DHI the first in Europe to define 'supplementary measures'

The Danish Spatial and Environmental Planning Agency (BLST) has published a report, which suggests that restoration of boulder reefs in coastal waters, a protected habitat type under the EC Habitats Directive, could also serve as a ‘supplementary measure’ and hence support implementation of the EU Water Framework Directive.

The title of the report is "Stenrev i Limfjorden: Fra naturgenopretning til supplerende virkemiddel", by Flemming Møhlenberg, Jesper H. Andersen, Ciarán Murray (DHI), Peter B. Christensen, Tage Dalsgaard, Henrik Fossing & Dorte Krause-Jensen (DMU). Click here (in Danish) to download the report.

2 km² of boulder reef = ÷ 800 tons N. How can that be?
As in many other EU countries, Denmark has focused exclusively on reduction of nutrient loads as a means to fulfil the environmental quality requirements of the Water Framework Directive. Meanwhile other, perhaps equally important methods are neglected in coastal waters. It is indisputable that there have been significant increases in nutrient loads but it is also very likely that the conditions in fjords and coastal waters have been affected by fishing and by extraction of raw materials. Large rocks/boulders have been deliberately removed e.g. for building harbours and smaller stones have been removed unintentionally by trawl-fishing.

Consequently there has been a less developed population of perennial macroalgae, which are often limited by access to hard substrates. Subsidiary effects have been reduced oxygen production in benthic waters and hence worsened hypoxia and fauna mortality.

We have used numerical modelling to investigate the effects on hypoxia and "internal N-loading" following a "virtual" re-establishment of hard substrate in Løgstør Bredning (an open, shallow area) in the Limfjord. A number of assumptions are made in the models, the most important of these based on observations and measurements in the Limfjord over the last 10 years (the same density and production of macroalgae as at present but over a greater area, reduction in N-release from sediment when oxygen concentration is increased, etc.)

Extent of hypoxia during a calm (wind-free) summer with no reef established (left) and after establishing 4km2 reef (with associated macroalgae) spread over 7 sub-areas in Løgstør Bredning (right)

Establishing boulder reefs reduces the extent and, in particular, the duration of anoxia and N-release from sediment. On the basis of a series of model scenarios, it was concluded that establishing 2km2 of reef spread over a number of smaller areas was most cost-effective.

Relationship between reef area and calculated reduction in N-flux from sediment for different reef configurations. A reef area of 2 km2 reduces the internal load by 400-600 tons N during the summer. It is estimated that a load reduction in the summer period is numerically worth twice as much as a reduction made primarily in the winter (which is true for N-reduction in catchment areas)

Where can I read more?
The report is available from BLST’s web page, click here (in Danish) to get a pdf. If additional information or comments are needed, please contact: Flemming Møhlenberg, head of innovation, flm@dhigroup.com or Jesper H. Andersen, head of unit (EU Water Policy), jha@dhigroup.com.