Assessing of long-distance background nutrient loading to the Archipelago Sea, northern Baltic, with a hydrodynamic model

Nutrient concentrations have increased in the Archipelago Sea during recent years. This is probably caused by growth in the loads from agriculture, air, fish farming and the Baltic proper. The Archipelago Sea acts Like a nutrient filter between the Baltic proper and the Bothnian Sea; part of some of the externally input nutrients are retained within this area. Previously only rough estimates of the amount of the background load have been given. In this study the amount of net nutrient inflow from the Baltic proper was estimated with the aid of a three-dimensional hydrodynamic model for the years 1993-1997. Our model suggests that the role df background nutrient load from the Baltic proper may sometimes be decisive in the eutrophication process. For example, in June 1993 the background load of phosphorus was 300 000 kg in one month, i.e. about threefold compared to the total phosphorus load from fish farming during this whole year. Nutrient inflow varies a lot during open water seasons and therefore more detailed studies are highly recommended. If all other local loading sources were included in the dynamic nutrient balance model we could calculate actual nutrient concentrations at any given time in the Archipelago Sea. (C) 1998 Elsevier Science Ltd. All rights reserved.