Biological Oceanography

(^) The difference divided by 7‰ gives the number of trophic steps to reach tuna, and 1
is added for the tuna themselves. For six stations running northeast to southwest
across the region of study (Fig. 9.3), the results (Table 9.2) for TLYFT(Glu (^) – (^) Gly)
averaged 5.2, higher than the bulk muscle δ^15 N, but not significantly so.
Table 9.2 Estimated δ^15 N ratios (‰, means with standard errors in parentheses) in
glutamic acid and glycine from white dorsal muscle of yellowfin tuna from the
eastern tropical Pacific, compared to δ^15 N of bulk muscle. Trophic level, TL, was
calculated with the McClelland and Montoya equation.
(From Olson et al. 2010.)
Hannides et al. (2009) applied CSIA to four copepod species and one euphausiid
from the North Pacific subtropical gyre (NPSG), using glutamic acid as the trophic
AA, and compared it to glycine and phenylalanine as source AAs. The differences
among the species (Table 9.3) were as expected from knowledge of these copepods
and their likely diets (Oithona and Neocalanus TL = 2.1 to 2.2; Euchaeta and
Pleuromamma TL ≈ 2.8 to 3.3), and all trophic-level estimates were surprisingly low,
given the known complexity of the picoplankton to microplankton food web in these
oligotrophic waters. We review that complexity below. There were substantial
seasonal differences in the δ^15 N of both the source and trophic AAs, but the variations
of both were in the same direction and amounts for all of the species, so that the TL
estimates were nearly constant. Seasonal variation seems to derive from the isotopic
variation of the available nitrogenous nutrients. Because the life cycles of at least the
copepods are on the order of <1 to ∼2 months, and given the almost daily turnover of
organic carbon at lower trophic levels, the δ^15 N of their AAs should, indeed, follow
seasonal changes in nutrient isotope composition. The CSIA-estimated TL should be
roughly constant despite the shifts in nutrient δ^15 N.
Table 9.3 Trophic levels (TL ± SE) estimated from δ^15 N differences (glutamic acid –
phenylalanine) and (glutamic acid – glycine), for four copepods and one euphausiid
(Thysanopoda),
(After Hannides et al. 2009.)