Importance of sympagic production to Bering Sea zooplankton as revealed from fatty acid-carbon stable isotope analyses

We analyzed the fatty acid (FA) composition and carbon stable isotope ratios of indi- vidual FAs (δ 13 CFA) of 3 zooplankton species (Themisto libellula, Calanus marshallae/glacialis, and Thysanoessa raschii) sampled from the Bering Sea during winter maximum ice extent, spring ice melt, and summer ice-free conditions in 2009 and 2010. Our goal was to assess diets of these eco- logically important species and estimate the proportional contribution of pelagic and sympagic carbon sources to their diets. FA profiles showed little variation in diet within species between ice conditions or years but revealed differences in diet among species. FA biomarkers confirmed that T. libellula was predominately carnivorous and that C. marshallae/glacialis and T. raschii were primarily herbivorous. Estimates from 4 stable isotope mixing models using combinations of δ 13 CFA values of diatom FA markers (16:1n-7, 20:5n-3), and a flagellate FA marker (22:6n-3) showed that substantial, albeit highly variable, proportions of these FAs originated from organic matter originating from sea ice algae (T. libellula 36 to 72%, C. marshallae/glacialis 27 to 63%, and T. raschii 39 to 71%). Our results suggest that ice algae may be an important food source for zooplankton when water column phytoplankton are not available during critical periods in their life history. Predicted increases in water column phytoplankton production in the Bering Sea may help offset the expected reduction in ice algal production and any detrimental effects that this might have on consumers such as zooplankton.