Daphnia growth rates in Arctic ponds: limitation by nutrients or carbon?

Arctic organisms with annual life cycles experience a strong selective pressure to fulfill their life cycle at low temperatures within a short seasonal window. Yet, apart from low temperature, the factors that constrain or promote growth rates in high arctic systems are still poorly understood. A substantial part of the freshwater systems in the arctic consist of shallow, fish-free ponds with the crustacean Daphnia as the key grazer. This grazer has high demands for phosphorus (P) for RNA-synthesis and subsequently protein synthesis for growth. In this study, we compared growth of juvenile Daphnia that were fed seston from two high-Arctic (79°N) ponds on Svalbard in 2004, which differed strongly in P-content and C:P-ratios. In both ponds, Daphnia growth was limited by food quantity (carbon) rather than by P or N. The study also suggests that in absence of predators, infection level of epibionts might be an important factor regulating growth rate and population development of Daphnia growth in these systems.

Competition in natural populations of Daphnia

I investigated the competitive relationships between two species of Daphnia, D. galeata and D. cucullata, and their interspecific hybrid. The term hemispecific competition was introduced to describe competition between parental species and hybrids. In eutrophic Tjeukemeer both parental species were found to compete with the hybrid, whereas competition between D. galeata and D. cucullata seemed limited. Although the effect of competition on life history traits of daphnids may be profound, the influence of the competitors on the seasonal dynamics of the Daphnia species seems limited. [KEYWORDS: COEXISTENCE, HYBRID, COMPETITION, DAPHNIA, HEMISPECIFIC]

Seasonal succession of cyanobacterial protease inhibitors and Daphnia magna genotypes in a eutrophic Swedish lake

Lakes are well known for having a pattern of seasonal succession of phytoplankton and zooplankton. The succession of different taxa of phytoplankton results in a succession of zooplankton taxa, and within the genus Daphnia, into a succession of different genotypes (clones). One cause for this succession of Daphnia clones might be the production of digestive protease inhibitors by cyanobacteria, which usually bloom in summer. Here we report seasonal changes in the frequency and the abundance of Daphnia magna haplotypes in a eutrophic lake, which developed a chymotrypsin-inhibitor-producing cyanobacterial bloom in May. These seasonal changes were not related to changes of biotic and abiotic lake parameters. However, a very high content of chymotrypsin inhibitors was observed in May (but not in other months). This was assumed to have exerted a strong punctual selection pressure on the Daphnia population and on the direct targets of the protease inhibitors, i.e. the digestive chymotrypsins of Daphnia. Actually, D. magna from before and during the cyanobacterial bloom showed a different protease pattern on activity stained SDS-PAGE in comparison to clones from the month after the bloom. However, no difference in tolerance, measured as IC50 values, to inhibition by natural lake seston from May was found between the clones from before and after the bloom. Thus, the hypothesis that a seasonal adaptation of D. magna subpopulations from either April/May or June might have occurred could not be proven. This suggests that the Daphnia population investigated here is locally adapted to cyanobacterial protease inhibitors