A main issue currently under investigation by a multi-agency taskforce in the Everglades is the cause of bioaccumulation of methylmercury (MeHg) up the food chain. A clear understanding of the aquatic food web is essential for determining the entry points and subsequent biomagnification pathways of contaminants. 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, invertebrates, and vertebrates have been used to establish relative trophic levels among various organisms because at each ascending trophic level, there is an increase in the carbon-13 (¹³C) and nitrogen-15 (¹⁵N) content of the organism due to selective metabolic loss of carbon-12 (¹²C) and nitrogen-14 (¹⁴N) during food assimilation. Thus, an organism is typically enriched in ¹³C and ¹⁵N relative to its diet by 1 to 3 parts-per-thousand (permil). There appears to be little or no enrichment in sulfur-34 (³⁴S) with increasing trophic level.

The nitrogen isotopic compositions (¹⁵N) of organisms in the Everglades range from -10 to +20 permil. These ¹⁵N values are in good agreement with suspected trophic position; primary producers have lower values than herbivores while omnivores and carnivores have successively higher values. Plants show an extremely large range of values. Most of the variability is in the macrophytes, and these almost always have lower values than algae. Among the invertebrates, insects show considerable overlap with crustaceans and small fish but generally have lower ¹⁵N values. Crustaceans have lower ¹⁵N values than most fish. Small omnivorous fish have lower ¹⁵N values than larger carnivorous fish. In general, the ¹⁵N values of gambusia (mosquitofish) are 7 to 9 permil higher than the values for co-existing periphyton mats. If bulk periphyton is an important component of the gambusia food chain, the gambusia values are equivalent to 2 to 3 trophic levels above the bulk periphyton.

The carbon isotopic compositions (¹³C) of organisms range from -40 to -15 permil. In general, 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. However, there is usually very good separation of the ¹³C values of macrophytes and algae; most macrophytes have higher ¹³C values than algae and other organisms. The generally high ¹³C values of the macrophytes are inconsistent with their being a major food source in most locations.

Few samples have been analyzed for sulfur isotopic compositions (³⁴S) but the preliminary data show a range of +5 to +30 permil. There appears to be little or no increase in ³⁴S with trophic level; instead, the main controls on the ³⁴S appears to be source and extent of sulfate reduction. Areas where the decreases in sulfate concentrations over time are caused primarily by sulfate reduction, show large increases in the ³⁴S of dissolved constituents. These isotopic patterns are then incorporated into the biomass.

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 a persistently higher value of the ¹⁵N of dissolved inorganic nitrogen (DIN) at the canal sites than at marsh sites. One likely explanation for this pattern is denitrification in anoxic waters and sediments in stagnant parts of the canals, which would cause the ¹⁵N of the resultant DIN to increase. Marsh sites near canals show a much larger range of isotopic compositions than more pristine marsh sites because of the periodic influxes of eutrophic canal waters.

These values provide valuable clues about trophic relations among consumers. Different sites appear to have different food chains, and some show evidence for some seasonal differences in the importance of different food sources. At some sites (for example, U3 in Water Conservation Area (WCA) 2), the isotopic data suggest that algae is a major food source to local food webs. In contrast, at other sites (for example, cell 3 in the Everglades Nutrient Removal (ENR) area), decaying cattails (and the microbes that live on them) appear to be a major food source.

The compositions and spatial distributions of the carbon, nitrogen, and sulfur isotopes suggest that the values reflect spatial variability in reducing conditions in the marshes that favor methane production, sulfate reduction, and perhaps denitrification. The isotopic compositions of aquatic plants appear to integrate the variability in water-column isotopic compositions due to redox reactions and other factors in the ecosystem, and these same patterns are incorporated throughout the food chain. Therefore, zones frequently dominated by particular redox reactions may be labeled by the isotopic compositions of local organisms. Furthermore, organisms that live in zones where geochemical conditions are different may have distinctive isotopic compositions.

The "isotopic labeling" of different environments implies that isotopic techniques might be useful for determining whether fish migrate from canals to marshes in response to changes in hydrologic or nutrient conditions. Largemouth bass at some sites (for example, the ENR outlet, L-7, and a mid-marsh site in WCA 1) have narrow and distinctive ranges in isotopic compositions. These compositions suggest that the bass at the WCA 1 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 are consistent with movement of bass between canal and marsh sites, probably in response to fluctuations in water levels. These data indicate that for adjacent canal and marsh sites where the primary producers have distinguishable isotope compositions, the isotope compositions of fish can be used to determine whether the fish migrate in and out of the marshes in response to changes in water levels or food availability. 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.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)

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