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 degrees 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.