosphere. Early theoretical methods of carbon sequestration simply involved pumping C02 into ocean depths of greater than 3700 m and letting it form dense lakes that wouldn’t be affected by the currents and take hundreds of years to disperse. This would work, as under these conditions the C02 could be piped down in liquid from and at this depth would be denser than the surrounding sea water and so would not periodically rise to the surface and re-enter the atmosphere. The problem with this method is that these “lakes” would suffocate any marine life in direct contact. Furthermore, the odour of dead marine life would attract scavengers, which would also perish. A contemporary solution would be to drill hundreds of meters into the seabed and deposit the carbon there. Where it would form hydrate ice crystals blocking the rock pores and take millions of years to leak out into the ocean.

Final Thoughts
The deep sea is so vast that human impacts will take a long time to manifest themselves on a wide scale. However, small localised habitats of high productivity and diversity (vent sites & cold water coral reefs) will be the most affected and change at a greater rate than the abyssal plain. Due to our limited knowledge of the deep, the ecosystem may be significantly altered before we’ve fully understood its natural state. Protecting the deep sea and its habitats will prove challenging. As it mostly lies outside countries territorial waters, making cooperation between nations essential.

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