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

Citation

Walsh, Jennifer; Reiss, Christian (2020), Lipid content and stable isotopes of zooplankton during five winters around the northern Antarctic Peninsula, Dryad, Dataset, https://doi.org/10.5061/dryad.hhmgqnkdx

Abstract

The Southern Ocean zooplankton community is diverse, yet most species are understudied, especially with respect to their overwinter feeding ecologies. This dataset describes body condition and trophic biomarker data (lipid content and stable isotopes of carbon and nitrogen) from 19 zooplankton species collected over five consecutive winters (August and September 2012 – 2016) around the northern Antarctic Peninsula. To complement these data and provide context for interpretation, we report environmental data (percent sea-ice cover, sea-ice type, water temperature, salinity, and integrated chlorophyll-a (to 100 m)) as well as species abundance data at each sampling location. For most species, these are the first winter measurements or time series of body condition, trophic position, and abundance in relation to environmental variables around the northern Antarctic Peninsula. 

Methods

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).

Usage Notes

For samples from pooled stations (denoted as “POOLED” in the station column), we estimated tow depth and environmental variables by calculating the mean of each variable for the sampling area and year. Reported abundance for pooled stations is the mean abundance for a given species within the sampling area and year. To estimate sea-ice type, we assigned a number to each ice type (0 = none, 1 = slush, 2 = thin, 3 = first, 4 = multi), calculated the mean for the area and year, and rounded the result to the nearest whole number. Station coordinates and daylight conditions are not reported for samples from pooled stations (denoted as “NA”).

In 2012, percent sea-ice cover and sea-ice type were not determined. Sampling stations were characterized as “ice” (100 %) or “no ice” (0 %). Where sea-ice conditions were not reported, "NR" appears in the dataset.

For most species in 2012 and for small species in other years, the number of individuals analyzed for lipids and stable isotopes were not recorded (denoted as "NR" in the dataset).

In a few cases, lipid was undetectable in samples. These are denoted “ND” (for none detected) in the dataset. Stable isotope values are reported for these samples.

In some cases, especially for gelatinous species, the calculated percent mass loss when wet samples were dried exceeded 100 %. In these cases, only percent lipid of wet mass is reported in the dataset.

For one sample in 2012 (2012TM02), we were unable to obtain stable isotope values. Stable isotopes are reported as “NR” for this sample, but lipid data for this sample are reported.

Lipid data are not reported for whole-animal (“WAN”) samples, since lipid was not extracted from these samples.