Terrestrial organic matter quantity or decomposition state does not compensate for its poor nutritional quality for Daphnia

The importance of allochthonous organic matter contributions to aquatic food webs has long been debated. We studied the aging process of terrestrial particulate organic matter (tPOM) in aquatic ecosystems by incubating senescent plant material in the presence of natural microbial communities in either lake water or artificial growth medium. We hypothesised that the aging would change the biomarker composition of tPOM, and would improve its nutritional quality for Daphnia due to microbial conditioning. We also hypothesised that when algal availability is low, Daphnia can partially compensate for the low quality of tPOM when more of this resource is available to them. The fatty acid biomarkers of tPOM changed very little during short- and long-term incubations. In particular, the proportion of long-chain saturated fatty acids was quite stable even during long-term incubations. A decrease in carbohydrate and omega-3 polyunsaturated fatty acid content, and an increase in the bacterial and fungal lipid biomarkers indicated that microbial decomposition of tPOM took place during the incubation. The delta N-15 value of aged tPOM increased, which indicated microbial colonisation, while the delta C-13 value did not change during aging. Daphnia magna survival and reproductive output were low when fed diets containing fresh or aged tPOM. The reproductive output of Daphnia was slightly enhanced in the presence of aged tPOM as compared to fresh tPOM, but only when tPOM comprised >= 95% of the diet. When Daphnia were offered low (0.5 mg C/L) concentrations of algae, even high additions of tPOM (up to 15 mg C/L) did not increase Daphnia growth rates. Daphnia reproduction increased with tPOM additions, presumably due to the sterols and omega-3 polyunsaturated fatty acid in the tPOM that Daphnia retained from their diet. Our results indicate that tPOM is a poor resource for pelagic cladocerans, even when aged in the presence of microbes or offered in large quantities.