2.3 Stable isotope analysis
Analyses of δ13C
and δ15N values in western sandpiper plasma,
invertebrates, biofilm, and microphytobenthos were conducted at the Port
and Airport Research Institute (Nagase, Yokosuka, Japan). Plasma samples
were homogenized with a microspatula, and invertebrate, biofilm, and
microphytobenthos samples were homogenized with a mortar and pestle.
Samples were weighed and packed into tin capsules. Isotope ratios were
measured with a Thermo Electron Delta Plus Advantage gas isotope ratio
mass spectrometer (Bremen, Germany) interfaced with a Thermo Electron
FlashEA 1112 elemental analyzer (Bremen, Germany). The long-term
analytical precision was < 2 ‰ for δ13C and
δ15N. Vienna Pee Dee Belemnite (VPDB) and air (AIR)
were used as standards for δ13C and
δ15N, respectively. L‐Histidine and L‐Alanine (Shoko
Science Co., Ltd., Yokohama, Kanagawa, Japan) standards and blanks were
included with each run to correct for drift. Isotope ratios are reported
in parts per thousand (‰) using delta notation with
δhN =
(Rsample /Rstandard -1) x
1000, where R is the ratio of enriched to depleted isotopes for
the sample or standard, N is the element of interest, andh is the mass of the enriched isotope. Most samples hadC:N ratios >4. Therefore, we normalizedδ13C values to account for variable lipid
content according to the following equation from Post et al. (2007):
\(\delta^{13}C_{\text{normalized\ }}=\ \delta^{13}C_{\text{original\ }}-3.32+0.99\ \times C:N\).
Normalized δ13C values were used in all
subsequent analyses.
Plasma was selected for stable isotope analysis because of its rapid
turnover rate. Turnover rates for carbon and nitrogen isotopes in
western sandpiper plasma are unknown; however, δ13C
and δ15N values reached equilibrium in dunlin
(Calidris alpina ) plasma after 10 days following a diet switch.
The dunlin is a close relative of the western sandpiper, and it is
likely that isotopic turnover occurs more quickly in western sandpiper
plasma because western sandpipers have 50% less body mass and faster
metabolic rates than dunlin
(Lourenço, Granadeiro, Guilherme,
& Catry, 2015). Western sandpipers captured in SF Bay in April could
have wintered in SF Bay or migrated to SF Bay from wintering areas to
the south. Migrants typically arrive in mid-March and early-April with
an average duration of stay in SF Bay of 12 days (Iverson et al., 1996).
The average departure date from SF Bay for western sandpipers is 28
April (range = 20 April - 9 May; Bishop et al., 2006). Our captures from
19-23 April coincided with the onset of western sandpiper departures
from SF Bay, at which time most birds would have plasma at isotopic
equilibrium with SF Bay prey.