Thus, an organism is typically enriched in ¹³C and ¹⁵N relative to its diet by 1 to 3 parts-per-thousand (). In other words, "you are what you eat plus a little bit". This "little bit" is called a trophic fractionation. There appears to be little or no enrichment in sulfur-34 (³⁴S) with increasing trophic level. The relative values of each organism on Figure 2 (below) reflect the differences in diet and trophic position of the organism.

The isotopic contents of materials are described using "delta values" (where  = delta). Hence, the ¹⁵N content is described using the term ¹⁵N, with ¹³C and ³⁴S referring to the ¹³C and ³⁴S contents, respectively. The units are in parts per thousand, or  (permil).

Figure 2 shows the average ¹⁵N (refers to the ¹⁵N content) and ¹³C (refers to the ¹³C content) values of selected fish plus the compositions of important insects and crustaceans and plants. These values are normalized to the compositions of mosquitofish, an important indicator species, to allow direct comparisons of samples collected at different sites and times. These values show the same general trophic relationships shown in Figure 1.

Nitrogen isotopes:  The ¹⁵N values are in good agreement with suspected trophic positions; primary producers have lower values than herbivores, while omnivores and carnivores have successively higher values.  The average nitrogen trophic fractionation is about 1.6  per trophic level.

Carbon isotopes:  The ¹³C values of algae, invertebrates, and fish show considerable variability with little or no consistent increase in ¹³C with increasing trophic level. Hence, bulk carbon isotopes are not very useful for determining trophic position. The generally high ¹³C values of the macrophytes (e.g., lily pads, sawgrass) are inconsistent with their being a major food source in most locations.

Figures 4 and 5 show that the isotopic compositions of organisms from areas of  high and low nutrient concentrations are very different.  We observe an increase in ¹³C and ¹⁵N with increasing trophic level at U3, a low-nutrient marsh site near the middle of WCA-2A (Figure 4). The foodweb at this site behaves in accordance with the isotopic theory shown in Figure 1. However, at  F1 (Figure 5), a high-nutrient site in WCA-2A near the Hillsboro Canal, the ¹³C values decrease with increasing trophic level, producing a isotopic foodweb structure that is not consistent with theory (i.e., it has a trend-line backwards from the expected pattern).

In general, organisms collected in high-nutrient sites near the Everglades Agricultural Area (EAA) have higher ¹⁵N values than ones collected in more pristine areas to the south. Near the EAA, organisms in the canals generally have higher ¹⁵N values than samples from adjacent marshes, and the ¹⁵N values decrease with distance from the canals. This difference probably reflects denitrification in anoxic waters and sediments in stagnant parts of the canals.

While is not yet clear what causes the striking isotopic difference between nutrient-impacted sites (e.g., ENR, E0, F1, L67) and more pristine sites (e.g., U3, 2BS, 3A-15, 3A-TH), isotopes appear to provide a quick and easy method for determining if high-nutrient areas might be causing significant changes in foodweb relations.

Largemouth bass at some sites (e.g., ENR, L-7, and a mid-marsh site in WCA 1) have distinctive ranges in ¹⁵N and ¹³C (Figure 6) and ¹⁵N and ³⁴S (Figure 7). These compositions suggest that the bass at the WCA 1 site do not migrate in or out of L-7, and that L-7 and the ENR cell 3 sites have significantly different environmental conditions. The larger and overlapping ranges in ¹³C and ¹⁵N values at sites in WCA 2 and 3 (green points in Figure 6) are consistent with movement of bass between canal and marsh sites, probably in response to fluctuations in water levels.

Spatial variability of ¹⁵N, ¹³C, and ³⁴S values in the Everglades reflects spatial variability of reducing conditions in the marshes that promote methane production, sulfate reduction and denitrification.  The isotopic compositions of aquatic plants integrate the variability in water column isotopic compositions and these same patterns are incorporated throughout the foodweb. Therefore, organisms that live sites where geochemical conditions are dominated by particular redox reactions have distinctive isotopic compositions. The "isotopic labeling" of different environments suggests that isotopic techniques could be useful for determining whether fish migrate in and out of the marshes in response to hydrologic or nutrient-level conditions.

Because MeHg concentrations are a function of local environmental conditions, these isotopic data should prove useful for determining where some populations of game fish are acquiring elevated levels of MeHg.

Stable carbon, nitrogen and sulfur isotope analyses provide valuable information regarding foodweb structure and trophic relationships, differences in chemical environment among site locations, and fish migration in the Everglades.  In general, the aquatic foodweb of the Everglades is consistent with that predicted from isotopic theory.  Nutrient-impacted sites are distinguishable due to their unique (albeit backwards) trophic structure as determined by ¹³C.  Stable isotope analyses distinguish fish populations and offer a more cost-effective alternative to tag-and-release programs for the determination of migration habits. Stable isotope analyses complement gut-content based foodweb studies, provide an independent check on diet estimates, and provide insight into the difference between what an organism eats and what it actually assimilates. Such an assessment should prove useful for the better regulation of hydrologic, geochemical, and biological conditions most favorable to the restoration of the Everglades.