Contrasting growth effects of prey capture in two aquatic carnivorous plant species

A detailed 11-day growth analysis was performed under greenhouse conditions on two aquatic, carnivorous plants in order to determine growth effects caused by supplemental feeding on zooplankton. The two species surveyed were the rare, stenotypic Aldrovanda vesiculosa and the more common, eurytopic Utricularia australis. While a highly significant increase in shoot biomass (by 60 %) was found for Aldrovanda plants supplemented with prey, no significant effect of prey addition occurred in Utricularia. However, main shoots of fed plants of both species were significantly longer, had more mature leaf nodes, and their mean apical growth rate was higher than in unfed plants. Branching was markedly supported by prey in Aldrovanda only. Feeding significantly increased the structural investment in carnivory in Utricularia but had no effect on Aldrovanda, although significantly increasing maximum trap length in both species. Measurements of total nutrient concentrations in fed Aldrovanda plants revealed much more total N, P, K, and Mg at the end of the experiment, compared with unfed plants. In Utricularia, however, fed plants contained only more N than unfed plants but the opposite was true for the total amount of P, K and Mg. In Aldrovanda, a large proportion of N, P, K, and Mg consumption could be covered from prey. The different growth effect of prey addition in the two species could be explained by the operation of a complex food web in Utricularia traps, which possibly substitute for prey capture in barren waters.