Abstract Seasonal variation in phytoplankton biomass imposes strong selection pressures on planktonic herbivores such as Daphnia, with both low and high food availability shaping life-history strategies. While the threshold food level (TFL) has been widely studied, the incipient limiting level (ILL)—the food concentration above which growth or reproduction no longer increases—remains poorly characterized at the intraspecific level. This study aimed to quantify clonal variation in ILL among four genetically distinct Daphnia magna clones originating from pond and lake habitats. Individuals were reared along a gradient of algal concentrations under standardized laboratory conditions, and somatic growth rate and intrinsic rate of increase were calculated. Results showed substantial among-clone variation in ILL for somatic growth rate, spanning ~ 0.7–1 mg CorgL−1. Similar patterns were observed for population growth rates, with one clone requiring significantly higher food levels (~1.3 mg CorgL−1) to achieve maximum fitness. These findings challenge the general assumption of a universal ILL and suggest that its clonal variation is generated by local adaptation to food regimes. Incorporating clone-specific ILLs into ecological models may improve predictions of population dynamics and community composition under fluctuating trophic conditions.
Bednarska et al. (Thu,) studied this question.