Fyn County

Nature Management and Water Environment Division

14 June 2004

Informal Meeting of EU Water Directors, Dublin, Ireland 22–23 June 2004

Odense Pilot River Basin – Pressures, Impact and Risk Assessment of Odense Fjord

Harley Bundgaard Madsen, Head of Hydrology and Marine Department,
Fyn County, Denmark

Introduction

Odense River Basin encompasses an area of 1,050 km2, corresponding to approximately one third of River Basin District Fyn. The latter is administered by Fyn County as the competent authority designated in connection with Danish implementation of the Water Framework Directive.

Odense River Basin consists of 1,100 km of watercourses, 2,600 lakes and ponds, aquifers that supply drinking water to the population in the area (especially to Odense city) and Odense Fjord, into which the watercourses in the basin discharge. Odense Fjord is connected to the North Sea/Baltic Sea Ecoregion.

There are approx. 250,000 inhabitants in the river basin with the population being dominated by Odense city, which has approx. 180,000 inhabitants. Approx. 68% of the basin consists of farmland.

The project

The purpose of the Odense Pilot River Basin project is to test a number of EU guidance documents through the preparation of a provisional Article 5 report in 2003 and a pilot river basin management plan before the end of 2006.

The first phase of the project was completed by answering a questionnaire about the 12 guidance documents and through publication of the provisional Article 5 report “Odense Pilot River Basin. Provisional Article 5 Report pursuant to the Water Framework Directive”.

The report concludes that due to excessive pressures from agriculture, households and industry, a large proportion of the watercourses, lakes and groundwater bodies and Odense Fjord are at risk of failing to meet the WFD’s environmental objective of achieving “good status” in 2015.

As regards Odense Fjord, preliminary reference conditions have been defined for the quality elements eelgrass, macroalgae (Ulva lactuca), Secchi depth, nitrogen and phosphorus content.

The reference conditions are estimated on the basis of historical data dating back to 1900, empirical modelling and dynamic 3-D modelling. As regards the quality elements eelgrass, macroalgae (Ulva lactuca) and Secchi depth, a dynamic fjord model has been used to establish relationships between nitrogen loading of the fjord and the nitrogen concentration of the fjord water, the depth distribution of eelgrass, the biomass of undesirable macroalgae and Secchi depth. The reference conditions for macroalgal biomass in the fjord were determined using a reference nitrogen load (estimated from measurements of diffuse loading from Danish nature areas). A provisional operational objective was subsequently defined for “good ecological status” with regard to macroalgae as the reference state plus 50%. As regards the nitrogen concentration in the fjord water, the depth distribution of the eelgrass and the Secchi depth, “good status” is defined as a 15–25% deviation from the reference conditions. Based on the provisional criteria for “good ecological status”, mathematical modelling has been used to estimate the corresponding objective for the maximum acceptable nitrogen load. Applying the “one out, all out” principle, the most restrictive load has been compared with the current loading of the fjord. This showed that in order to meet the objective, loading will have to be reduced by about 50% relative to the current level. As the measures already planned are not expected to yield a reduction of this magnitude it is concluded that Odense Fjord is at risk of failing to meet the WFD’s environmental objective of good ecological status unless further measures are taken to reduce nitrogen loading.

As regards the watercourses, lakes and groundwater, more qualitative risk assessments have been made on the basis of comparisons of the current conditions and the criteria for “good status”.

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