ABSTRACT
Besides the increased abundance and biomass, the zoobenthic communities also showed an altered community composition between the 1970s and the 1990s. The increased biomass was mainly because of increased abundance of Macoma balthica. A shift in relative importance of species was evident for stress-tolerant taxa, such as chironomid larvae and M. balthica, which were more abundant in the 1990s, while the abundance of Monoporeia affinis had decreased (Bonsdorff et al. 1991). In the 1990s the benthic communities were dominated by Macoma balthica and Chironomidae in the inner archipelago zone, whereas M. balthica and Monoporeia affinis were dominant in the middle archipelago. The distribution of M. affinis had moved further towards the open sea areas, where they were often found to be the dominant species (Bonsdorff et al. 1990, 1991, 1996). About 40% of the species composition had changed since the 1970s, and a shift from suspension feeders to deposit feeders had occurred, indicating functional disturbances as well (Bonsdorff & Blomqvist 1993, Bonsdorff et al. 1997a,b). A reinvestigation of the entire area in the year 2000 revealed a decrease in abundance and biomass, corresponding to the reduction in nutrient loading and benthic hypoxia (Nummelin 2000, Perus et al. 2001). The species composition had not changed, however, indicating that the previously noted functional shifts have persisted in this system.
