Assessing Recent Eutrophication in Coastal Waters of the Gulf of Finland (Baltic Sea) using Subfossil Diatoms |
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Authors: | Kaarina Weckström |
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Institution: | (1) Department of Biological and Environmental Sciences, Division of Aquatic Sciences, University of Helsinki, P.O. Box 65, FIN-00014, Finland |
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Abstract: | Marine eutrophication of estuaries and coastal waters is considered to be a significant problem worldwide. In the semi-enclosed
Baltic Sea, where the nutrient load has strongly increased from its natural level, this has led to marked changes in the coastal
ecosystems. A key to successful management of coastal waters is reliable scientific evidence of their past state. The palaeolimnological
record of subfossil diatoms was used to study the rate and magnitude of eutrophication over the last ca. 200 years in two
urban and three rural sites. The urban sites showed marked increases in the percentage abundance of planktonic diatoms (from
<50 to ca. 90% and from <5 to ca. 70%) and diatom-inferred total dissolved nitrogen (from <800 to ca. 3000 μg l−1 and from <400 to ca. 800 μg l−1), and a decrease in species richness starting in the 19th – early 20th century with increased urbanisation. At both sites
a clear recovery was observed after the cessation of waste water loading by the mid 1980s. The present planktonic diatom assemblages
of these embayments, however, show no change back to the pre-disturbance diverse benthic communities. In contrast, the changes
observed in the rural sites were only moderate and occurred later starting in the 1940s. No marked increases in diatom-inferred
total dissolved nitrogen were seen, however, all sites showed an increase in small planktonic taxa (from ca. 1–6% to 8–36%)
indicating increased nutrient enrichment and turbidity. These small floristic changes could be seen as an early warning signal
despite little change in the inferred nutrient concentrations. The results have implications for the European Water Framework
Directive, which requires European surface waters to be of good ecological status, defined both by biological and chemical
quality elements. |
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Keywords: | Baltic Sea Diatoms European Water Framework Directive Eutrophication Species richness Transfer function |
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