Interannual variability in the Northern California Current food web structure: Changes in energy flow pathways and the role of forage fish, euphausiids, and jellyfish
The Northern California Current (NCC) is a seasonally productive and open ecosystem. It is home to both a diverse endemic community and to seasonally transient species. Productivity and food web structure vary seasonally, interannually, and decadally due to variability in coastal upwelling, climate-scale physical processes, and the migratory species entering the system. The composition of the pelagic community varies between years, including changes to mid-trophic level groups that represent alternate energy-transfer pathways between lower and upper trophic levels (forage fishes, euphausiids, jellyfish). Multiple data sets, including annual spring and summer mesoscale surveys of the zooplankton, pelagic fish, and seabird communities, were used to infer NCC trophic network arrangements and develop end-to-end models for each of the 2003-2007 upwelling seasons. Each model was used to quantify the interannual variability in energy-transfer efficiency from bottom to top trophic levels. When each model was driven under an identical nutrient input rate, substantial differences in the energy available to each functional group were evident. Scenario analyses were used to examine the roles of forage fishes, euphausiids, and jellyfish (small gelatinous zooplankton and large carnivorous jellyfish) as alternate energy transfer pathways. Euphausiids were the more important energy transfer pathway; a large proportion of the lower trophic production consumed was transferred to higher trophic levels. In contrast, jellyfish acted as a production loss pathway; little of the production consumed was passed upwards. Analysis of the range of ecosystem states observed interannually and understanding system sensitivity to variability among key trophic groups improves our ability to predict NCC ecosystem response to short- and long-term environmental change.