Effects of dietary phosphorus deficiency on the abundance, phosphorus balance, and growth of Daphnia cucullata in three hypereutrophic Dutch lakes

We studied population dynamics, phosphorus balance, and individual growth to evaluate limitation of Daphnia cucullata by dietary phosphorus (P) in three interconnected, hypereutrophic Dutch lakes. Consistent with the P-limitation hypothesis, declines in egg production and population density coincided with seasonal increases in the seston C:P ratio from 300 to 400-500 (molar) in all three lakes. Adult Daphnia collected from Lake Breukeleveen exhibited a linear increase in specific P content during the first 36 h of feeding on a P-rich diet in the laboratory (r(2) = 0.99), which confirms that field Daphnia experienced P deficits in their tissues. Laboratory experiments tested the growth of juvenile D. cucullata in controls with natural seston and in seston supplemented with phosphate or the P-rich cyanobacterium Synechococcus. Growth was also estimated in filtered lake water with Scenedesmus (2 mg C L-1) as food. This design allowed us to partition differences between the control and Scenedesmus growth rates into limitation by phosphorus, energy and residual factors. In each of 10 experiments, D. cucullata exhibited improved growth in response to the phosphate and Synechococcus supplements. In agreement with predictions, Daphnia exhibited the lowest control growth rate and the strongest response to P addition in seston from the lake with the highest seston C:P ratio. However, animals in seston from two lakes with very similar seston C:P ratios differed in their growth responses. Our results confirm dietary P limitation but show that energy limitation and residual factors also contributed to between-lake differences in Daphnia growth.