JPR Advance Access originally published online on February 2, 2006
Journal of Plankton Research 2006 28(4):399-406; doi:10.1093/plankt/fbi125
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Environmentally controlled Daphnia spring increase with implications for sockeye salmon fry in Lake Washington, USA
1 University of Idaho, Department of Fish and Wildlife, Moscow, ID 83843, USA, 2 University of Washington, School of Aquatic and Fishery Sciences, Seattle, WA 98105, USA, 3 University of Lund, Department of Limnology, Lund, Sweden and 4 Washington Department of Fisheries and Wildlife, 600 Capitol Way North, Olympia, WA 98501, USA
* Corresponding Author: shampton{at}uidaho.edu
Received October 16, 2005; accepted in principle December 14, 2005; accepted for publication January 26, 2006; published online February 2, 2006
Communicating editor: K.J. Flynn
In Lake Washington, juvenile sockeye salmon (Oncorhynchus nerka) strongly prefer Daphnia over other prey, switching uniformly to Daphnia when the threshold abundance of 0.4 Daphnia L–1 is achieved. Using long-term Lake Washington data (1978–2001) and fry trap data (1992–2001) from a major tributary, we examined the following: (i) factors that predict Daphnia pulicaria and Daphnia thorata increase to this threshold "switching" abundance, (ii) trends in Daphnia dynamics that may affect sockeye foraging and (iii) temporal correspondence of Daphnia increase and fry arrival. The winter abundance of D. pulicaria, in combination with basic parameters of spring conditions, was an important predictor of the date of D. pulicaria spring increase, indicating greater reliance on pelagic population dynamics (versus diapause hatch) than D. thorata exhibited. In addition, D. pulicaria was a more consistent prey than D. thorata, the latter exhibiting larger population fluctuations. Thus, recently increasing D. thorata prominence could decrease diet consistency for sockeye fry. Additionally, the timing of sockeye arrival to Lake Washington and Daphnia’s increase to the switching threshold has become less concordant, so that fry in recent years have had to rely upon less profitable prey for longer periods. Long-term trends and species-specific differences in Daphnia phenology may affect fry through altering diet composition, with additional implications for other zooplankton withstanding greater predation pressure in Daphnia’s absence. Recent decades of warming in Lake Washington are consistent with the warming of lakes worldwide, and complex phenological responses such as those reported here may be common as the climate continues to change.