Marked early 20th century pollution and the subsequent recovery of T��l� Bay, central Helsinki, as indicated by subfossil diatom assemblage changes

Abstract

Diatoms were used to infer the trophic status history of a Baltic Sea inlet, Toolo Bay, located in central Helsinki. The waterbody received partially treated sewage effluent from the local sewage works and from a sugar factory from ca. 1890 until ca. 1960 when primary sewage treatment and the diversion of the effluent began. The increased nutrient input is shown in the diatom stratigraphy in the form of a rapid increase in planktonic eutrophic forms particularly Cyclotella cf. meneghiniana, and a reciprocal destruction of the periphytic flora. The overall diversity decreased markedly at this time. There is a gradual recovery of the system to the predisturbance conditions from 1960 onwards, as indicated by the re-emergence of the mesotrophic periphytic diatoms. Detrended correspondence analysis (DCA) is used to summarize the responses of the diatom assemblages to changing nutrient concentrations.