Andre presfaktorer end næringsstoffer og klimaforandringer – vurdering af de invasive arter amerikansk ribbegople og sortmundet kutling

As part of a larger project funded by the Danish Environmental Protection Agency (“Effects on the quality elements defined by the EU Water Framework Directive (WFD) of other pressure factors than excess nutrient load and climate change”) a number of environmental pressure factors other than excess nutrient loading and climate change have been identified as potential risks to all of the quality elements of the WFD (Petersen et al. 2018). In a larger review, a number of invasive species were identified as potential pressure factors on the quality elements of the WFD. In the review, it was estimated that the invasive species Mnemiopsis leidyi and Neogobius melanostomus may have potential significant effects on the quality elements phytoplankton and benthic fauna, respectively (Petersen et al. 2018). Both species are known to be aggressive invaders and in international per-reviewed studies, effects on the ecosystem have been documented but for none of the species, sufficient data were available to perform a scientific analysis at the level of water bodies as defined in the WFD (Petersen et al. 2018). However, the Danish Environmental Protection Agency wanted an expert assessment of potential effects of the two invasive species based on the data available at present. 
Mnemiopsis leidyi possess many characteristics of invasive species such as rapid growth, high dispersal potential, high fertility, short generation time, high competitiveness and high tolerance to environmental conditions. Mnemiopsis leidyi was first observed in northern European waters in 2005 and major occurrences were observed in Limfjorden in 2007. Since then, it has been observed in most Danish waters. The expected effect of Mnemiopsis leidyi in the ecosystem is that it eats zooplankton, thereby reducing grazing pressure on phytoplankton, which can then grow uncontrolled and thereby lead to a number of biological cascade effects. As data on occurrence of jelly fish including Mnemiopsis leidyi is not a part of any national Danish monitoring program, only data from scientific publications are available. These have not hitherto been collected systematically and data is primarily available from Limfjorden up to 2011. Most other data for the analysis, e.g. concentration of Chlorophyll a, derives from the national environmental monitoring program. Overall, the analysis demonstrated a negative correlation between the occurrence of Mnemiopsis leidyi and copepod biomass in Limfjorden. There could, however, not be demonstrated a relation between Mnemiopsis leidyi and concentrations of Chlorophyll a. One reason for the lack of a cascade effect of the grazing on zooplankton is probably that other suspension-feeders - especially blue mussels - can affect the concentration of Chlorophyll a and thus disguise the effect of reduced concentrations of zooplankton. No unequivocal effects of the occurrence of Mnemiopsis leidyi can thus be demonstrated for the WFD quality element phytoplankton. In order to demonstrate any effect, a more elaborate monitoring program including Mnemiopsis leidyi, as well as other jelly fish like Aurelia aurita, needs to be established.
Neogobius melanostomus is found in coastal shallow waters during the spring, summer and fall, while in winter it hibernates in deeper water. The species has many invasive characteristics such as high competitiveness of territory and food, a broad diet, high dispersal potential and high tolerance to fluctuations in temperature, oxygen content and salinity. Neogobius melanostomus was first observed in Danish waters at Bornholm in 2008, and the following year the first individuals were found at Guldborgsund and subsequently along the coasts of Lolland, Falster, Møn and Zealand's southwest and southeast coast and also in the later years at the coasts of Fyn. The expected effect of invasion of Neogobius melanostomus is a selective predation pressure on the benthic fauna leading to a reduced diversity and potentially cascade effects at selective feeding on benthic suspension-feeders. The analysis was performed as a BACI-analysis using data from the national environmental monitoring program on benthic fauna. From the monitoring data, species that are known prey for the fish were selected for the analysis. For Neogobius melanostomus only presence/absence data was used, as no quantitative data are available. The selected 'impact' areas are respectively Gulborgsund (Neogobius melanostomus first observed 2009) and Stege Bugt (Neogobius melanostomus first observed 2011) and as 'control' area was used the inner part of Isefjord. Overall, the results of the analysis showed that there was no statistical evidence of a negative effect of Neogobius melanostomus on bottom fauna in the two selected 'impact' areas. Some variation in data was found, and the primary cause of a 'significant positive effect' found in some 'impact' areas was mainly a negative development of the species in the 'control' area. In order to demonstrate a potential effect of Neogobius melanostomus on bottom fauna, sampling with a better representation than in the present environmental monitoring of epibenthic and in particular mobile epibenthic species is required. 
We have not been able to demonstrate significant effects of neither Mnemiopsis leidyi on the quality element phytoplankton nor Neogobius melanostomus on the quality element benthic fauna. For both species, part of the explanation is that there is an insufficient data basis for the analysis. Basically, Mnemiopsis leidyi have not been monitored for a sufficiently long period of time and with sufficient geographical coverage, and it is not possible to separate the effects of Mnemiopsis leidyi from the effects of other jellyfish. For Neogobius melanostomus, quantitative data on the distribution of the species are not available, and the monitoring of benthic fauna does not consider the preferred prey for Neogobius melanostomus. The present analysis thus illustrates current knowledge gaps in our understanding of the potential impact of invasive species for the coastal environmental condition. However, it cannot be concluded that if only data were sufficient, an effect could have been documented. Both analyses demonstrate large variations in the measured quality elements that cannot in themselves be explained by the presence of invasive species.