A good understanding of the mechanisms and magnitude of impact of invasive alien species on ecosystem services and biodiversity is a prerequisite for efficient prioritization of actions for prevention of new invasions or for mitigation measures. In this review, we have identified marine alien species of high-impact on ecosystem services and biodiversity in the European seas, classified the mechanisms of impact, commented on the methods applied for assessing impact and the related inferential strength, and reported gaps in available information. Furthermore, we have proposed an updated inventory of 86 marine species in Europe, representing 12 phyla, with documented high impact on ecosystem services or biodiversity. Food provision was the ecosystem service that was impacted by the highest number of alien species (in terms of both positive and negative impacts). Following food provision, the ecosystem services that were negatively affected by the highest number of species were ocean nourishment, recreation and tourism, and water purification, while the ecosystem services that were more often positively impacted were water purification, cognitive benefits, and climate regulation. In many cases, marine aliens impact keystone/protected species and habitats. Almost 30% of the assessed species affected entire ecosystem processes or wider ecosystem function, more often in a negative way. Fifty of the assessed species have been reported to be ecosystem engineers, fundamentally modifying, creating or defining habitats by altering their physical or chemical properties. Positive impacts of alien species are probably underestimated, as there is a persistent perception bias against alien species. We conclude that the “native good, alien bad” philosophy is a misconception and the role of most of the alien species in marine ecosystems is rather complex. Among the species herein assessed as high-impact species, 15 had only negative and 8 only positive impacts; for the majority (64 species) both negative and positive impacts have been reported, and the overall balance is often unknown. Although there is no doubt that invasive species have modified marine ecosystems, evidence for most of the reported impacts is weak, as it is based on expert judgement or dubious correlations, while only ~10% of the reported impacts were inferred via manipulative or natural experiments. A need for stronger inference is evident, to improve our knowledge base of marine biological invasions and better inform managers.JRC.H.1-Water Resource

Effective identification of species using short DNA fragments (DNA barcoding and DNA metabarcoding) requires reliable sequence reference libraries of known taxa. Both taxonomically comprehensive coverage and content quality are important for sufficient accuracy. For aquatic ecosystems in Europe, reliable barcode reference libraries are particularly important if molecular identification tools are to be implemented in biomonitoring and reports in the context of the EU Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD). We analysed gaps in the two most important reference databases, Barcode of Life Data Systems (BOLD) and NCBI GenBank, with a focus on the taxa most frequently used in WFD and MSFD. Our analyses show that coverage varies strongly among taxonomic groups, and among geographic regions. In general, groups that were actively targeted in barcode projects (e.g. fish, true bugs, caddisflies and vascular plants) are well represented in the barcode libraries, while others have fewer records (e.g. marine molluscs, ascidians, and freshwater diatoms). We also found that species monitored in several countries often are represented by barcodes in reference libraries, while species monitored in a single country frequently lack sequence records. A large proportion of species (up to 50%) in several taxonomic groups are only represented by private data in BOLD. Our results have implications for the future strategy to fill existing gaps in barcode libraries, especially if DNA metabarcoding is to be used in the monitoring of European aquatic biota under the WFD and MSFD. For example, missing species relevant to monitoring in multiple countries should be prioritized for future collaborative programs. We also discuss why a strategy for quality control and quality assurance of barcode reference libraries is needed and recommend future steps to ensure full utilisation of metabarcoding in aquatic biomonitoring.

Over the past century, the potential for aquatic species to expand their ranges in Europe has been enhanced both as a result of the construction of new canals and because of increased international trade. A complex network of inland waterways now connects some previously isolated catchments in southern (Caspian, Azov, Black, Mediterranean seas) and northern (Baltic, North, Wadden, White seas) Europe, and these waterways act as corridors for nonnative species invasions. We have developed a conceptual risk assessment model for invasive alien species introductions via European inland waterways, with specific protocols that focus on the development of environmental indicators within the socioeconomic context of the driving forces-pressures-state-impact-response framework. The risk assessment protocols and water quality indicators on alien species were tested for selected ecosystems within 3 main European invasion corridors, and these can be recommended for application as part of the Common Implementation Strategy of the European Commission Water Framework Directive, which aims to provide a holistic risk-based management of European river basins. The conceptual structure of the online Risk Assessment Toolkit for aquatic invasive alien species is provided and includes 3 main interlinked components: online risk assessment protocols, an early warning system, and an information transmitter for risk communication to end users.