Comparing biogenic blue mussel (Mytilus edulis) reef definitions in Northern Europe: implications for management and conservation

To combat biodiversity loss, the European Union (EU) established a network of protected areas to be implemented by 2000, termed Natura 2000 (N2000). However, several N2000 sites remain unmapped, and various habitats remain undefined. Delayed implementation allows continued habitat degradation and biodiversity loss. Targeting biogenic reefs by blue mussels (Mytilus edulis and Mytilus trossulus), this study 1) compared blue mussel reef definitions across EU member states surrounding the North Sea and Baltic Sea, 2) scrutinized biological mechanisms underpinning the habitat definitions, and 3) provided suggestions for harmonized habitat definitions, respecting spatial and biological variation. By comparing official definitions of blue mussel reefs applied in Denmark, Sweden, The United Kingdom, and Germany, this study revealed A) decadal delay in implementation, B) reef definitions varying significantly regarding number of parameters used for identification (e.g., mussel seabed area (m²), and proportion seabed mussel coverage (%)) and the associated parameter limits (e.g., 2.500 m², and 30%, respectively). Specifically, parameter limits for identification of reefs range between 1 and 10,000 m2 for mussel area and between 5% and 30% seabed mussel coverage. The study failed to identify biological mechanisms justifying this variation. Variable habitat definitions, unjustified by biological and spatial variation, may result in uneven protection levels across borders, potentially compromising connectivity of the N2000 network. To avoid this scenario, our study highlights the need for compatible biogenic reef definitions reflecting spatial and biological properties and suggests moving from protection of individual reefs to protection of areas where habitat forming blue mussels are regularly occurring.