The Tomlinson Pollution Load Index applied to heavy metal, ‘Mussel-Watch’ data: a useful index to assess coastal pollution

The Tomlinson Pollution Load Index (PLI) is calculated as the chemical product concentrations that are measured in marine mussels. It can be used as an index of bioavailability of pollutants for mussels in coastal waters. For the computations, published data from ‘Mussel-Watch’ programs have been used. The PLI standardizes the data by using the quotients obtained by dividing each concentration by a baseline concentration for each chemical, that is, by the lowest concentration of each chemical found in mussels and listed in the literature. The results indicate that the PLI has a normal distribution with a marked positive asymmetry. The exceptional values that surpass a PLI of 100 correspond to very contaminated sites and require immediate intervention; a PLI > 50 requires a detailed survey. The PLI does not show any relationship with either the year when specimens were sampled or with mussel species but it does detect an annual cycle in metal concentrations in mussels. It is also observed that the number of measured metals influences the PLI, because the pollution of a site is usually due to the presence of a few pollutants in large concentrations. The PLI is proposed as a standardized system for detecting pollution which permits a comparison of pollution levels between different sites and at different times.