MULTIVARIATE-ANALYSIS OF THE PLANKTON COMMUNITIES IN THE LOOSDRECHT LAKES - RELATIONSHIP WITH THE CHEMICAL AND PHYSICAL-ENVIRONMENT

Data on the water temperature, Secchi depth, water chemistry and phytoplankton and zooplankton of the Loosdrecht lakes, collected during an eight-years monitoring study (1983-1990), were examined using Principal Components Analysis (PCA) and Redundancy Analysis (RDA). The Loosdrecht lakes are highly eutrophic lakes, in which several restoration measures were taken from 1970 to 1986. These measures included the diversion of sewage by the construction of sewerage and additional reduction of external phosphorus-loading by supplying from 1984 onwards water from the Amsterdam Rhine Canal, which had phosphorus removed from it, and which replaced the inlet of highly eutrophic water from the River Vecht. The descriptive statistical models based on this study show that, although the total P in the lake was decreasing, probably because of the reduced external loading, the sum of the densities of cyanobacteria and Prochlorothrix hollandica was increasing. The dominant species of the zooplankton community (rotifers and crustaceans) did not show changes related to the restoration measures. It is concluded that the available data are insufficient to predict future changes, but that the changes in the past can be described well by a simple linear ordination model.