animals from all ocean areas and depths – checking gut contents is an almost reflexive
observation for biologists. It generates good and often true stories. Some results for
zooplankton were discussed in Chapter 7. The tuna results are quite usual in that, like
tuna, most animals eat anything within suitable size limits that presents itself and
moves slowly enough to overtake and ingest. As both the chaetognath (Fig. 9.2) and
tuna data (Table 9.1) show, the size of prey tends to increase as a predator grows, but
food less than the maximum ingestible size continues to be part of the diet.
Trophic-Level Assignment by Stable Isotope proportions
(^) Above we mentioned “food-web level”, for which the standard jargon is “trophic
level”. Much attention has been given to determining the trophic levels of marine
organisms: from gut-content analysis and in recent decades from shifts in the relative
abundances of the stable isotopes of carbon and nitrogen (e.g. Fry & Sherr 1984).
However, trophic level is an abstract concept; trophic levels do not swim about in the
sea, available for collection and study as such. Primary producers might seem a
possible exception, being identified by their photosynthetic pigments, but many of
them (dinoflagellates, other flagellates, ciliates with borrowed chloroplasts, ...) are
actually “mixotrophs”, getting nutrition both auto- and heterotrophically. Most pelagic
consumers feed at more than one trophic level. For example, ciliated protists can
ingest Synechococcus or a prymnesiophyte in one hour and flagellated
microheterotrophs in the next, and a tuna can eat a mackerel in one swallow and a
chain of salps in the next. Thus, few animals “belong to” a single, integral trophic
level. Nevertheless, like some other abstractions that do not exist (an ideal gas, an
infinitely dilute solution, a stable age distribution), trophic levels are a useful notion
for evaluating how much primary production is metabolized by an ecosystem in
producing its higher-order carnivores, i.e. for evaluating the transfer efficiency of the
food web.
(^) As carbon and nitrogen bound in organic matter are passed progressively up a food
web, the tissue that omnivores and carnivores construct from them becomes
progressively enriched in the less-abundant stable isotopes ^13 C and ^15 N relative to the
dominant ^12 C and ^14 N. That happens primarily because compounds of the lighter
isotopes fit somewhat more readily into the active sites of metabolic enzymes, and are
preferentially removed from the tissues. The rarer isotopes amount to only several
atoms per thousand, and “per mille” (‰) units are the favored expression of their
abundance. Generally, those ratios are presented as comparisons to the ratios in
standard substances, symbolized δ^13 C and δ^15 N (Box 9.1). Use of the standards
simplifies the mass spectrometry required to determine the isotopic ratios.