The indicator is based on concentrations of nine hazardous substances measured in eight species of mussels and oysters. The nine substances are cadmium (Cd), mercury (Hg), lead (Pb), Copper (Cu), hexachlorobenzene (HCB), lindane (gamma-hexachlorocyclohexane, HCHG), DDT (using the breakdown product p,p'-DDE; DDEPP), benzo[a]pyrene (BAP) and 2,3',4,4',5-Pentachlorobiphenyl (CB118). For each station, the 90% percentile over the 10-year period 2010-2019 was used to classify concentrations in ‘low’, ‘moderate’ and ‘high’ classes. These classifications are based on OSPAR's Background Assessment Concentrations (BAC) criteria, the Environmental Quality Standards (EQS) from the EU Water Framework Directive (WFD) and the Maximum Permissible Concentration (MPC) for food.
Concentrations of the nine hazardous substances were collected from two databases: ICES (www.ices.dk) and EMODNET Chemistry (https://www.emodnet-chemistry.eu/). The databases contain measurements for a large number of species, but to achieve a relatively homogenous dataset, only data from bivalves of the following species were used: Cerastoderma edule (common cockle), Mya arenaria (softshell clam), Ruditapes philippinarum (Manila clam), Mytilus edulis (blue mussel), M. galloprovincialis (Mediterranean mussel), Crassostrea gigas (Pacific oyster), Ostrea edulis (native oyster) and Macoma balthica (Baltic clam). This left us with approximately 300,000 measurements. Approximately 10% of the stations had data in both databases and needed database cleaning to avoid using the same measurement from both databases. Furthermore, only data from the late summer and autumn were used, as concentrations in mussels in the spring are affected by spawning. For status, the 90% percentile over the 10-year period 2010-2019 was used to classify concentrations in 'low', 'moderate' and 'high' classes using two threshold values per contaminant (some thresholds also differ between blue mussels and oysters). The lower of the two thresholds was mostly OSPAR's BAC criterion; the EQS for cadmium and mercury was used. The higher threshold value was either the EQS from the WFD or MPC for seafood security. In order to calculate status, at least 1 year (Mediterranean) or 3 years (other areas) of data since 2010 were needed. For trends, dry-weight concentrations for metals and wet-weight concentrations for organic substances were used. Data before 2005 were excluded and trends were calculated for time series of 5-10 years that ended in 2015 or later. If data series had no median concentrations under LOQ, we used linear regression; otherwise, a Bayesian model (with non-informative priors) implemented in JAGS (Plummer 2021) was used.
Database extractions taken in early 2021 were used. However, due to the time lag in reporting, most data series stop in 2019 — so the analysis was restricted to the period 2010-2019. Even so, we do not expect that including later data would have substantially changed the conclusions.
The MSFD websitestates that ‘human activities affect the marine environment through the release of chemical contaminants, which degrade the state of marine waters and can cause serious damage to its functioning’. The indicator is based on bivalves, which live as water filterers and are very effective ‘harvesters’ of any contaminants found in the water. In our process of selecting contaminants, we wanted to have contaminants that have been recorded in as many parts of Europe as possible — as well as contaminants that span a range of sources and behave differently in the environment. For instance, mercury is very volatile, which makes the atmosphere (including rain) an important source. Many of the other contaminants are mostly found in the aquatic environment. Many of the contaminants — such as DDT, lindane — represent ‘legacy’ contaminants. These are now largely banned but still exist in the environment, including in ocean sediments.
There is large variation in coverage between regions and parameters — with especially low coverage in the Black Sea. Also, ‘novel’ contaminants such as per- and polyfluoroalkyl substances (PFAS) were not included in the indicator because there was no coverage in the Mediterranean and Black Sea regions.
