Nutritional value of algae: a critical control on the stability of Daphnia-algal systems

In Daphnia–algal systems, the effect of nutrient enrichmenton stability is an important ecological issue. Here I considera system of Daphnia and two potential prey; one prey termedprimary algae, which are preferentially consumed, and the othersecondary algae, which yield less nutrition and are more resistantto the grazer. The presence of secondary algae is a key to thestability, but their nutritional value has not been clearlydefined in the previous theory and the actual value varies.Here I use a simple mathematical model defining explicitly thenutritional values of algae and examine the stability of thesystems as a function of phosphorus enrichment. Whether or notall three species can stably coexist depended on the combinationof the algal species used for simulation. In systems where allthe species coexist in a stable manner, in which enrichmentdoes not necessarily lead to destabilization, there is alwaysa critical nutritional value of the secondary algae. Empiricalwork supports the possibility that the unknown nutritional valueof secondary algae takes a value close to the critical one.Furthermore, at the critical nutritional value, the populationresponse in the systems to enrichment is consistent with theobserved trend in natural systems. This suggests that Daphnia–algalsystems in nature can maintain stability in the face of enrichment,without requiring specific assumptions such as spatial heterogeneity.