ABSTRACT
Improved sewage treatment (biological treatment and P removal) was introduced during the period 1968-73. Hereafter, the P load decreased significantly from 3.3 t day−1 to 0.8 t day−1. An improved water quality in the inner part of the Stockholm Archipelago and a decreased volume of sulphur bacteria (Beggiatoa spp.) mats were later recorded (Brattberg 1986). The decreased P concentration and the enhanced N : P ratio were considered to be the most essential factors responsible for this improvement. Contrary to P, the input of N continued to increase gradually in spite of the improved sewage treatment and was estimated at that time to be 25 t day−1. Consequently, P replaced N as the most limiting nutrient in this area (Brattberg 1986).
