Zustandsentwicklung schleswig-holsteinischer Seen zwischen 1983 und 1993: Eine Betrachtung unter Anwendung der Clusteranalyse |
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| Authors: | Gerald Schernewski Ulrike Schulz |
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| Institution: | Institut für Meereskunde der Universität Kiel, Germany |
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| Abstract: | In 1983 the regional authority in Schleswig-Holstein (North Germany) started their long term monitoring programme in 29 lakes. During the following years the number increased to 68 lakes in 1993. The monitoring programme includes the parameter carbon (DOC, TOC), nitrogen (NH4-N, NO2-N, NO3-N, TDN) phosphorus (PO4-P, TDP), conductivity and temperature. Water samples are taken only once a year during late winter overturn in the main outflow of every lake. In this study TOC, TDN and TDP are chosen as most independent variables by correlation tests. Average concentrations of the years 1983 to 1985 and 1992 to 1993 as well as the concentration difference between 1992/93 and 1983/85 are clustered using the Average Linkage Method. It is necessary to use averages of a few years to reduce effects of interannual variability.Between 1983/85 and 1992/93 the average TDP-concentration in all 29 lakes was reduced by 40% from 0.68 to 0.041 mg/l. TDN-concentrations declined by 13% from 2.6 to 2.3 mg/l. The strong reduction of phosphorus concentrations clearly reflect the success of local sewage purification during the last two decades. The high importance of diffuse input sources for lakes is the reason for the less pronounced decline in TDN concentrations.The increase in TOC-concentrations by 14% (from 9.3 to 10.6 mg/l) between 1983/85 and 1992/93 is no indicator for an increased average annual algae biomass in the lakes. It is a result of unusual algae blooms during the warm winters between 1989 and 1993.Beside some similarities, the dendrogram on the basis of 1983/85 data and the one using 1992/93 values show that a lot of lakes are located to different clusters. The classification on the basis of concentration differences (1992/93–1983/85) yield three larger groups of lakes with a similar development and 5 groups containing only one or two lakes with extraordinary changes in their properties during the decade. To a certain degree, TOC, TDN and TDP can be seen as trophic state indicators. During the decade most lakes have been subject to significant changes and some main cluster with different directions of the trophic development become obvious. On the other hand a considerable number of lakes show a very special behaviour.The results underline that cluster methods are a useful tool to discover and analyse different developments in lakes. The calculation of T-values on the basis of classification results permit the identification of most representative individuals for every group of lakes. Representatives are valuable for a detailed discussion of driving forces and future investigations. They allow to handle a reduced number of lakes with minimal information loss about the entity.Lake Belau is the best representative for the average TOC-, TDN- and TDP-changes in all lakes. Its changes in water qualtity during the decade is discussed in detail. The high interannual variability of Chlorophyll a-, TDP- and TDN-concentrations in this lake clearly reflect the problems of long term analysis on the basis of this very limited monitoring programme. |
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| Keywords: | Cluster analysis trophic state winter algae bloom nutrients annual variability |
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