More than 70% of the Earth is covered by water, and most of the sea floor consists of soft sediments, such as mud, clay and sand. Thus, soft sediments form one of the most important habitats on Earth, with a high diversity of organisms. However, the sediments in coastal areas are often subject to anthropogenic pollutants, and current remediation methods are often both costly and destructive on the ecosystem. Benthic macrofauna, i.e., sediment-living invertebrates larger than 1 mm, sustain a variety of ecosystem functions in the sediment by their activities; they oxygenate the sediment, redistribute particles and nutrients, and are also important secondary producers. Therefore, it is essential to maintain healthy benthic ecosystems and to study effects of various disturbances on benthic species composition and their recovery trajectory after disturbance, a process called ecological succession.

The aims of this PhD thesis were to investigate: 1) how benthic marine invertebrate communities in a Norwegian fjord respond to thin-layer capping with activated carbon, a recently proposed technique to remediate contaminated sediments in situ in a more cost-efficient and less ecosystem destructive manner, 2) how benthic community composition in sediments from the Baltic Sea is explained by environmental variables and concentrations of polycyclic aromatic hydrocarbons (PAHs) and metals, and 3) how well current benthic quality assessment indices as well as functional indices represent these disturbances.

The remediation method with activated carbon had long-term negative effects on the benthic macrofauna (paper I-III). Up to nine years after the capping, the communities were still severely affected, with lower species diversity, abundance and biomass. Recolonization of key species, such as the brittle star Amphiura filiformis, did not occur in the capped fields. Further, functional indices calculating bioturbation and bioirrigation estimated significantly lower activities in the communities exposed to activated carbon. Other studies have shown promising results in reducing contaminant fluxes and bioaccumulation of dioxins, but the long-term side-effects observed here on the benthic communities after capping show that the remediation method needs to be improved before application on a large scale. 

The benthic community composition was also studied in the Baltic Sea (paper IV). The significant factors to explain the benthic community structure were PAHs, even at medium to low concentrations in the sediment, together with the environmental variables salinity, temperature and depth. These variables also explained the Benthic Quality Index (BQI), used for ecological status assessments in coastal areas of Sweden and in the open Baltic Sea.

 However, several ecological indices commonly used to assess the benthic environmental status were not able to detect the disturbance caused by activated carbon in the Norwegian fjord (paper III), although clear effects on the benthic community in terms of lower species diversity and abundance were evident. Thus, current status assessment indices may need to be revised, or at least used with caution, in management evaluation of the sediments after disturbances other than eutrophication.