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No. 428: Development of a management strategy that takes into account the entire ecosystem in De Østlige Vejler

Lauridsen, T.L., Bruhn, D., Clausen, P., Andersen, L.H., Pertoldi, C., Jeppesen, E. Søndergaard, M., Levy, E., Fox, A.D., Balsby, T.J.S., Bahrndorff, S., He, H., Pedersen, C.L. & Nielsen, H.H. 2021. Udvikling af en forvaltningsstrategi, der tilgodeser hele økosystemet i De Østlige Vejler. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 186 s. - Videnskabelig rapport nr. 428. http://dce2.au.dk/pub/SR428.pdf

Summary

The Eastern Vejler is a unique nature conservation area consisting of brackish water lakes, reed beds, meadows and saltmarshes in northern Jutland, Denmark, recognized for its internationally important breeding and staging migratory birds. Over the course of recent decades, various management-planning procedures have been implemented at the site, which, in recent years, have concentrated on attempting to secure good water quality as well as improving conditions for endangered species of nesting meadow birds. However, there has been no overall evaluation of the success and efficacy of the currently implemented management practices. This report presents an assessment of the recent past and current status of nature conservation interests in the Eastern Vejler area, concentrating on plants and vertebrates throughout the entire area, but with particular emphasis upon how water quality and inundation levels affects key plant and animal species in the reed beds and meadows.

 

Lakes

 

Monitoring data from Eastern Vejler lakes show a general deterioration in freshwater conditions during 1999/2000 to 2019, with the exception of Lund Fjord. Phosphorus concentrations have increased significantly in Selbjerg Vejle and Glombak, and are increasing in Han Vejle. The increase in nutrient concentrations has resulted in a shift in fish community composition, with the former dominance of European Perch Perca fluviatilis now replaced by a predominance of Common Roach Rutilus rutilus, a trend also observed in several of the smaller lakes. There have been no changes in observed in conductivity and salinity over the same period of years.

 

The change in the composition of the fish community has resulted in a zooplankton community composed of smaller average sized individuals than previously, which consequently are less efficient as algae grazers. As a result, chlorophyll measurements have increased, reducing visibility in the water column, which has led to impaired growth conditions for submerged vegetation. This is reflected in the decline in the former abundance of Charophytes, whereas the more tolerant Eurasian Water-milfoil Myriophyllum spicatum has become more dominant in most of both the large and small lakes.

 

Using the results of stable isotope analysis from the investigated lakes, it was evident that the lakes most affected by high phosphorus concentrations (Selbjerg Vejle, Glombak and Lund Fjord) were also those that were most affected by the influence of human/agricultural activity, based on isotope ratios at different trophic levels. The more isolated systems, including Han Vejle and the small lakes, were in contrast far less affected.

 

Taken as a whole, therefore, the monitored lakes over this period have thus moved from a state of good water quality, characterised by clear visibility in the water column, to a state where water column turbidity has worsened considerably and is now characterised by increased chlorophyll and nutrient content.

 

Meadows and reed beds

 

Bygholm Meadows are unusual in showing no clear environmental gradients in relation to moisture, salinity and nutrition, but retains a complex mosaic of habitat with a varied topography. For this reason, it is therefore also difficult to come with clear management recommendations. However, given the current status, the existing grazing pressure seems appropriate in relation to maintaining plant biodiversity at its present level.

 

If the management objective for Bygholm Meadows is to keep them in their current condition as relatively poor saltmarsh habitats, the area should no longer be flooded (irrigated) with freshwater from the Lund Fjord canal. If, on the other hand, the objective is to create a meadow that is better example of a classic botanical saltmarsh locality, it is recommended that Bygholm Meadows be flooded more with saltwater from the Limfjord. However, this needs to be carefully considered in the context of other aquatic interests in the immediate vicinity, as this management will pose a risk to the water quality in the lakes.

 

In relation to erosion risk and the adverse effects of water level on the vegetation, it is difficult to recommend a specific formula for the management of water levels on Bygholm Meadows. This is primarily because Bygholm Meadows vary greatly in their surface topography, with the result that under all water level conditions, areas are created where the critical bottom speed required for sediment erosion is exceeded within the water column. Areas with lowest inundation levels thus experience the greatest sediment transport of, among other things, organic material, which is why conditions for vegetation can deteriorate locally in relation to water levels.

 

Harvested reed bed areas of different age since last harvest have been investigated to compare their plant communities. Each of the investigated areas contained unique plant species, while only one third of all plant species were present across all investigated areas. The simplest way to maintain the greatest possible plant diversity in a reed bed with the greatest possible habitat heterogeneity is therefore to harvest different areas each year, at the same time leaving some completely unharvested.

 

At the same time, the reed bed management also affects invertebrate biomass, such that reed harvest increases the biomass of both flying and surface-living invertebrate species. The effect of reed harvest and time since last harvest also affect the invertebrate community composition, as different groups of invertebrates thrive at each different reed bed age. The results again confirm the need to maintain the greatest possible overall diversity in reed bed areas of different growth age for maximum effect.

 

Birds

 

The section on the Eastern Vejler birds presents the importance of the area for breeding and staging migrating birds, and their changes over time, both from an international and national perspective. Based on the number of different species mentioned as the designation criteria for Eastern Vejler’s designation as an EU Bird’s Directive Special Protection Area, the area ranks in Denmark's top-seven sites. There follows descriptions of a number of selected key species that are associated with the lakes, meadows and reed beds of the area and a review of changes in abundance over time to the present.

 

Numbers of non-breeding Mute Swans Cygnus olor and Bewick’s Swans C. columbianus bewickii as well as Coot Fulica atra have declined dramatically in the Eastern Vejler, probably due to declines in biomass of favoured submerged plant species and a shift in the species composition of underwater plant communities to less favoured food species. This was especially the case in Selbjerg Vejle and Glombak, which were previously the most important foraging areas for the three herbivorous waterbird species mentioned above. Management aimed to improve water quality in the lakes will therefore most likely be beneficial to the lost bird communities formerly associated with these lakes.

 

The existing wealth of studies that recommend optimal management of meadows such as those at Bygholm to provide ideal conditions for breeding wader species is reviewed in the report in relation to providing optimal conditions there. Modelling is presented to show that numbers of dabbling ducks staging in both spring and autumn are favoured by maintaining relatively high water levels when these birds pass through. On this basis, it was recommended that spring water level be maintained at a higher level than was formerly the case, a recommendation that was considered to not only be more favourable for dabbling duck species, but also for breeding meadow birds as well. Maintaining higher local densities of these species will also be beneficial for several species of birds of prey associated with the Eastern Vejler, which prey on such species.

 

For breeding bird species listed on Annex 1 of the EU Bird Directive associated with reed beds during the breeding season, there are indications for Bittern Botaurus stellaris and Marsh Harrier Circus aeruginosus that both predator abundance and water level conditions can affect local abundance of summer breeding numbers. In the case of the Bittern, the preceding winter weather also plays a role by affecting survival, although the report was not able to investigate fully the relative importance of these various factors. Nevertheless, under all circumstances, it is recommended, where possible, to maintain the highest water levels in those parts of the Eastern Vejler with greatest densities of these species, in order to protect the birds' nests from predation.

 

The markedly increase in numbers of geese seen at, and especially roosting on, the Eastern Vejler in recent years has led to speculation as to whether nutrients added to the lakes from overnight deposition of bird faeces may have contributed to the deteriorating water quality in the lakes. The amounts of nutrients that come from faeces deposited from all over-night and daytime active waterbirds to the large lakes, Lund Fjord, Selbjerg Vejle and Glombak, are small and typically less than one percent of the amounts of nutrients that come from the lakes' catchments and from the atmosphere. In Han Vejle, calculations showed that the contributions from birds can be more significant for the lake's water quality. Bird counts showed that the vast majority of geese fly to spend the night at Bygholm Meadows and that was confirmed by telemetry data for Pink-footed Geese Anser brachyrhynchus, one of the two most numerous species in the area, which come in to roost and remain on Bygholm Meadows throughout each night.