A long-term record of human impacts on an urban ecosystem in the sediments of Töölönlahti Bay in Helsinki, Finland

Abstract

Ecological impacts of urbanization are receiving increasing scientific attention, yet few data sets permit long-term effects on terrestrial and aquatic ecosystems to be assessed. Töölönlahti Bay, in the centre of Helsinki, Finland, provided on opportunity to characterize recent human impacts especially by means of chemical and biostratigraphical analyses of a sediment core. Periods of coniferous forest, forest clearance, urbanization and the development of parks, can be distinguished in the pollen record of the core. Palynological diversity was highest before the forest clearance at the turn of the century. The character of the sediment and the water have changed substantially in response to rapid population growth, the construction of sewage systems and building within the catchment of the bay. This is reflected in marked increases in organic matter, phosphorus and heavy metal (Cd, Cu, Ni, Zn and Pb) concentrations between 1890 and 1960, accompanied by a rapid increase in diatom species indicative of eutrophication and a decline in diatom species diversity.

Since the cessation of waste-water disposal in the 1960s, concentrations of a number of pollutants have declined and water quality has gradually improved, but conditions are still affected by internal and atmospheric loadings. As a consequence of land uplift (2 mm per year) and the rapid sedimentation rate (6 mm per year), the volume of the bay is decreasing. Within 200 years, the shallow bay, which is skirted by extensive parks and famous cultural buildings such as the Finlandia and the Opera Houses, will fill with sediment unless it is dredged.