Plants and animals produce countless chemical substances as part of their life processes. For the purposes of the Ocean Health Index, ‘chemical’ refers to a compound or substance that has been purified or manufactured by human sources.
More than 100,000 chemicals are used commercially (Daly 2006), and many enter the marine environment via atmospheric transport, runoff into waterways, or direct disposal into the ocean.
Three general categories of chemicals are of particular concern in the marine environment: oil, toxic metals, and persistent organic pollutants.
How Was It Measured?
It is not yet, and may never be, possible to measure actual concentrations of the numerous substances found throughout the ocean, so the following proxy measures were used. Heavy metal pollution in coastal fish was selected as proxy for chemical pollution from land-based chemicals. Ocean-based pollution was modeled from data on commercial shipping tracks and ports.
These models only provide rough estimates of pollution intensity. They do not represent all chemicals and they do not distinguish between chemicals that are more or less toxic.
The total amount of oil entering the ocean has been estimated, but global data on the size and geographic distribution of oil spills are not available, so oil pollution could not be included as a separate category within the Ocean Health Index. However, oil would be among the substances contained in runoff from impervious surfaces and released by shipping and ports.
‘Oil’ is the general term for any thick, viscous, typically flammable liquid that is insoluble in water but soluble in organic solvents. Plants and animals produce a variety of natural oils, but the Clean Waters goal is primarily concerned with oil derived from geological deposits of petroleum (crude oil) for use as a fuel or lubricant.
Natural oil makes up 47% of the oil in the ocean. About 600,000 metric tonnes of oil enters the ocean naturally each year by seepage through many cracks in the seafloor (NRC 2003), but input from each is typically slow (Wells 1995) and natural seepage is not considered to be pollution.
The other half of the oil comes from anthropogenic sources, including boats, land-based runoff and, to a lesser degree, oil spills. These sources pose a greater threat to marine environments as the oil enters the ocean in concentrated areas at a high rate of flow.
The largest sources of human oil pollution are urban-based runoff and operational discharge of fuel from boating traffic and port operations. Discharge associated with boats constitutes 24% of the total amount of oil in the ocean (UNEP/GPA 2006).
Only 8% of overall oil ocean pollution is a result of spills during transportation or production. However, the toxicity levels of these spills tend to persist over time and have been linked to highly visible local and regional disasters.
After 20 years, oil pollution from the 1989 Exxon Valdez spill persists and, in some areas, is nearly as toxic as initial levels (Exxon Valdez Trustee Council 2009; Raloff 2009).
Metals are chemical elements that are typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity. Metals are toxic if they change the structure and function of proteins and enzymes (GESAMP 2001).
Metals found in the ocean that are highly toxic on their own include mercury, cadmium, lead, arsenic, tin, copper, nickel, selenium, and zinc. Mercury, cadmium, and lead can become even more highly toxic in combination with organic compounds. For example, mercury can form neurotoxic compounds such as methylmercury (CH3Hg), when combined with carbon.
Arsenic, copper, nickel, selenium, tin, and zinc are not highly toxic by themselves but are able to react with organic materials, creating very toxic compounds (UNEP 2006).
Many metals occur naturally in the environment, but anthropogenic emissions from industrial and mining activities can increase concentrations of many to toxic levels.
96% of mercury enters the ocean via atmospheric input (GESAMP 2001).
While some metals are deliberately dumped in the ocean, most are found downstream from their sources, including waste dumps, industrial areas, mining operations and metal processing areas.
Persistent Organic Pollutants [POPs]
Persistent Organic Pollutants (POPs) are chemical compounds that are toxic to humans and wildlife.
POPs include pesticides such as DDT, herbicides, PCBs (a component found in many coolants, flame-retardants, adhesives), and BPA (a compound found in plastics – primarily in plastic bottles).