Zooplankton trophic structure and ecosystem productivity

Abstract

The number of trophic steps within a plankton food web plays an important role in determining the energy available to support higher-level consumers by affecting trophic transfer efficiency (TE): fewer steps can enhance TE by decreasing respiration and predation losses. In this study, trophic structure within the zooplankton community was investigated using stable isotopes in size-fractionated mesozooplankton, and related to 2 biomass proxies related to TE: the normalized biomass size spectra (NBSS) and the ratio of zooplankton:phytoplankton biomass (log10 (zoo:phyto)). Four regions were compared: the California Current Ecosystem (CCE - most productive), the Equatorial Pacific (EqP), the Costa Rica Dome (CRD) and the North Pacific Subtropical Gyre (NPSG - least productive). Compound-specific isotope analysis of amino acids confirmed large differences (~3‰) in the isotopic baseline among ecosystems. EqP and NPSG had low and distinct source δ15N values, while CRD/CCE had high and overlapping values. Trophic differences indicated that the CCE had the lowest number (0) of trophic differences within the 4 zooplankton size classes; NPSG and EqP had the highest number (3), and CRD was intermediate (1). NBSS slopes confirmed the CCE and NPSG as extremes and statistically different from each other. TE patterns estimated from log10(zoo:phyto) suggested EqP was the least efficient, while the other 3 ecosystems (despite large ranges in zooplankton and phytoplankton biomass) had similar TEs. The inverse relationship between food chain length and system productivity, a paradigm originally formulated for microbial food webs, holds for the mesozooplankton assemblage at the productivity extremes.