The mean concentrations of total phosphorus (P) and total nitrogen (N) in the polder lake Tjeukemeer during the period April - September (summer season) between 1968 and 1982 were 0.215 and 3.8 mg/l, respectively. These values exceeded those laid down by the Dutch water authorities for standard water quality. Though the mean summer chlorophyll-a (chl-a) concentration of 116 µg/l during 1968 - 1982 was comparable with that for the basic quality standard, the mean concentrations between 1977 and 1982 were from 1.3 to 2.1 as high.

Multiple regression analysis of the mean concentrations of total P and total N and climatological and hydrological data demonstrated that the P and N concentrations in Tjeukemeer in summer and in winter were significantly correlated with the deficit or surplus of water and consequently with the man-made regime, namely the inflow and outlet of water in SW Friesland.

Between 1979 and 1982 the ratios (w/w) of the mean concentrations of total N and total P and those of particulate N and particulate P in summer were less than 16 and 10, respectively, indicating that phytoplankton growth was N rather than P limited.

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