Gut passage of phosphorus-limited algae through Daphnia: do they take up nutrients in the process?

Nutrient-limited algae are known to be a food source of inferior quality for zooplankters. Three factors are thought to determine this poor quality: direct elemental limitations of the algae, biochemical limitations and an increased resistance to digestion because of an increase in cell wall thickness. Thus far, most studies have concentrated on the effect of the algae on the daphniids. It has recently been hypothesized, however, that while going through the digestive tract of herbivorous zooplankters the digestion resistant nutrient-limited algae might actually take-up nutrients, in a similar way as it has been described for gelatinous alga such as Sphaerocystis. In this study, we present results of different experiments investigating whether nutrient-limited algae are indeed more resistant to digestion, and whether nutrient-limited algae take-up the limiting nutrient in the guts of their predators. We observed that digestion resistance is not very important, and that it can only be observed at high food levels. As a result, we could not find any evidence for nutrient uptake of these algae when they pass through the daphniids. We did find that animals adapted to low-P environments have a higher incorporation efficiency for P, and conclude that digestion resistance in nutrient stressed algae is of very limited ecological relevance.