## 1 Introduction
These include legacy contaminants such as polychlorinated biphenyls (PCBs), dioxins, dichlorodiphenyltrichloroethane (DDT), and metals, but also emerging contaminants such as pharmaceuticals, perfluoroalkyl substances, and flame retardants. Increasing our currently poor understanding of the potential effects of these emerging contaminants is essential for making robust assessments of environmental quality and regulatory decisions. In the Baltic Sea, HELCOM's list of eleven substances of \"specific concern\" includes HBCDD (hexabromocyclododecane) and it is one of the substances in the set of HELCOM core indicators for hazardous substances [9], which are used to assess the state of the Baltic marine environment and the EU Marine Strategy Framework Directive (MSFD) qualitative descriptors (\"contaminants in environment\"). HBCDD is a brominated flame retardant (BFR) used widely in building materials, polystyrene, textiles, and electronics. It is used additively, i.e., it is not chemically bound in the matrix, and is thus gradually released from the products to which it has been added or from waste. It may therefore enter the marine environment directly, via atmospheric fallout or runoff from land. It is classified as a Persistent, Bioaccumulative, and Toxic (PBT) substance and a \"Substance of very high concern\" [15]. In 2013, HBCDD was added to Annex A of the Stockholm Convention on Persistent Organic Pollutants (POPs), a global treaty to protect human health and the environment from environmentally persistent chemicals by for example reducing production, import and export, use, and release to the environment. HBCDD is also listed in Annex XIV list of the European Union Regulation REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals). The main source of HBCDD to the Baltic is \"leakage from products (e.g., insulation, electrical products, and textiles) via rivers, atmosphere, or wastewater treatment plants\" [9]. HBCDD concentrationsare measured in a range of species as part of the Swedish national monitoring program and temporal trends vary spatially and temporally and with the organism measured [16]. For example: concentrations in herring muscle and cod liver are generally higher in the Baltic Proper than on the Swedish West coast; concentrations in guillemot eggs have been decreasing over the past decade from their peak in the mid-2000s, but steadily increasing since 1980 in cod liver from SE Gotland. Concentrations may also vary widely between samples from the same species, site, and time point [16]. The effects of HBCDD used in its classification as a PBT were determined using standard ecotoxicological tests (for a summary see [15]). For aquatic species, reduced size and reproductive output was seen in _Daphnia magna_ (21-day exposure, Lowest Observed Effect Concentration (LOEC) 5.6 \(\mu\)g L\({}^{-1}\)), and reduced growth rates were seen in three microalgae (72-h 50% Effect Concentration (EC\({}_{50}\)) c. 40-50 \(\mu\)g L\({}^{-1}\)). No effects were seen in growth of the amphibod _Hyalella azteca_ at concentrations up to 1000 mg-kgdw\({}^{-1}\) but numbers of the worm _Lumbriculus variegatus_ were reduced at 50 mg-kgdw\({}^{-1}\). No measureable effect was seen on eggs, larvae, or adult rainbow trout at concentrations up to 6.8 \(\mu\)g L\({}^{-1}\). Assessments are often complicated by the low solubility of HBCDD and the different solubilities and toxicities of the different diastereomers present in the technical mixture of HBCDD. A few experiments have also been done using Baltic species, applying HBCDD in a more realistic manner, associated to suspended organic matter; [PERSON] and [PERSON] reported cytotoxic and genotoxic effects (significant increases in nuclear and nucleolar abnormalities and frequency of dead cells) of HBCDD in the bivalve _Limecola bahlhica_[17]. [PERSON] et al. suggested that copepods may be a particularly sensitive part of the Baltic plankton [18]. [PERSON] et al.