Modelling nutrient retention in the coastal zone of an eutrophic sea

The Swedish Coastal zone Model (SCM) was used at a test site, the Stockholm archipelago, located in the northern part of the central Baltic Sea, to study the retention capacity of the coastal filter on nitrogen (N) and phosphorus (P) loads from land and atmosphere. The efficiency of the coastal filter to permanently retain nutrients determines how much of the local nutrient loads actually reach the open sea. The SCM system is a nutrient-phytoplankton-zooplankton-detritus-type model coupled to a horizontally integrated, physical model in particular suitable for estuaries. In this study the Stockholm Archipelago, consisting of 86 sub-basins, was divided into three sub-areas: the inner, the intermediate and the outer archipelago. An evaluation of model results showed that the modelled freshwater supply agrees well with observations. The nutrient, salinity and temperature dynamics simulated by the SCM are also found to be in good or acceptable agreement with observations. The analysis showed that the Stockholm Archipelago works as a filter for nutrients that enter the coastal zone from land, but the filter efficiency is not effective enough to retain all the supplied nutrients. However, at least 65 and 72% of the P and N, respectively, are retained during the studied period (1990-2012). A major part of the retention is permanent, which for P means burial. For N, almost 92% of the permanent retention is represented by benthic denitrification, less than 8% by burial, while pelagic denitrification is below 1 %. Highest total amounts of P and N are retained in the outer archipelago, where the surface area is largest. The area-specific retention of P and N, however, is highest in the smaller inner archipelago and decreases towards the open sea. A reduction scenario of the land loads of N and P showed that the filter efficiencies of N and P increase and the export of N from the archipelago decreases. About 15 years after the reduction, the export of P changes into an import of P from the open sea to the archipelago.