Nutrient Dynamics in Southwestern Kattegat, Scandinavia: Modelling Transport, Budget and Consequences of Reduced Terrestrial Loads

Abstract

Geografisk Tidsskrift, Danish Journal of Geography 97: 1–10, 1997.

The nutrient transport through southwestern Kattegat was predicted for a 4 year period using the numerical, hydrodynamic model “MIKE12”. Model results were validated through direct measurements of nutrient concentrations in the study area, and through measurements of total-P and total-N deposition and resuspension rates. The local terrestrial supply of nutrients makes up only 2–5% of the total transport through the area, which was dominated by fluxes over Little Belt and Great Belt boundaries to the study area. A mass balance for 1990 showed that ∼30% of the external N load and ∼100% of the external P load was deposited in the sediment. This is in reasonable agreement with field measurements of yearly net deposition rates in the study area. Resuspension rates in the area were however much higher (on average 17 times) than net deposition rates.

Two scenarios were simulated for 1990, one with the actual local load of external nutrients and one with a local load reduction according to the national “Action Plan for the Aquatic Environment”. A comparison between the two scenarios showed that after reduction, nutrient concentrations dropped significantly (same magnitude as load reduction) in the inner parts of fiords close to the nutrient sources. In the open water parts of the study area the resulting decrease in nutrient concentrations (15% for N and 2–5% for P) was mostly manifested in the spring after high fresh water discharges.

Keywords:

• Numerical model
• nutrient budget
• load reduction
• Kattegat