For the purposes of the Ocean Health Index, ‘chemical’ refers to a compound or substance that has been purified or manufactured by humans. It does not include the many chemicals that marine plants, animals and microbes produce as part of their normal life processes.
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. Although some countries possess good data on the occurrence and concentrations of such chemicals in their waters, such data are not available globally. Therefore, proxy methods were used to estimate the intensity of chemical pollution based on the amount of pesticides used by each country.
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. Because of limited data availability, we measured the chemicals component as the average of land-based organic pollution, land-based inorganic pollution and ocean-based pollution from commercial shipping and port, using the following proxy measures:.
Land-based inorganic chemical pollution used raw data were drawn from modeled plumes of land-based inorganic pollution, as developed in Halpern et al. (2008) that produced intensity of pollution at 1km2 resolution. The model used impervious surface area within watersheds as a proxy measure for the likely amount of this pollution runoff that reached river mouths. Plumes from the rivers were distributed using a diffusive model. Each ocean pixel was then rescaled 0-1 based on the global maximum pixel value.
Land-based organic chemical pollution was modeled from data on agricultural pesticide use. Data on the amounts (tons) of pesticides used in each country were obtained from FAO. For each country, the amount of pesticide used was distributed onto land use classes and then summed by watershed as a proxy measure for the likely amount of pollution that reached river mouths. Plumes from those rivers were then distributed using a diffusive model that produced intensity of pollution at 1 km2 resolution. Each ocean pixel was then rescaled 0-1 based on the global maximum pixel value.
Ocean-based chemical pollution is assumed to come from commercial and recreational ship activities, but since no global data for the latter are available a combination of commercial shipping and ports and harbors data were used. Data on the density of commercial shipping for 1 km2 cells were gathered from World Meteorological Organization Voluntary Observing Ships Scheme. The amount of ship traffic is used as an estimator of the amount of ship-based pollution. Port-derived pollution is modeled as a diffusive plume from each port out to a maximum distance of 100 km. Data for 618 global ports comes from World Port Ranking, U.S Port Ranking, American Association of Port Authorities, Australia ports database and Lloyds List database.
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. Focal studies at the country level may be able to use more detailed data, but they do not yet exist at the global level. Details about the models used are provided in online Supplementary Information for Halpern et al. (2012).
The Clean Water goal is unusual because its four components--Chemical Pollution, Nutrient Pollution, Pathogen Pollution and Trash Pollution--indicate both Status and Pressure. Low levels of those factors produce a high goal score, but high levels produce a low score. For example, perfectly clean water has no chemical pollution, so Status for this component is expressed as 1 - Chemical Pollution. Status for the other components is similarly expressed. Input data for calculating Status and Pressure for each component is listed in Table S23 of Halpern et al. 2015. The overall goal score is the geometric mean of the scores for the four components, which are weighted equally.
Use of the geometric mean magnifies the importance of a very bad score for any one of the components, matching public perception that very high levels of a single pollutant would make waters seem ‘too dirty’ to enjoy for recreational or aesthetic purposes.
Chemical pollution is a pressure for nearly all of the Ocean Health Index goals. All pressures, including chemical pollution, have different effects on different goals. For each goal, the effect of each pressure is weighted 'low' (1), 'medium' (2) or 'high' (3). The actual data-derived value of the pressure is then multiplied by the weight assigned to it for that goal. That process is repeated for each pressure-goal combination. The sum of those values divided by 3 (the (the maximum pressure-goal value) expresses the total affect of that pressure on the goal.
Chemical pollution has high effect (weight = 3) for Tourism & Recreation, Sense of Place (Iconic Species) and Clean Waters (where it is also a Status component). It has medium effect (weight = 2) on Natural Products (Aquarium Trade, Fish Oil and Seaweed), Carbon Storage (Seagrass), Coastal Protection (Seagrass), Livelihoods & Economies (fishing, Mariculture and Aquarium Trade), and Sense of Place (Lasting Special Places). Its effects on other goals are low (weight = 1).