Can phaeopigments be used as markers for Daphnia grazing in Lake Constance?

Abstract

The formation of chlorophyll a degradation products was measured with natural phyto- plankton from Lake Constance and Daphnia magna and native Daphnia as grazers in grazing experi- ments during spring bloom conditions using high-pressure liquid chromatography (HPLC). Chlorophyll a start concentrations were between 1.2 and 16.3 ug 1'; phaeopigment weights constituted -5% of chlorophyll a weight. Only phaeophorbide a was a marker for Daphnia grazing; concen- trations of other phaeopigments (phaeophytin a, chlorophyllide a and two unidentified phaeo- pigments) did not increase during Daphnia grazing. Conversion efficiencies (chlorophyll a to phaeophorbide a) were between 0 and 43% on a weight basis, and between 0 and 65% on a molar basis. Conversion efficiencies were highest at high grazer density (40 Daphnia I1) and after a 24 h exposure time. Grazing by microzooplankton probably led to the formation of the two unidentified phaeopigments. In Lake Constance, Daphnia density was significantly positively correlated with the phaeophorbide a/chlorophyll a ratio when it was <5000 Daphnia m~3. However, when higher Daphnia densities were included in calculations, then Daphnia density was positively, but insignificantly, correl- ated with the phaeophorbide a/chlorophyll a ratio. This suggests that when the level of food per Daphnia is low, then grazing is more efficient with less production of phaeophorbide a and a higher production of colourless products.