Lipid content and stable isotopes of zooplankton during five winters around the northern Antarctic Peninsula

The U.S. Antarctic Marine Living Resources (AMLR) Program conducted five winter surveys (August - September 2012 - 2016) around the northern Antarctic Peninsula and South Shetland Islands. A description of the survey area and environmental and zooplankton sampling methods can be found in Walsh et al. (2020). Briefly, we surveyed a grid of 110 historical fixed stations around the South Shetland, Elephant, and Joinville Islands. We conducted a Conductivity-Temperature-Depth (CTD) cast and an Isaacs-Kidd Midwater Trawl at each station. Upper mixed layer (UML) depth was defined according to Mitchell and Holm-Hansen (1991). UML temperature and salinities were defined to be means over the depth range of the UML. Water samples from CTD casts were processed for chlorophyll-a (integrated to 100 m, mg chl-a m-2) according to Holm-Hansen et al. (1965) and Holm-Hansen and Riemann (1978). We defined daylight conditions according to three categories. Day (D) was defined as one hour after local sunrise to one hour before local sunset; night (N) was defined as one hour after sunset to one hour before sunrise; and Twilight (T) was defined as one hour before and after sunrise and sunset. Sea-ice percent cover and type were classified according to a modification of the standardized visual approach from the Scientific Committee on Antarctic Research Antarctic Sea Ice Processes and Climate Program (ASPeCT) (Worby and Allison 1999). We extracted lipid from zooplankton according to a modified approach from Folch et al. (1957) as described by Budge et al. (2006). We analyzed lipid-free zooplankton for stable isotopes at the University of California Davis Stable Isotope Facility (https://stableisotopefacility.ucdavis.edu/ ). We also analyzed a subset of whole-animal zooplankton to address the issue of skewed nitrogen stable isotope values resulting from lipid extraction. References: Budge, S.M., Iverson, S.J. & Koopman, H.N. Studying trophic ecology in marine ecosystems using fatty acids: A primer on analysis and interpretation. Mar. Mammal Sci. 22, 759-801 (2006). Folch, J., Lees, M. & Sloane Stanley, G. A simple method for the isolation and purification of total lipides from animal tissues. J. Biol. Chem. 226, 497-509 (1957). Holm-Hansen, O., Lorenzen, C.J., Holmes, R.W. & Strickland, J.D.H. Fluorometric determination of chlorophyll. J. Cons. perm. int. Explor. Mer. 30, 3-15 (1965). Holm-Hansen, O. & Riemann, B. Chlorophyll a determination: improvements in methodology. Oikos 30, 438-447 (1978). Mitchell, B.G. & Holm-Hansen, O. Observations and modeling of the Antarctic phytoplankton crop in relation to mixing depth. Deep-Sea Res. 38, 981-1007 (1991). Walsh, J., Reiss, C.S. & Watters, G.M. Flexibility in Antarctic krill Euphausia superba decouples diet and recruitment from overwinter sea-ice conditions in the northern Antarctic Peninsula. Mar. Ecol. Prog. Ser. 642, 1-19 (2020). Worby, A. & Allison, I. A technique for making ship-based observations of Antarctic sea ice thickness and characteristics. Part I: observational technique and results. Research Report No. 14. Antarctic Cooperative Research Center, Hobart, Tasmania (1999).