The history of eutrophication in the sea area of Helsinki in the 20th century. Long-term analysis of plankton assemblages.

The aim of this study is to present a historical continuum of nearly 100 years of environmental investigations and changes in trophic levels in the sea area of Helsinki. The survey is based on a re-examination of original planktological and meteorological data; the principle being that eutrophication can be detected in plankton assemblages despite changes in methodology. The bays around Helsinki were found to be moderately eutrophic to hypereutrophic at the beginning of the 20th century. There were considerable differences in the development of the bays both in terms of degradation and recovery. The efficiency of wastewater treatment and especially the introduction of sea outfall have played an important role in decreasing eutrophication in the bay area of Helsinki.