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mapping spatial variability in marsh redox conditions using biomass stable isotopic compositions
Mapping Spatial Variability in Marsh Redox Conditions Using Biomass Stable Isotopic CompositionsCarol Kendall, Steven Silva, Daniel Steinitz, Erika Wise, Cecily Chang (USGS, Menlo Park CA); Jerry Stober, Phyllis Meyer (USEPA, Athens GA)
Zones frequently dominated by particular redox reactions appear to be labeled by the C, S, and N isotopic compositions of local organisms. Isotopic compositions of biomass may prove to be more cost-effective and reliable indicators of prevailing environmental conditions that favor methylmercury production than other parameters currently being considered because biomass isotopic compositions are much more difficult to perturb than the more transient concentrations of aqueous species.
As part of a collaboration between the USGS and the USEPA, periphyton, mosquitofish (gambusia), and sediment samples were collected during September 1996 at over 100 REMAP marsh sites throughout the Everglades to assess the local and regional ranges in their C, N, and S stable isotope ratios. These organisms were chosen because periphyton communities play an important role in the ecosystem, and the ubiquitous mosquitofish might be a useful indicator species. Archived periphyton samples collected in May 96 were also analyzed for isotopes to assess possible seasonal changes. The periphyton samples were composites of floating mat samples; samples were vapor acidified before analysis to remove carbonate. Five gambusia were collected at each site; they were filleted and analyzed individually. The sediment samples were composites of material collected in the top 0-10 cm below the floc layer; they were vapor acidified before analysis to remove carbonate.
Biochemical reactions such as denitrification and methyl mercury (MeHg) production in the shallow subsurface sediments in aquatic systems and watersheds can have profound effects on surface-water quality. These processes are controlled by redox conditions in the sediments or water column and, hence, may be strongly affected by transient changes in oxygen, nutrient, and trace element levels caused by seasonal changes in hydrology and anthropogenic inputs. Because of the ephemeral nature of chemical signals produced by these reactions, it is often difficult to assess the spatial and temporal extent of environmental conditions that favor these reactions, or the degree to which these reactions affect ambient water quality, by conventional chemical or isotopic measurements of dissolved species. Environmental Indicators: We are attempting to use the 15N, 13C, and 34S of biota in marshes and canals as (1) indicators of local environmental conditions that may impact water quality, and (2) indicators foodweb structure (see below). The theoretical basis of this study is that the isotopic compositions of non-fixing, primary producing plants reflect to a large extent the isotopic compositions of the dissolved N, C, and S in the environment that is being utilized by the biota, as modified by various possible fractionating mechanisms in the plants. The isotopic compositions of the primary producers are then reflected by higher level organisms such as invertebrates and fish, as modified by mixed diets, trophic enrichments, and the larger foraging areas and lifetimes of the higher level organisms. Hence, under favorable conditions, the biomass isotopic compositions can reflect (and integrate) the extent of such processes as denitrification, sulfate reduction, and methane oxidation in the sediments and water column that affect the isotopic compositions of the dissolved species. Fig. 1 (below) is a schematic for how the isotopic compositions reflect various biogeochemical processes and nutrient sources in the ecosystem.
The 15N, 13C, and 34S of aquatic plants (e.g., phytoplankton and algal mats) and animals (e.g., invertebrates and fish) reflect the isotopic compositions of the dissolved N (in nitrate and ammonium), C (in bicarbonate), and S (in sulfate) in the environment as modified by mixed diets and trophic enrichments (about 2-3 per trophic level for 15N and 0-1 per trophic level for 13C). This conceptual model is illustrated in Fig. 2 (below).
The five gambusia from each site showed little variability. Fig. 3 (below) shows histograms of the standard deviations of the average Delta 15N and Delta 13C values of the sets of 5 or 10 gambusia collected at each site. The average 1 sigma variations in 15N and 13C were 0.4 and 0.7 , respectively. The good agreement in their compositions indicates that gambusia collected at the same place exhibit a well-mixed, consistent diet, over the life-time of the gambusia (despite the large range in their observed gut contents).
This puzzle is a topic of active research; at some sites, we have strong evidence for possibilities #1 and/or #2. To further explore possibility #1, we are sampling alternative C sources and assessing possible variability in the mats by picking them apart and subsampling. In addition, collaborations with Paul McCormick (SFWMD) have generated an interesting set of data; algae were collected from sets of growth plates set at 3 sites at site U3 (in WCA2A) for 9 different 1-week time periods. The large range in isotope and elemental values over time and space as different organisms colonized them indicated that it might be difficult to resolve which types of algae are actually being assimilated by local organisms. Recent attempts to use compound-specific stable isotope techniques to trace individual molecules up the foodchain will provide more insight into #2, and planned controlled growth experiments (in the lab and in mesocosms) will help resolve possibility #3. Over 50% of the sites have gambusia with 13C values that are lower than the corresponding periphyton 13C values. The distribution of these sites is shown in Fig. 5 (below). Sites are divided into 3 categories, based on the relative 13C values of gambusia and bulk periphyton collected at the same site: (1) sites where the gambusia 13C values are higher than the periphyton 13C values (blue), consistent with theory, (2) sites where the gambusia 13C values are roughly equivalent to periphyton 13C values (green), and (3) sites where the gambusia 13C values are considerably lower than the periphyton 13C values (yellow), inconsistent with theory. This model represents a "testable hypothesis" for the spatial distribution of sites dominated by algal-based foodwebs vs sites dominated by detrital-based or other types of foodwebs. Sampling locations are shown as "dots."
POM (particulate organic matter) isotopes:
In the Everglades, both periphyton and mosquitofish show wide ranges in isotopic composition throughout the marshes, with about a 10 range in values for each isotope and species (Figs. 6-8). However, the spatial contour plots of the isotopic compositions display intriguing regional patterns that show little change with time or species. This is strong evidence that the environmental patterns reflected by the primary producers are being incorporated up the foodweb. The isotopic values show a general correspondence with peat thickness, with the lowest 13C values and highest 34S values along the major axis of the flow system. Areas with high MeHg contents commonly have high 15N and 34S values; these are areas where sulfate reduction and denitrification are dominant processes in the sediments. Zones frequently dominated by particular redox reactions appear to be "labeled" by the 13C, 15N, and especially the 34S of local organisms.
One overly simplistic explanation for the lower 13C values where peats are thickest (i.e., where the peats contain the most reduced C) is because this is where methane production (followed by oxidation, which produces lower 13C values -- see Fig. 1), is likely to dominate. The 1 lower periphyton 13C in May vs September is consistent with the idea that redox chemistry may be an important control on 13C values because it is likely that the system is more reducing when the waters are shallower and more clogged with rotting vegetation. However, as shown in Fig. 1, there are a number of processes that can affect the 13C of organisms. Fig. 6 shows that the spatial distribution of 13C values in periphyton is very similar to the 13C patterns seen in gambusia. Although Fig. 4 shows that the relationship between the 13C values of gambusia and periphyton is neither constant nor simple, on the "gross" scale of Fig. 6, both organisms appear to reflect a similar spatial distribution of environmental biogeochemical factors.
Fig. 7 shows quite good agreement between the spatial distributions of high 15N values of periphyton and gambusia. As shown in Fig. 4, the actual corresponding 15N values at any site are about 7 different because of the 2-3 trophic levels between periphyton and gambusia. The strong spatial correspondence of high values in gambusia and periphyton suggests that denitrification (or some other process producing high 15N values) has caused the entire foodweb at these sites to be isotopically "labeled."
The high 34S values are oriented roughly parallel to L67, from WCA-2BS to Shark River. Hence, not only are the environmental isotope patterns persistent over time, but they have been incorporated up the foodchain from periphyton to gambusia. There is a slight enrichment in the gambusia (fractionation? differences in diet?) relative to the periphyton, and the May algae 34S values are also a little higher than the September values, consistent with a greater "redox" signal in the dry season as seen in the C isotopes. When the biomass 34S values along several likely water flowpaths were compared with the REMAP sulfate concentrations (Stober et al., 1998) measured at the same sites and times, there was absolutely no hint of the kind of inverse relation of 34S vs sulfate that would be expected for progressive sulfate reduction in the water column as the waters slowly move SW (as observed in the canal 34S data in Orem et al., 1998). This is moderately good refutation of the idea that the 34S values are the result of progressive water-column processes during movement of the sulfate "plume" southward, and supports the hypothesis that the biomass 34S values reflect sulfate reduction (and other S cycling) in the local sediments (and maybe in the periphyton).
The general similarities in the spatial distribution of 15N, 13C, and 34S values for periphyton and gambusia suggest that the isotopic compositions of the entire foodweb are being affected by the biogeochemical processes described above. Hence, in order to interpret differences in isotopic composition of organisms in terms of trophic levels, one must first "remove" the isotopic effects of these biogeochemical processes. One way to do this is by normalizing (by subtraction) all the organism isotope data to the isotopic composition of some widely-distributed "indicator species" that appears to have a relatively constant diet over time. Since gambusia collected at the same place and time (Fig. 3) show a small range in 15N and 13C values, they are a suitable candidate for a "normalizing agent." In a sense, normalizing organism data to the 15N and 13C values of gambusia removes the effect of the spatial patterns seen in Figs. 6-7, making it easier to interpret the "residuals" as possible trophic differences.
So why aren't we seeing the same patterns in the sediments (Fig. 9) as in the biomass? One possibility is that the isotope patterns seen in the algae and fish samples are simply not present in the macrophytes that are preserved as peat, and since the algae and fish are not major contributors to the formation of peat, their isotope patterns are simply not preserved. Preliminary data from USGS sites suggests that macrophytes do have a broad range of 34S values at some sites but it is not yet clear whether these values correspond with the 34S values of periphyton and gambusia. However, a more complete set of macrophyte samples (from REMAP II) will need to be analyzed to address this question. Another explanation is that the isotope patterns, especially the very striking 34S patterns seen in the current biota samples, reflect recent changes in environmental conditions that affect the biogeochemistry of the marshes, caused by increases in sulfate (and nutrients) derived from the EAA. The difference in the 34S patterns in modern biota and older sediments (mostly >50 years ??) is strong evidence for a significant increase in sulfate reduction in the marshes in the last few decades, probably in response to increased S loading from the EAA.
Spatial Variation in Foodwebs: The discrepancies between the 15N and 13C data suggest that different C sources are being utilized by different organisms, suggesting more than 1 foodchain at some locations or tremendous temporal/spatial variability in 13C. With the combination of 13C and 15N data, we can positively rule out some organisms as significant food sources to others (e.g., adult dragonflies). It is not yet clear whether there are different foodwebs in the N vs S Everglades. However, nutrient-impacted sites certainly have different isotopic compositions than more pristine sites, and we can see that some places have isotope values consistent with periphyton being an important C source (i.e., U3) and others where detrital materials appear to be very significant (i.e., ENR Cell 3).
The compositions and spatial distributions of the C, N, and S isotopic compositions 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 more variable water-column isotopic compositions produced by redox reactions (and other factors) in the ecosystem, and these same patterns are incorporated throughout the food chain. These compositions are relatively stable over time because the biomass remains in the system and is actively recycled without significantly affecting the isotopic compositions of the residual material. Therefore, zones frequently dominated by particular redox reactions appear to be labeled by the C, S, and N isotopic compositions of local organisms. Isotopic compositions of biomass may prove to be more cost-effective and reliable indicators of prevailing environmental conditions that favor MeHg production than other parameters currently being considered because biomass isotopic compositions are much more difficult to perturb than the more transient concentrations of aqueous species (like sulfate or sulfide). Hence, the spatial isotope patterns are likely to provide a valuable integration of long-term environmental conditions in the Everglades.
Orem, W.H., Bates, A.L., Lerch, H.E., Corum, M., and Boylan, A. (1999c) Sulfur contamination in the Everglades and its relation to mercury methylation. USGS Open-File Report 99-181, U.S. Geological Survey Program on the South Florida Ecosystem, Proceedings of South Florida Restoration Science Forum, May 17-19, 1999, Boca Raton, FL, pp. 78-79. Stober, J., Scheit, D., Jones, R., Thornton, K., Gandy, L., Trexler, J., and Rathbun, S. (1998) South Florida Ecosystem Assessment Monitoring for Adaptive Management: Implications for Ecosystem Restoration, Final Technical Report - Phase I, EPA#904-R-98-002, vol. 1.
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