generalist feeders can vary wildly (they eat everything of suitable size that they can
overtake), very large numbers of full stomachs should be studied. Observers must be
able to recognize at least general categories of prey from torn and digested remains,
perhaps just a jaw or some fin rays. In some instances, suitable experts could be
harder to find or fund than a mass-spectrometry laboratory. Also, some predators
regurgitate gut contents at capture, so trophic placement requires other methods.
When δ^15 N is the method of choice, replication at both the field and laboratory levels
should be applied to overcome the substantial inter-individual variability of the
isotopic ratios (Post 2002). In any case, δ^15 N-based estimates have been made for
marine biota ranging from zooplankton to albatross, including polar bears. The results
are of uneven quality and should be used bearing that in mind.
Primary Source Identification for Organic Matter from
(^13) C/ (^12) C Ratios
(^) The change in δ (^13) C at each trophic transfer is also an enrichment on average, but
much less than for δ^15 N. Mean (δ^13 Cconsumer − δ^13 Cdiet) ≈ +0.39‰, with a standard
deviation of 1.3, based on a summary of 107 studies reported by Post (2002). Thus,
the shift can be either positive (a small majority of cases) or negative, ranging from
−3 to +4, providing minimal (not useful) trophic-level information. However, in
nearshore and estuarine waters, δ^13 C has some value for determining the
photosynthetic source of organic matter. There are two distinct sets of photosynthetic
carboxylation reactions: the Calvin–Benson or C3 cycle typical of algae and some
higher plants, and the Hatch–Slack or C4 cycle of grasses (including reeds and many
crops – sugarcane, maize, wheat). The δ^13 C values of C3 photosynthate are in the
range from −24 to −34‰, whereas C4 values range from −6‰ to −13‰. Cacti and
other xeric plants have an alternate C3 system, CAM, with δ^13 C from −10 to −22‰,
but their carbon mostly does not reach estuaries or oceans. For seagrasses, δ^13 C
ranges from −3 to −24‰, likely because isotope ratios of seawater carbonate available
for photosynthesis are different from and more variable than those of CO 2 in air (Lin
et al. 1991). For nearshore animals, snails or sea urchins for example, the difference
of δ^13 C from −24‰ can indicate the importance of algal vs. seagrass carbon. Also,
despite the seagrass values, δ^13 C greater than ∼ –24‰ can be indicative of a partially
terrestrial source for organic matter in the marine food web, while most marine
organic matter will have considerably more negative δ^13 C. Possibly more useful,
marine organic matter in riparian zones, sometimes delivered by salmon or other
anadromous fish and distributed by large animals like bears and river otters, can be
detected by unusually negative δ^13 C. There are examples of both applications in the
literature, sometimes supplemented with other tracer isotopes such as sulfur-34