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Last updated: January 15, 2013
South Florida Restoration Science Forum

Mercury

Tracing Foodweb Relations and Fish Migratory Habits in the Everglades with Stable Isotope Techniques

Poster presented May 1999, at the South Florida Restoration Science Forum

By: Carol Kendall, Cecily Chang, Robert Dias, Steven Silva, Daniel Steinitz, Erika Wise, Eric Caldwell  (USGS, Menlo Park CA);  Paul Garrison  (WI DNR., Monona WI);  Ted Lange  (FL GFWFC, Eustis FL) .


Abstract

A clear understanding of the aquatic foodweb is essential for determining the entry points and subsequent biomagnification pathways of contaminants such as methylmercury (MeHg) up the food chain. This poster shows how stable isotopes can be used to estimate foodwebs for marsh and canal sites, identify nutrient-impacted sites with non-normal foodwebs, and reconstruct the migration habits of large fish.

How can isotopes be used to determine who is eating who?

conceptual model
Figure 1
(Click on image for larger version.)

 
The traditional method of food web investigation focused on the determination of gut contents (literally, "who ate what"), and is still used today.  More recently, stable carbon, nitrogen and sulfur isotope analyses of plants and animals have been used to establish relative trophic (predator) levels among various organisms because at each ascending trophic level (from prey to predator), there is an increase in the carbon-13 content (13C/12C ratio) and nitrogen-15 content (15N/14N ratio) of the organism due to selective metabolic loss of carbon-12 (12C) and nitrogen-14 (14N) during food assimilation and growth (Figure 1).

Thus, an organism is typically enriched in 13C and 15N relative to its diet by 1 to 3 parts-per-thousand (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.

Isotopic compositions of selected marsh organisms

isotopic compositions of organisms

Figure 2
(Click on image for larger version.)

 
How can isotopes be used to determine predator-prey relations in the Everglades?
The isotopic contents of materials are described using "delta values" (where delta symbol = delta). Hence, the 15N content is described using the term delta symbol15N, with delta symbol13C and delta symbol34S referring to the 13C and 34S contents, respectively. The units are in parts per thousand, or parts per thousand (permil).

Figure 2 shows the average delta symbol15N (refers to the 15N content) and delta symbol13C (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 delta symbol15N 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 parts per thousand per trophic level.

Carbon isotopes:  The delta symbol13C values of algae, invertebrates, and fish show considerable variability with little or no consistent increase in delta symbol13C with increasing trophic level. Hence, bulk carbon isotopes are not very useful for determining trophic position. The generally high delta symbol13C values of the macrophytes (e.g., lily pads, sawgrass) are inconsistent with their being a major food source in most locations.


 
Is there a difference between what a fish eats and what it assimilates?
The left-most "pie diagram" in Figure 3 shows the estimated average diet of marsh mosquitofish based on examination of the stomach contents of several hundred fish. The delta symbol15N values of organisms can be used to test the diet estimates determined by gut contents analysis. The expected delta symbol15N value of the fish for this average diet can be calculated by multiplying the delta symbol15N values of each of the diet items (e.g., zooplankton) on Figure 2 by the relative amounts of each diet item, and assuming an average trophic fractionation of 2 parts per thousand. This calculation produces a delta symbol15N value of -3 parts per thousand for mosquitofish, which is 3 parts per thousand lower than it should be if the fish assimilated all the food in its diet.

Average Diet for Marsh Mosquitofish based on gut contents analysis
(Paul Garrison, unpubl. data, 1999)
Average Diet if Plants are not Digested (and hence do not contribute to growth)
pie diagram
pie diagram
Figure 3
(Click on image for larger version.)

 
Why is the calculated value too low?  One possibility is that although 40% of the gut contents is periphyton, the fish may not be assimilating this material.  If periphyton is removed from the diet under the assumption that it is not assimilated by the fish (as shown in the right-most pie diagram), then the calculated delta symbol15N value is much better (only 0.5 parts per thousand too low).  Therefore, although algae (periphyton) often is a major component of the stomach contents of fish, it appears to be only a minor component of what is actually digested. This is an important observation since the MeHg content of algae can be high in some environments; however, it is not known whether MeHg within the algal mats can be absorbed by the fish even if the algae is not assimilated.

 
Are foodwebs different at different sites within the Everglades?

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 delta symbol13C and delta symbol15N 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 delta symbol13C 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).

Site U3 isotopic compositions
Figure 4
(Click on image for larger version.)
Site F1 isotopic compositions
Figure 5
(Click on image for larger version.)

 
In general, organisms collected in high-nutrient sites near the Everglades Agricultural Area (EAA) have higher delta symbol15N values than ones collected in more pristine areas to the south. Near the EAA, organisms in the canals generally have higher delta symbol15N values than samples from adjacent marshes, and the delta symbol15N 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.

Do isotopes provide a fingerprint for fish migration in the Everglades?

Largemouth bass at some sites (e.g., ENR, L-7, and a mid-marsh site in WCA 1) have distinctive ranges in delta symbol15N and delta symbol13C (Figure 6) and delta symbol15N and delta symbol34S (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 delta symbol13C and delta symbol15N 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.

largemouth bass isotopic compositions
Figure 6
(Click on image for larger version.)
largemouth bass isotopic compositions
Figure 7
(Click on image for larger version.)

 
Spatial variability of delta symbol15N, delta symbol13C, and delta symbol34S 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.

Summary

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 delta symbol13C.  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.

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