Heavy metals in various Belgian benthic invertebrates
Context of the study
In the Belgian part of the North Sea, most contamination derives from dredging disposals. Many species (flatfish, mussels,...) are studied continuously as part of monitoring programs. In addition to this, benthic communities are monitored as well. 
Content of the study
This study analyzed heavy metal contents in benthic invertebrates between 1981 and 1996. The animals were caught at 10 locations in the Belgian part of the North Sea. The study aimed to detect trends in the concentrations of mercury (Hg), cadmium (Cd), lead (Pb), copper (Cu), zinc (Zn) and chrome (Cr) in the benthic organisms. The following animals were sampled: starfish, brown shrimp, hermit crab, swimming crab and cut trough shell.
Main results of the study
In almost all locations, the concentrations for all heavy metals in the animals showed a decreasing trend over the 15 year study period. At one location (the Bligh Bank) however an increase in copper and zinc concentrations was observed in brown shrimp and hermit crabs. The reason for this increase in unclear, but might be related to the dumping of waste products in the Bligh Bank. The overall decrease in contamination is very likely the result of both the lesser input of metals form the Scheldt river and reduced contamination by dredging activities. Concentrations for most metals were also comparable between the different sampling sites. Chromium however did seam to have lower concentrations in 3 of the 5 species sampled at the Bligh Bank. In contrast, copper concentrations in brown shrimp appeared to be higher at this location. The reason for not finding higher concentrations in animals at dumping sites of dredged soils might be that these soils are dumped over a large area and are dispersed widely by strong ocean currents.
The study concluded that heavy metals, in benthic animals, are declining over the entire Belgian part of the North Sea. It was also concluded, despite the dumping of dredged soils at certain locations, that the Belgian part of the North Sea can be seen as an unity for heavy metal pollution.