We have found no satisfactory field-based studies of stable-isotope amplification in
the marine pelagic microbial food web; possibly we missed them. There are many
studies of δ^15 N in particulate organic matter (e.g. Bode et al. 2007) and some culture
studies. Hannides et al. (2009) suggest that their low trophic-level estimates for NPSG
copepods, using CSIA-δ^15 N, likely result from low amplification due to intense
recycling. Virtually all organic nitrogen can be recycled to the inorganic form at high
frequencies, likely every few days. Thus, amplification of stable isotopes may only
take a permanent hold on nitrogenous organic compounds after they are transferred to
moderately long-lived animals. Bode et al., working with “plankton” collected with
20 μm mesh and then partitioned with nested sieves, found no mean or median
differences among three size classes from 20 to 500 μm, mean δ^15 N = 5.6‰. Plankton
(actually particles) <20 μm were only 0.52‰ less, while plankton 500–1000 μm were
0.80‰ greater. Plankton retained by 1 mm, but passing 2 mm screens, had δ^15 N =
6.8‰, very close to values for three copepod species. These small shifts among small
but distinct size categories are in agreement with the proposal of Hannides et al. that
rapid recycling of nutrients from small organisms reduces amplification of heavy
isotope fractions. Bode et al. found δ^15 N of 9.3 to 11.3‰ in muscle of planktivorous
fish and 13.1‰ in the piscivorous common dolphin. Apparently, amplification occurs
mainly in longer-lived, more-stable tissues. However, the results from Hoch et al.
(1996) for a flagellate (Pseudobodo) and ciliate (Uronema) feeding on bacterial
cultures, suggest that enrichment on the order of 3‰ can occur. Possibly, if organisms
like heterotrophic flagellates, ciliates, and dinoflagellates can be separated in
sufficient quantity from bulk POM, a single-step enrichment will be discernible. Such
separations are a task for flow cytometry without the initial use of filters.
(^) Estimates by Olson et al. (2010) of yellowfin tuna trophic level from both whole-
muscle δ^15 N and CSIA-δ^15 N were reasonably close to an analysis based on mass-
weighted averages of approximate TL values from gut contents of tuna from the same
catches: TLYFT(gut content) ≈ 4.6. You are entitled to ask: Why would one bother with
all the extraction of tissue nitrogen or with CSIA chemistry, and then with the mass
spectrometry in both methods, when gut-content values give the same results? For
tuna, perhaps, isotope evaluations can be taken as mainly an expensive check.
However, the level of expertise required to identify and evaluate gut contents with
results resembling quantification should not be underestimated. Because diets of