One of the main objectives of the Trilateral Wadden Sea Cooperation laid out in the ‘Joint Declaration on the Protection of the Wadden Sea’, which was first signed in 1982 and updated in 2010, is to enable ‘a natural ecosystem, its functions and characteristic biodiversity’. This conservation target is challenged by the introduction of alien species (for definitions used in this thematic report, click here). Biological invasions can affect biodiversity negatively, e.g. by means of interspecific competition, alteration of habitat, effects on food-web properties, hybridisation and the transfer of disease (e.g. Gollasch & Nehring 2006, Molnar et al. 2008, Savini et al. 2010, Rabitsch et al. 2013, Katsanevakis et al. 2014), though alien species can also have positive impacts (e.g. Katsanevakis et al. 2014). Aside from effects on biological diversity, alien species can also have adverse or favourable effects on society or specific economic sectors, e.g. fisheries and shipping (Molnar et al. 2008, Vilà et al. 2010, Rabitsch et al. 2013, Katsanevakis et al. 2014).
Major pathways for introductions of species to the North Sea are shipping (both via ballast water and ship-fouling) and aquaculture operations (Gollasch et al. 2009). For the trilateral Wadden Sea, Buschbaum et al. (2012) described 66 alien taxa, including 17 tentative cryptogenic alien macrobenthos, though this number has since increased (pers. comm. C. Buschbaum and D. Lackschewitz, Alfred Wegener Institute, A. Gittenberger, GiMaRIS). As small alien organisms have been poorly studied and are easily overlooked, the number of alien taxa may be even higher (Reise et al. 1999, Wijnhoven & Hummel 2009). Earlier inventories have shown that most introduced species found are epibionts (organisms living attached to the surface of a host) or fouling organisms (organisms attached to non-living substrata), while a few live in sediments, or are holoplanktonic or parasitic (Reise et al. 1999). Buschbaum et al. (2012) conclude in their recent overview that most alien species belong to the macrobenthos. An additional 14 alien phytoplankton taxa and two alien zooplankton taxa have been identified (Gollasch et al. 2009, Reise et al. 1999). Before 2012, permanently established populations of alien fish were unknown (Buschbaum et al. 2012), but since then the Round Goby (Neogobius melanostomus), an euryhaline species, has been found in German estuaries (Hempel & Thiel 2013).
Most alien species in the Wadden Sea arrived through secondary spreading, i.e. they spread from adjacent coasts to the Wadden Sea via regional shipping or through inland canals and rivers, but also by natural dispersal from the sites of primary introduction or translocations of shellfish (Buschbaum et al. 2012). Management efforts for controlling alien species therefore need to address the entire European Atlantic coast (Buschbaum et al. 2012). At the North Sea coast, most alien species have become established in brackish environments, harbours and in the vicinity of oyster farms (Reise et al. 1999, Gollasch & Nehring 2006, Reise et al. 2006). For the Dutch Wadden Sea, Gittenberger & Rensing (2012) made a detailed comparison of the number of alien species found in three hard-substrate habitats, namely marinas, mussel banks and/or oyster riffs, and dikes outside the harbours. Their results showed that most alien species were found in marinas, specifically on floating docks, indicating that hull-fouling on recreational boats may be an important (secondary) vector for bioinvasions to the Wadden Sea. Other authors suggest that bio-fouling organisms with limited self-dispersal capacity can more easily spread via small boats, and the management of alien species therefore needs to address the small boat vector (e.g. Zabin et al. 2014). Pictures below by NIOZ, A. Gittenberger and D. Thieltges.
Invasive alien species may pose a threat to the typical Wadden Sea biodiversity, by altering habitats and having long-lasting effects on native biota in the Wadden Sea. The often rather rapid and long-range spread of some alien species in the Wadden Sea, as witnessed for the Pacific Oyster (Crassostrea gigas), has made policy makers aware of the timely manner and larger scale (not just national) required for the management of alien species. At the 12th Trilateral Governmental Wadden Sea Conference in Denmark in February 2014, the Strategic Framework for Alien Species was therefore discussed and further steps were taken towards the development of a trilateral alien species management and action plan. The overall objective of the Strategic Framework is ‘to prevent threats to the Wadden Sea ecosystem and biodiversity through alien species’. This shall be done by taking precautionary measures against their introduction, by minimising their spreading once they have been detected in the Wadden Sea, and through eradication measures where feasible. Two of the key elements that the Strategic Framework emphasises are the set-up of early warning and detection by means of a monitoring programme, as well as activities to raise awareness amongst managing authorities, companies engaged in shipping and aquaculture, researchers, and the public. This is also in line with a request from the UNESCO World Heritage Committee, as mentioned in the Wadden Sea Plan (Common Wadden Sea Secretariat 2010).
The establishment of a trilateral monitoring programme for alien species presents a number of challenges, as it requires collaboration between a diverse group of governmental agencies that have different jurisdictional authorities, funding sources and mission statements. This is further complicated by the fact that the three Wadden Sea countries use different national strategies and legislations when it comes to the implementation of international policies and guidelines with regard to alien species (Bouma et al. 2011). Ideally, a future trilateral monitoring programme for alien species will meet the requirements of TMAP (Trilateral Monitoring and Assessment Programme), which aims to be ‘a harmonised and effective monitoring and assessment programme, based on sound scientific evidence, that serves the needs of policy making at all levels, the commitments ensuing from relevant Directives and conventions, as well as the World Heritage status and that supports the management of the Wadden Sea as an ecological entity’.
For the management of alien species in the trilateral Wadden Sea area, a number of national, European and global regulations, policies and guidelines are of special importance (for details see chapter 2). The EU Habitats Directive is one of the main reasons for this management as its aim is ‘to contribute towards ensuring biodiversity through the conservation of natural habitats and of wild fauna and flora in the European territory of the Member States to which the Treaty (EEC Treaty Treaty establishing the European Economic Community) applies’ (see Article 2, paragraph 1).
European legal instruments, policies and guidelines of specific relevance for alien species management are (in chronological order)
A relevant global policy instrument is
The Dutch Wadden Sea is being managed under the WFD (Wadden Sea Board 2012), which does not explicitly mention alien species. On the other hand, the Dutch part of the North Sea is being managed under the MSFD, which does address alien species in its descriptor 2 (see ‘Information Needs and Monitoring questions’ for more in-depth information). However, for the Dutch Wadden Sea, the ‘implementation of particular aspects of the environmental status not covered by the WFD or other EU legislation will be addressed within the framework of the Trilateral Wadden Sea Cooperation, not requiring the formal application of the MSFD to the Wadden Sea’ (Wadden Sea Board 2012, p. 5).
Currently, the regular WOT (Wettelijke Onderzoekstaken, statutory research tasks) and MWTL (Monitoring Waterstaatkundige Toestand des Lands (Milieumeetnet Rijkswaterstaat), Monitoring Scheme for the management of the Dutch Fresh and Marine Waters by Rijkswaterstaat, which is part of the Dutch Ministry of Infrastructure and the Environment) monitoring programmes are being used to additionally screen for alien species (pers. comm. S. Smolders, Team Invasive Alien Species, Ministry of Economic Affairs, and S. H. Kabuta, Rijkswaterstaat). This holds true for the Dutch Wadden Sea as well as the Dutch part of the North Sea (see Marine Strategy for the Netherlands). Findings of alien species are reported to the Ministry of Economic Affairs by Rijkswaterstaat and can, given their significance, be addressed accordingly (pers. comm. S. H. Kabuta, Rijkswaterstaat).
In 2009 and 2011, more in-depth inventories of the alien macrofauna and macroflora were performed within the framework of risk analyses of the South-North transport of mussel seeds from the Oosterschelde to mussel farming plots in the Wadden Sea (Gittenberger et al. 2009 and 2012). In 2014, this alien species inventory was repeated using the same hard substrate locations that had previously been sampled and was complemented by an additional alien species inventory of soft sediment locations (Gittenberger et al. 2015). All these inventories offer the opportunity to develop an alien species baseline for a future monitoring programme in the Dutch Wadden Sea. The Team Invasive Alien Species of the Dutch Ministry of Agriculture, Nature and Food Quality (since 2012 Ministry of Economic Affairs) explores the suitability of such kind of inventories for early detection of alien species to meet the obligations resulting from the regulations (pers. comm. S. Smolders, Team Invasive Alien Species, Ministry of Economic Affairs). The ‘Directorate Nature and Biodiversity’ (Dutch: Directie Natuur en Biodiversiteit) of the Ministry of Economic Affairs will make the final decision on how to proceed with the alien species monitoring in the Dutch Wadden Sea.
Contrary to the Netherlands, the German and Danish Wadden Sea are subject to the MSFD (Marine Strategy Framework Directive). Whereas the WFD (Water Framework Directive) assesses the chemical and ecological status of each individual coastal water body, the MSFD’s goal is the achievement of Good Environmental Status (GES) at the greater assessment scale of (sub)region (the Greater North Sea and the Celtic Seas).
In Germany, rapid assessments of alien macrobenthos have been performed on a yearly basis since 2009 (Lackschewitz et al. 2015). In the summer and autumn of 2014, two additional rapid assessments of alien species were conducted in the Wadden Sea of the States of Schleswig-Holstein and Niedersachsen in line with the Marine Strategy Framework Directive’s requirements and in accordance with the methods described in the Joint HELCOM/OSPAR Guidelines (pers. comm. C. Buschbaum, Alfred Wegener Institute, and G. Scheiffarth, National Park Wadden Sea of Lower Saxony). The results of these rapid assessments contribute to an alien species baseline, which will be used to compare and evaluate any future monitoring and control measures implemented to fulfil the MSFD requirements in the German Wadden Sea. Furthermore, in 2012 and 2013 an inventory focused on alien species, commissioned by the shellfish industry in the form of a risk assessment for shellfish translocations was conducted by GiMaRIS throughout the Schleswig-Holstein Wadden Sea (unpublished report by Gittenberger et al., status March 2015).
In order to control the target setting under the MSFD, the ‘Facharbeitsgruppe (FAG) Neobiota’ worked on a proposal for a system of indicators to analyse trends in alien species in the Wadden Sea (“trend indicator” for descriptor 2.1 of the MSFD) and is considering a corresponding monitoring programme (information from FAG Neobiota meeting, Nov. 2014, and pers. comm. K. Hoppe, Leibniz Institute for Baltic Sea Research, Warnemünde, 30 April 2015). Special focus is given to the management of the key pathways and vectors of alien species introductions, enabling the implementation of any necessary measures to prevent further introductions. In October 2014, the German monitoring handbook was reported to the EU Commission in the frame of the MSFD implementation, but this did not include specifications on alien species monitoring. The German proposal for the system of indicators has been reported to OSPAR and HELCOM (for the Baltic Sea) in the meantime though (pers. comm. K. Hoppe, Leibniz Institute for Baltic Sea Research, Warnemünde, 30 April 2015). It is unclear when the indicators will officially be reported to the EU Commission. In the ongoing indicator revisions, it is being considered to eliminate descriptor 2.2 (impacts of alien species) as chances for successful measures against marine invasive alien species are extremely small and are counterproductive to the use and accomplishment of the GES.
In Denmark, a proposal for a cost-effective monitoring programme, which fulfils the obligations under the MSFD and the Regulation of the European Parliament and the Commission on the Prevention and Management of the Introduction and Spread of Invasive Alien Species, has recently been completed for Danish marine waters (Andersen et al. 2014). The proposal is a result of the study ‘Monitoring of non-indigenous species in Danish marine waters (MONIS)’ initiated by the Danish Nature Agency. Its focus was on taking the greatest possible advantage of existing monitoring activities, and assessing the applicability of existing monitoring guidelines and contemporary biomolecular technologies. The Danish proposal covers all Danish marine waters and is not exclusive to the Danish Wadden Sea.