CALCULATING THE LOAD OF ORGANIC-CARBON TO THE APHOTIC ZONE IN EUTROPHICATED COASTAL WATERS

The load of particulate organic carbon (POC) during lengthy time intervals to the aphotic zone and the bottom of coastal areas of the boreal North Atlantic can be expressed by the power model PE=0.049 · PT1.41 (n=16; r2=0.94), where PE is the sedimentation of POC out of the euphotic zone and PT is the total primary production. The load of POC to the aphotic zone increases, thus with the power of PT1.41 and can be calculated with high precision when PT is known and horizontal transport of water is of minor significance. The trend in annual PT estimates from the Kattegat was studied for a time interval of 40 years. Substantial increase of PT was recorded in the southern Kattegat and interpreted as a consequence of eutrophication. PE increased by about 140 and 250% during that time period. The increase in the POC flux to the aphotic zone is sufficient to reduce the oxygen concentration in a 8 m deep layer of bottom water to zero. The wide spread occurrence of low oxygen concentrations and anoxia can thus be easily explained by increased export of organic matter to the bottom layer due to eutrophication. Combined primary production and sedimentation measurements as well as the use of mathematical models for the estimation of the organic carbon load to the aphotic zone represent a new and promising approach to study, monitor, and control marine eutrophication. © 1990.