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According to modern oceanographic perspectives that emphasize microbial pathways, phagotrophic protists comprise one to several levels of intermediate consumers between phytoplankton and larger metazooplankton (copepods and krill). However, recent attempts to quantify pelagic trophic structure in the open ocean using nitrogen stable isotope techniques have brought into question whether such measurements adequately account for protistan trophic steps. Here, we use a two-stage chemostat system, with Dunaliella tertiolecta and Oxyrrhis marina as a predator-prey model, to address this question experimentally. To investigate 15 N trophic discrimination under different conditions of nitrogen availability and recycling, Oxyrrhis was fed in the light and in the dark on phytoplankton provided with high and low nutrient ratios of N : P. We used both bulk and amino acids-compound specific isotopic analysis (AA-CSIA) to distinguish trophic fractionation from changes in the d 15 N values of phytoplankton (isotopic baseline). Results demonstrate that protistan consumers are not, in fact, significantly enriched in 15 N relative to their prey, a marked departure from the general findings for metazoan consumers. In addition, we show that changes in the isotopic baseline propagate rapidly through the protistan food chain, highlighting the need to account for this variability at ecologically relevant time scales. If protistan trophic steps are largely invisible or significantly underestimated using nitrogen isotope measurements, research that utilize such measurements in ecological, fisheries, and climate change studies may miss a large part of the ocean's variability in food-web structure and ecosystem function.

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Nitrogen isotopic compositions of zooplankton in the California Current Ecosystem (CCE) are known to vary over inter-annual scales of climate variability, but the extent to which those changes are driven by variations in baseline phytoplankton d 15 N values vs. zooplankton trophic position (TP) is poorly resolved. We use field samples collected during a large natural environmental perturbation, the 1998-1999 alternation between El Niñ o and La Niñ a states, to test the ability of large dominant CCE zooplankton Euphausia pacifica and Calanus pacificus to alter their TPs in response to environmental variability. To distinguish trophic changes from variations of d 15 N values at the base of the food web, the zooplankton were assayed by Compound Specific Isotope Analysis of Amino Acids (CSIA-AA). Linear mixed-effect models were developed to utilize data from all amino acids (AAs), providing greater statistical power than the typical CSIA-AA approach of using only phenylalanine and glutamic acid. We confirm a significant 15 N enrichment of , 2% at the base of the food web for all AAs and all zooplankton groups during the 1998 El Niñ o. This baseline enrichment in 15 N has been speculated to occur during El Niñ o events but never conclusively shown. We also demonstrate a significantly elevated TP, implying increased carnivory during 1998, for E. pacifica while C. pacificus did not alter their TP between years. Lastly, TPs calculated from the standard CSIA-AA equation with laboratory-derived constants gave unrealistically low estimates, suggesting an assessment of these variables in situ is needed for an accurate application in natural systems.