Die Entwicklung und Männchenbildung derDaphnia-Population im Bodensee-Obersee 1956–1980

Besides the old-settledDaphnia hyalina a ‘galeata’-type ofDaphnia immigrated into Lake Constance (Upper Lake) in the beginning of the fifties. Since this time a great number of phenotypes with a high degree of morphological variations was registrated. Since about 1970 a smallDaphnia type became more and more numerous, especially in the eastern part of the lake. An exact determination is not yet possible. In 1972 an extreme maximum ofDaphnia males was found. The share of males increased from 3 to 4% of the population to 30% in autumn 1972. In the following years the percentage decreased again to the formerly observed 3–4%. The relationship to the phytoplankton biomass is discussed.