The effect of riverine nitrate loads on winter concentrations in the Great Belt, Denmark

This paper presents a method that models the fraction of the NOx concentrations (nitrate and nitrite) in a marine recipient resulting from riverine runoff from land. The analysis is performed on the surface waters in the Great Belt, Denmark, during winter seasons. The method is based on simulation of two independent NOx fractions: First, the "background" fraction, which results from the oceanographic mixing in the Belt Sea between Kattegat water and Baltic Sea water, and second, the nutrient load from land. The calculation of the background fraction is based on the finding that the Kattegat water has high salinity and high NOx contents whereas the Baltic Sea water has low salinity and low NOx contents. By means of an empirical relation between salinity and NOx a specific salinity measurement in the mixing zone can be related to a "background" NOx concentration. Measurements in the Great Belt show to be higher than the calculated "background" concentrations, which indicates the presence of a second NOx fraction. The second fraction is defined as the difference between the calculated background concentration and the measured NOx concentration. The hypothesis of the present paper is that this second fraction is dominated by the river runoff. Cross correlation analysis between the time series of the river NOx load and the time series of surplus concentration in the Great Belt reveals a time lag between maximum river load and maximum surplus concentration during winter seasons of approx. 1 month. An empirical transfer function has been developed in order to connect the daily NOx river loads with the surplus concentration in the Great Belt. The development of the NOx-concentration in the Great Belt due to river run off is modelled for 8 specific winter periods of 3 months (December - February) between 1988/1989 and 1995/1996. The model simulates concentrations on a weekly basis and quantifies the NOx-increase during each winter based on the river load data. The analysis indicates that the NOx concentration can be doubled during winters with high runoff, giving rise to an increase of approx. 60 mu gN/l at the end of February. In winters with minimal runoff no significant increase above the background level is found.