|
Cultural eutrophication from over-fertilization of coastal waters by humans has adversely affected ecosystems worldwide. Scandinavian examples include negative effects on fisheries in the Baltic, Swedish coast, and the inner Oslofjord during the 1950's and 60's. Conditions in the Oslofjord improved with improved sewage treatment, but the fishery still has not recovered, and there is concern for Norwegian coastal waters, given the importance of fisheries and aquaculture. The southern and western coasts are particularly vulnerable; fjords with restricted water circulation make it difficult to reverse negative trends in dissolved oxygen, once started, and the coastal current distributes harmful algal blooms from the Skagerrak for long distances along the coast. Identification of past, present, and future cultural eutrophication is one of the critical requirements for the safe and sustainable management of coastal waters. However, eutrophication is a process of change, often over decades, and it is difficult to demonstrate its development until critical thresholds are exceeded, due to lack of long enough records of environmental data. There is therefore a need for new methods to identify the extent to which a given site is affected by cultural eutrophication, and to estimate how much nutrient loading the site can sustain before causing environmental problems.
Here, we present results from the south coast of Norway aimed at developing such methods by integrating environmental micropaleontological data with some of the few available long-term fisheries data. Scientists at the Institute of Marine Research in Flødevigen have for the past 80 years collected environmental data (e.g. dissolved oxygen, temperature, salinity, nutrients and fish abundance) for sites in the Skagerrak and the adjoining coast. They show a general regional increase in nutrients and lowered oxygen content over time, and the progressive collapse of local fisheries in various fjord systems at different times, starting with the inner Oslofjord in the 1950's and including the latest example from the 1990's.
Eutrophication was suggested as a possible cause. At the University of Oslo we previously identified eutrophication signals in assemblages of fossilizeable dinoflagellate cysts in cored bottom sediments from the inner Oslofjord and several other fjords with histories of eutrophication. Analysing for these signals in cyst records in well-dated sediment cores from sites studied by the fisheries scientists, we were able to check the postulated link between cultural eutrophication and the collapse of local fisheries. Cyst records strongly supported this link - maxima in the eutrophication signals coincided with the dates of the observed fisheries collapse in all four systems studied. These encouraging results demonstrate the potential for developing dinoflagellate cysts as indicators for use in «state of the environment» assessments of cultural eutrophication.
|
