A LONG-TERM RECORD OF HUMAN IMPACTS ON AN URBAN ECOSYSTEM IN THE SEDIMENTS OF TOOLONLAHTI BAY IN HELSINKI, FINLAND

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. Toolonlahti Bay, in the centre of Helsinki, Finland, provided on opportunity to characterize recent h uman impacts especially by means of chemical and biostratigraphical an alyses of a sediment core. Periods of coniferous forest, forest cleara nce, urbanization and the development of parks, can be distinguished i n the pollen record of the core. Palynological diversity was highest b efore the forest clearance at the turn of the century. The character o f the sediment and the water have changed substantially in response to rapid population growth, the construction of sewage systems and build ing within the catchment of the bay. This is reflected in marked incre ases in organic matter, phosphorus and heavy metal (Cd, Cu, Ni, Zn and Pb) concentrations between 1890 and 1960, accompanied by a rapid incr ease in diatom species indicative of eutrophication and a decline in d iatom species diversity. Since the cessation of waste-water disposal i n the 1960s, concentrations of a number of pollutants have declined an d water quality has gradually improved, but conditions are still affec ted by internal and atmospheric loadings. As a consequence of land upl ift (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 su ch as the Finlandia and the Opera Houses, will fill with sediment unle ss it is dredged.