PARENT SESSION
Contributed Oral Session 10: Aquatic Trophic Systems I
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 519 B, Level 5, Palais des congrès de Montréal
Long-term phenological changes among Lake Washington zooplankton and match-mismatch with juvenile sockeye salmon.
Hampton, Stephanie*,1, Scheuerell, Jennifer2, Romare, Pia3, 1 University of Idaho, Moscow, ID, USA2 University of Washington, Seattle, WA, USA3 University of Lund, Lund, Sweden
ABSTRACT- Spatial and temporal overlap of juvenile fish with appropriate prey is vitally important for their growth and survival. In Lake Washington, juvenile sockeye salmon (Oncorhynchus nerka) strongly prefer highly profitable prey Daphnia over other zooplankton, switching uniformly and nearly completely to Daphnia at the threshold abundance of 0.4 Daphnia/L. Using long-term Lake Washington data and fry trap data from its major tributary, we examined 1) the factors that predict the increase of Lake Washington D. pulicaria and D. thorata to this threshold "switching" abundance in spring, 2) intra-annual and long-term trends in dynamics for each that may affect sockeye foraging patterns, and 3) long-term trends in the correspondence of timing for Daphnia threshold achievement with the arrival of sockeye fry to Lake Washington from the main spawning habitat. Threshold onset for D. pulicaria was easily predicted by winter pelagic abundance and basic parameters of spring conditions, indicating a greater reliance on pelagic dynamics rather than spring diapause hatch than D. thorata exhibited. Daphnia pulicaria appeared to be a more consistent and reliable food resource than D. thorata which displayed conspicuously flashy dynamics in all years. Thus the increase in D. thorata prominence in more recent years may be of concern, in that a switch to dominance by D. thorata could reduce the consistency of spring and summer diet for sockeye fry. Additionally, the timing of sockeye arrival to Lake Washington and daphnid increase to the switching threshold has become less concordant; fry in later years had to rely on less profitable, less preferred prey for longer periods of their development. Thus, long-term trends and species-specific differences in phenology of Daphnia may affect growth and survival of sockeye fry through altering diet composition, with additional implications for population and community dynamics of other, less preferred zooplankton taxa that withstand greater predation pressure in Daphnia's absence.
Key words: planktivory, ephippia, cladoceran