Marine pollution is, by definition, damaging to marine organisms and ecosystems and may interfere with legitimate uses of the sea. In accordance with Part XII of the Law of the Sea Convention and various other international agreements, contracting parties are obliged to prevent, reduce and control pollution of the marine environment. An update of the 2009 report on Pollution In The Open Oceans by the Group of Experts on Scientific Aspects of Marine Protection (GESAMP) has been carried out by the Transboundary Water Assessement programme – see here the full report (2015) and the chapter.

Monitoring the deep ocean (deeper than 200m) is difficult and costly. Pollution monitoring by coastal states tends to be focused on the shallower shelf sea areas which are often most affected by contamination from land-based sources. The amount of scientific information relating to conditions in the open ocean is small in comparison to near-shore areas. However, scientific knowledge of pollution in the open ocean is steadily improving and some important advances have been made in the past five years. There are no fixed criteria for classifying marine pollution. For the most part, pollution monitoring involves measurements of particular substances in samples of water, sediment, biological issues and atmospheric deposition. Substances routinely monitored tend to be those with hazardous properties, known to arise from human activities, and for which suitable analytical methods exist.

The mere presence of a substance introduced by human activity is not always harmful and does not necessarily constitute pollution. Environmental concentrations that approach or exceed those known to be harmful (effect levels) are important indicators, but such levels are seldom found in the open ocean. Evidence of biological effects is an obvious indicator of pollution but to date the techniques and opportunities available for recording biological impacts in the open ocean are limited.

A useful indicator is a trend in either inputs of contaminants or their environmental concentrations. Trend monitoring requires repetitive measurements over long periods of time. For the open ocean, trend monitoring of inputs is restricted to atmospheric deposition (i.e., measurements at island stations and on ships). Recent scientific literature was reviewed on some of the principal ocean contaminants resulting from human activities. These are specifically: nutrients, carbon dioxide, mercury, marine debris, Persistent Organic Pollutants (POPs), noise and radioactivity. It was found that the deep ocean, occupying about 65% of the Earth’s surface, is significantly contaminated with the by-products of human activities; all major ocean basins are affected. Substantial quantities of contaminants are introduced from land, through shipping, and via the atmosphere.

No early decline in the bio-availability of mercury is predicted and without mitigation atmospheric inputs of CO2 will increase acidification of surface waters. Inputs of the nutrient nitrogen, which are already significantly elevated downwind of industrialized regions, are predicted to increase by the end of the century. In the Arctic, environmental levels of some recently-manufactured Persistent Organic Pollutants (POPs) are on the increase. Various taxonomic groups are adversely affected by noise generated from shipping, sonar devices and seismic surveys. There is a high incidence of marine debris, such as nets on the seabed that cause entanglement, and there is increasing evidence of ubiquitous contamination by plastic fragments that are ingested by many different species.

Depending on the substance or disturbance, particular sectors such as water, sediments and organisms are far more exposed than others. The effects of the contaminants are not always visible or easily detectable. Cause and effect can only be readily demonstrated in the cases of marine debris, human-made noise and acidification by atmospheric CO₂. Nevertheless, it is very likely that substantial and progressive changes in the physical and chemical properties of ocean ecosystems will, in time, produce a biological response.

Some changes, such as increasing inputs of nitrogen to low-nutrient waters, or increasing acidity, could trigger changes in primary production that influence entire food chains including the production of fish, birds and mammals. Other changes such as high levels of mercury in fish may impact on food safety. Issues such as POPs in the tissues of whales and dolphins could cause changes in the behaviour and survival of sensitive species.

The 2015 update shows that atmospheric inputs of CO₂ and nitrogen, as well as the extent of solid debris such as plastics and netting in the water column and on the seabed, are matters of special concern. Attention is also drawn to another, rapidly emerging threat which is the exploration and extraction of minerals and hydrocarbons on or within the deep ocean seabed. The potential of such activities for large-scale uncontrollable impacts, as shown by the recent oil leakage in the Gulf of Mexico, is substantial and not sufficiently recognized.

Information on the extent and trends of contaminants in the deep ocean is sparse but in most cases, through deduction and modelling, is enough to determine general patterns. There is a pressing need for time-series datasets from strategically selected sites to more accurately discern trends; this requires greater commitment to long-term funding for such measurements.

Although the effects of certain contaminants on species and communities can be seen locally, or shown experimentally, the real impact at ecosystem level is largely unknown. Indeed, taking into account the complex relationships within ocean ecosystems, it is likely that such understanding will remain beyond the capabilities of science for the foreseeable future. Nevertheless, it is reasonable to assert that the cumulative effects of multiple stressors on some ocean communities will eventually force changes in the structure and function of those communities. This will be damaging and possibly irreversible and in some instances the stressors may already be forcing these changes. This scenario is even more likely when taking into account other major changes such as those that result from fishing pressure and the upward trend in water temperatures. There is a strong case for more effective measures to reduce inputs of contaminants to the ocean.