Food web structure (food web base, number of trophic steps) can influence mercury distribution and bioaccumulation within aquatic food webs. Understanding variations in food web structure over spatial and temporal scales may help explain mercury patterns in Everglades biota.

The nitrogen (¹⁵N) and carbon (¹³C) isotopic composition of tissues are integrated measures of diet that can be used to distinguish the relative trophic positions of biota.

Laboratory and field studies demonstrate increases in ¹⁵N (~2-3‰) and ¹³C (~0-1‰) between consumers and their diet.

Plots of ¹⁵N versus ¹³C for biota representing a range of trophic positions should therefore show a positive slope (Figure 1).

Figure 1. Predicted increase in the stable nitrogen and carbon isotopic composition (15N and 13C, respectively) of tissue from biota in an aquatic ecosystem. Higher isotopic values should be found for tissues of organisms occupying relatively higher trophic positions. [larger image]

Figure 2. Map of southern Florida, showing collection sites for biota analyzed in this study. Samples discussed here were collected during September 1997, January 1998, and June 1998. [larger image]

We investigate three main questions in this study:

• Can isotopes resolve relative trophic level differences at the food web scale in the complex Everglades ecosystem?
• Does food web structure (as determined by isotopes) vary over space?
• Does food web structure vary over time?

Plots of ¹⁵N versus ¹³C for biota tissues are used to identify relative trophic levels. Variations in the ¹⁵N:¹³C slope (or lack thereof) among collection sites and dates are used to address the questions above.

Plants, invertebrates, and fish were collected from 16 well-studied USGS ACME (Aquatic Cycling of Mercury in the Everglades) sites throughout the Everglades during 1995-1999 as part of a collaboration between the USGS and the Florida Fish and Wildlife Conservation Commission (FFWCC).

Within this data set, we focus on biota collected from six sites during several sampling periods when a sufficient number and variety of aquatic organisms were collected (Figure 2).

Isotope ranges vary by site. ¹⁵N:¹³C patterns also group by site:

U3 and L35B: Positive ¹⁵N:¹³C slopes (¹⁵N and ¹³C resolve trophic level differences across the food web (Figure 3).

F1: Negative slopes (¹⁵N and ¹³C resolve the food web) (Figure 4).

3A-15: No significant slopes (only ¹⁵N resolves the food web) (Figure 5).

L67 and Cell 3: No significant slopes (neither isotope adequately resolves the food web) (Figure 6).

Color symbols on the graphs are used here to suggest general trophic groups. The groups are modified slightly from those of Loftus et al. (1998), which were derived based on stomach contents data obtained in the Shark Slough region of Everglades National Park, south of our study area. Symbols are medians, with the number of separate isotopic analyses shown in parentheses.

Figure 3.
Positive 15N:13C Slope
note: click on each of the graphs to view a larger version

Figure 4.
Negative 15N:13C Slope
note: click on each of the graphs to view a larger version

Figure 5.
Indeterminate 15N:13C Slope
note: click on each of the graphs to view a larger version

Biota ¹⁵N:¹³C patterns also vary by collection date at a given site:

U3 (Sep 1997) and L35B (January 1998) ¹⁵N:¹³C slopes are significant, whereas these sites do not show significant slopes for their other collection dates (Figure 3).

At 3A-15, L67, and Cell 3, the degree to which ¹⁵N and ¹³C values show separation among trophic positions varies collection date (Figures 4-6).

Figure 6.
Indeterminate 15N:13C Slope
note: click on each of the graphs to view a larger version

At the food web scale, nitrogen and carbon isotopes of tissue can discriminate among relative trophic level positions at some Everglades sites and collection dates, but not well at others. Isotopic differences among trophic positions vary primarily spatially, but also temporally for the sites and dates we investigate in this study.

Possibly, spatial and temporal changes in biogeochemical reactions and/or food web base contribute to the different patterns observed here.

Site F1 shows unexpected negative ¹⁵N:¹³C slopes, which contradict expected patterns based on previous laboratory and field studies. One explanation for this pattern is the existence of multiple food webs with different food web bases.

Loftus, W.F., Trexler, J.C. and Jones, R.D., 1998. Mercury Transfer Through an Everglades Aquatic Food Web. Final Report, Contract SP-329, Florida Department of Environmental Protection, Homestead, Florida.

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Related information:

SOFIA Project: Application of Stable Isotope Techniques to Identifying Foodweb Structure, Contaminant Sources, and Biogeochemical Reactions in the Everglades