Oxygen, nutrients, carbon and water exchange in the Skagerrak Basin

Abstract

The mean oxygen depletion rate at 600 m depth in the Skagerrak basin water was 0.04 ml l −1 per month between 1955 and 1991 and in the mean oxygen minimum concentration 5.6 ml l −1. No indication of long-term increase in oxygen depletion rate or decrease in oxygen minimum concentration was observed. The average period between major inflows to the Skagerrak Basin was about 25 months. The extra low oxygen minimum level observed in 1990 (4.8 ml l −1) was explained by the prolonged residence time (40 months) of the basin water between 1987 and 1991. The volume mean oxygen consumption rate ( CONS) was 0.055 ml l −1 per month during the stagnation period 1987–1991 and the volume mean oxygen depletion rate ( DEPL) contributed about 75% to CONS. The vertical diffusive flux of oxygen ( DIFF) into the basin water then reduced the oxygen depletion rate with about 25%. The total supply of particulate carbon below the 350 m level was estimated to about 430 000 ton y −1 (43 ton km −2 y −1), the total nitrogen supply about 80 000 ton y −1 and phosphorus 11 000 ton y −1. Assuming that the organic matter is composed according to Redfield ratios, we also found that a substantial denitrification process must take place in the bottom sediments (30–50%). The main source of organic matter to the Skagerrak Basin Water seems to be the North Sea.