Prey dominance shapes trophic structure of the northern California Current pelagic food web: evidence from stable isotopes and diet analysis

Eastern boundary current (EBC) upwelling zones are among the most productive of marine ecosystems globally and have been generalized in terms of their food web structure. Little empirically based evidence exists to suggest that there is any one form of trophic control of EBC systems and, because of logistical constraints, knowledge of food web structure is limited in these large marine ecosystems. To determine principal trophic links within the pelagic food web, we combined stable isotope and diet analysis of 21 dominant species of nekton found within the northern California Current, a highly productive EBC upwelling ecosystem. Samples of nekton and zooplankton were collected in June and August 2000 and 2002 from the shelf-slope waters of northern California to Washington. Results showed most nekton consumed mixed diets of zooplankton and larval-juvenile or adult fishes, indicating the importance of omnivory in this system. Euphausiids were overwhelmingly the most prevalent zooplankton taxa in the diets of nekton across multiple trophic levels, accounting for >50% of the index of relative importance in 10 of 21 nekton species analyzed. Nitrogen stable isotopes (delta N-15) reflected trophic patterns observed in the diets, with most nekton species (similar to 62%) falling between trophic levels (TL) of full zooplanktivores (TL = 3) and full piscivores (TL = 4). The prevalence of omnivory observed here is probably due to high primary production that supports an abundance of large zooplankton prey, such as euphausiids, which may be a more available prey resource for a broad range of predators. A modification of this form of bottom-up control, termed bottom-up omnivory, is presented.