Differing nutritional value and assimilation rates of terrestrial diets by the herbivorous zooplankton <i>Daphnia magna</i>

Abstract Subsidies from surrounding land areas can be important sources of aquatic food webs in shallow regions of streams and lakes. We measured nutritional value (elemental ratios, fatty acids, sterols, amino acids, energy content) of terrestrial tree leaves, i.e. birch (Betula sp.), aspen (Populus tremula) and alder (Alnus sp.), in comparison with green algae (Chlamydomonas sp.), for supporting a primary consumer, Daphnia magna. The overall nutritional value of biofilm on decomposing leaves, leaf leachates with microbes and leaf particles was lower than that of algae. Most terrestrial-origin diets could support the survival of Daphnia, but biomass growth and reproduction were minor compared with those of the green algal diet. Although fungi in the biofilm community could provide sufficient sterols, microbial processing of terrestrial leaves did not markedly increase Daphnia biomass. Moreover, aspen leachates seemed to be toxic to Daphnia. Tests with liposome supplements indicated that Daphnia growth was not primarily limited by the lack of cholesterol and/or eicosapentaenoic acid in terrestrial diets, but by energy availability. Tree leaves provided two to four times less bioavailable energy than the algal diet. Our findings suggest that Daphnia can survive on terrestrial subsidies, but achieve effective population growth when feeding mainly on algal diets.