Stæhr P.A., Jakobsen H.H., Hansen J.L.S., Andersen P., Storr-Paulsen M., Christensen J., Lundsteen S., Göke C., Carausu M.-C. 2016. Trends in records and contribution of non-indigenous species (NIS) to biotic communities in Danish marine waters. Aarhus University, DCE – Danish Centre for Environment and Energy, 45 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 179. http://dce2.au.dk/pub/SR179.pdf
The objective of this report is threefold: 1) to contribute to the Danish official contribution to the Marine Strategy Framework Directive (MSFD) reporting on non-indigenous species (NIS), 2) to be an independent report on NIS in the OSPAR and HELCOM regions and 3) to act as a baseline in future assessments of NIS in Danish waters. The report is the first national assessment of the occurrence and decadal trends of NIS and how NIS contributes to changes in marine community structures. We also investigate possible relationships between NIS abundance and key water quality conditions characterizing the marine environment. Marine species were grouped into five categories: Phytoplankton, zooplankton, macroalgae, benthic invertebrates and fish. The analyses cover the period 1989 to 2014 for five marine regions in the North Sea (OSPAR) and the Baltic Sea-North Sea transition zone (HELCOM and OSPAR): 1) the Norths Sea including Skagerrak, 2) the Kattegat, 3) Limfjorden, 4) the Belt Sea and 5) the western Baltic Sea.
Since the initiation of the Danish marine monitoring programmes in the 1990s, the number of marine and coastal NIS recorded has increased from 21 to 85 in 2014. Of the 85 marine NIS recorded in total, 63 species appear in the monitoring programmes, while the remaining 22 are documented in various reports (see table 6.2). Several NIS were not detected in the monitoring programme because of limited spatio-temporal sampling. Although monitoring programmes have recorded several NIS, inadequacies in the current sampling technique and strategy are responsible for lack of information on some groups of NIS including gelatinous zooplankton, shallow water fish, parasitic invertebrates, and flowering plants from the littoral zone. We calculated older introduction rates to be approx. two new NIS per decade from 1900 to 1980 followed by a dramatic increase of 16 new NIS per decade from 1980 to 2014 when combining information from monitoring and other documented records. Similar exponential increases in NIS numbers has been observed in other countries and related to a combination of intensified ship traffic, breakdown of dispersal barriers and climatic changes. Parallel to the introduction of NIS, there has been a gradual increase in the total species number sampled following the initiation of the monitoring surveys in the 1980s. However, as the relative number of NIS was not related to sampling effort, changes in this over time are not likely to bias our interpretations of NIS impact. Most of the recent increase (from 1989 to 2014) has occurred within the phytoplankton (45 %), followed by benthic invertebrates (26 %), macroalgae (14 %), fish (5 %), parasites (5 %), zooplankton (4 %) and flowering plants (1 %). Today, most NIS are established in the large sound Limfjorden (30 NIS), and fewest are found in the brackish western Baltic Sea region (11 NIS).
Significant and exponential increases in records of NIS relative to the total species records in the database (hereafter %NISrecord) were observed for phytoplankton and benthic invertebrates in all five regions. Only two records of fish NIS existed in the database indicating that trawl surveys primarily conducted in deeper waters over sandy sediments are not suited to monitor NIS of fish, which typically are adapted to shallow vegetated waters. For macroalgae and zooplankton, there was no clear temporal trend in %NISrecord. We also analysed changes in relative abundances of NIS compared to the native community (hereafter %NISabundance) standardised by biomass for phytoplankton, number of individuals for benthic invertebrates and percent cover for macroalgae (no abundance data were available for zooplankton). This analysis showed a decrease in %NISabundance for phytoplankton, but an increase for benthic invertebrates and macroalgae (only in Limfjorden for macroalgae).
Multivariate statistics was used to quantify the impact of NIS on community structures. This analysis showed that in regions where NIS were recorded, their contribution to total community similarity increased over time. This indicates that NIS have, over time, become a more characteristic component of Danish marine communities. The largest recent (i.e. from 2006 to 2014) contribution of NIS to community similarity was observed for phytoplankton where NIS contributed with ~ 10 percent of the similarity (compared to 5, 4, and 2 percent contribution from benthic invertebrate, macroalgal and zooplankton NIS, respectively). Furthermore, the relative importance of NIS to community similarity generally declined from the open saline waters toward the brackish Baltic Sea suggesting that many NIS have been introduced from neighbouring waters rather than long dispersal via ballast water. The stronger influence of NIS in the north-western regions is likely driven by a combination of factors, for example (1) high hydrodynamic connectivity between the Baltic Sea – North Sea transition zone, (2) introduction of NIS generally has to travel through this north-western North Sea Kattegat opening, (3) this region includes large estuarine systems (e.g. Limfjorden), and (4) this region has subsystems characterized by both euhaline waters (for stenohaline NIS) and estuarine brackish water (for euryhaline NIS). While NIS generally explained less than 10 percent of the year-to-year changes in the community similarity for these large-scale regions, NIS often dominated locally and seasonally. Examples of particular dominant NIS include Pseudochattonella during early spring in the Kattegat (phytoplankton), Sargassum muticum in the Limfjord (macroalgae), Ensis directus in all regions (benthic invertebrates), Mnemiopsis leidyi in the Belt Sea (zooplankton) and Neogobius melanostomus in the Belt Sea (fish).
We performed a univariate correlation analysis to evaluate if changes in %NISrecord over time were related to changes in environmental conditions (water clarity, salinity, temperature, primary production, concentration of chlorophyll and total nitrogen) in the five studied regions. Overall low correlations were found, suggesting that changes in environmental conditions such as those related to climate change were of small importance for changes in NIS number. A multivariate correlation analysis furthermore indicated that changes in species composition within groups (phyto-, zooplankton, macroalgae, benthic fauna) were partly related to %NISrecord but strongly related to salinity, highlighting a general positive relationship between salinity and marine species richness in Danish waters. To further resolve and disentangle the underlying causes for the increasing contribution to, and effect of NIS to Danish marine communities, analyses are needed at a more local scale where the occurrence of NIS (either as records or abundance) can be linked to environmental variables with greater certainty.