We have been investigating foodweb structure in the entire Everglades ecosystem with isotopic techniques since 1995. This study was intended to aid in the understanding of spatial, temporal, and species-specific variations in methyl mercury in game fish.

USGS Everglades Sites USEPA REMAP Sites (1999) USEPA REMAP Sites (1996)

To our immense surprise, almost none of our initial assumptions (which were based on literature reviews of previous isotope studies, most of which were in smaller and/or less complicated ecosystems in lakes and estuaries), turned out to be applicable to the Everglades. Hence, what started as a simple foodweb study at a few "typical" sites has turned out to encompass data from some 6000 samples collected from >300 sites, 15 of which were sampled intensively 1995-1998.

Our Everglades foodweb study is probably the largest isotope investigation of a wetlands ecosystem ever attempted. Thus far we have analyzed some 6000 samples for ¹³C and ¹⁵N, with another 1000 or so analyzed also for ³⁴S. Sample locations are shown in this figure. The samples cover the entire Everglades marsh area, south of the agricultural areas south of Lake Okeechobee, and west of the urban coastal corridor.

Most of the work has been focused on some 15 USGS sites where we attempted to study the food chain up to gar by intensive sampling at each site, a few times a year from 1995-98.

We also collaborated with the USEPA’s REMAP program and obtained samples of mosquitofish (Gambusia), periphyton (algal mats), and sediment from spatial synoptics in 1996 and 1999 (only the 1996 REMAP data will be discussed here).

Oligotrophic marsh sites (like U3, below and left) show the expected pattern (shown by the black arrow). However, all the nutrient-impacted (i.e., more eutrophic) marsh and canal sites (like F1, below right) show anomalous ¹³C values. These ¹³C values show a backwards trend that "appears" to indicate decreasing values up the food chain (as indicated by the black arrow).

U3	F1

The most likely explanation for the anomalous "backwards" ¹³C trend is that it is caused by seasonality in the ¹³C of the base of the foodweb. In particular, seasonal changes in the relative contributions of algae vs. macrophyte detritus to herbivores and omnivores would explain part of the pattern observed. It is NOT caused by lack of sampling of some critical plant with a very low ¹³C present at the sites -- we have tried and tried at many sites and many dates to find a "missing endmember", and looked and looked for plankton, but almost never found any present at the sites.

The mechanism by which seasonally variable mixing could cause the anomalous ¹³C pattern is illustrated by the set of small red foodweb arrows on the figure for F1.

Expectation: Trophic differences are the main control on ¹⁵N (and maybe ¹³C) values of organisms.

Reality: Wrong! The plots below show the spatial variations in the ³⁴S, ¹⁵N, and ¹³C of Gambusia collected at REMAP sites (9/96).

The 6 ‰ range in ¹⁵N values and the 12 ‰ range in ¹³C values are NOT caused by spatial differences in trophic levels of the fish. Instead, the patterns are caused by spatial differences in biogeochemical reactions that affect the ³⁴S, ¹⁵N, and ¹³C of the biota.

The color-contoured area is the marsh part of South Florida; the light blue denotes non-marsh areas. The dots show locations of Gambusia samples (5-10 per site).

Expectation: Disturbed sites (ones affected by high nutrients from agriculture) have shorter food chain lengths than more pristine sites.

The figures below (combined from Cabana and Rasmussen, 1994) show how ¹⁵N can be used to explain differences in Hg concentrations of the same species of fish in different ecosystems, by differences in food chain lengths.

The ¹⁵N values can distinguish between "Class 1" sites with short food chains (e.g., large fish eat zooplankton, so the fish are 1 trophic level above zooplankton) from "Class 3" sites with longer food chains (e.g., large fish eat smaller fish that preyed on insects that preyed on zooplankton, so that the large fish are 3 trophic levels above zooplankton). Shorter food chains result in less opportunity for biomagnification of contaminants.

Reality: Wrong! The diagram below (left) shows that there are NO consistent differences in food chain length (as indicated by the difference in ¹⁵N values between carnivorous fish and various possible diets) for nutrient-impacted sites and non-impacted (oligotrophic) marsh sites.

The diagram above (right) summarizes the differences in food chain lengths for different diets for both ¹⁵N and ¹³C, and shows that there ARE consistent differences in the food chain lengths for C, but not for N, between impacted and unimpacted (relatively pristine marsh) sites.

Expectation: Temporal variations in ¹³C and ¹⁵N are minimal so that sampling organisms at each site 2-3 times a year will be sufficient to define foodweb relations.

Reality: Wrong! The plots below show that there is considerable temporal variation in the ¹³C and ¹⁵N of organisms at some sites (this is oligotrophic site 3A-15). There is a strong correlation between seasonal changes in marsh water levels and the values of algae.

Small seasonal changes in water level may change the balance of photosynthesis, respiration, and atmospheric exchange reactions in the water column that control the ¹³C of DIC. Such changes will probably also affect the ¹⁵N of dissolved inorganic N (DIN) because of corresponding changes in N uptake and redox conditions.

The magnitudes of temporal changes in the ¹³C and ¹⁵N of biota collected at USGS sites (above) are similar to the ranges of values seen at the spatial scale at REMAP sites (left). The values of organisms collected at the same site usually show an inverse relation between ¹³C and ¹⁵N over time, especially in more pristine environments.

Expectation: Temporal variations in ¹³C and ¹⁵N will be largest at the base of the foodweb and will decrease with increasing trophic level, as shown in the schematic of the "foodweb triangle" below (left).

Reality: Wrong! In the Everglades, the spatial variability in ¹³C and ¹⁵N (spatial variability data above) results in a "foodweb hourglass" instead of the expected triangle, as shown in the schematic above (right).

This hourglass shape reflects the differences in migration areas, ages, and integration times for organisms of different trophic levels.

Contrary to expectation, the bass at the top of the food chain do not show well-integrated ¹³C and ¹⁵N values.

Expectation: Algae (in the form of the ubiquitous periphyton mats) is the dominant base of Everglades foodwebs.

Reality: Wrong! The figure below suggests that bulk periphyton (or algae with a similar average ¹³C value) is generally NOT the base of the foodweb leading to Gambusia at most sites sampled by the REMAP program in 9/96 because the ¹³C of periphyton is almost always higher than the corresponding ¹³C of Gambusia, in contradiction to the expected pattern.

Note that there is a much better correlation between the ¹⁵N values of Gambusia and periphyton, than the correlation between the ¹³C values. The average ¹³C value is -1.8 ‰.

(samples collected on plastic plates Oct.-Dec. 1997)

Expectation: Algal mats will show little temporal variation in ¹³C and ¹⁵N, and so will the organisms that live in them and eat algae.

Reality: Wrong! The plot on the right shows how the ¹³C and ¹⁵N vary over 3 months for 3 adjacent sites near U3 (an oligotrophic marsh site). Some of the variability is due to species succession, but most is due to changes in the ¹³C and ¹⁵N of water column DIC and DIN due to changing water levels.

During the dry season, the marshes probably become more anoxic because of shallower water levels, less photosynthesis, and increased quantities of decaying vegetation. Note that the ¹³C and ¹⁵N of algae show strong positive correlations with water level fluctuations (figures above). These oscillations are substantially damped up the food chain, probably because of the longer integration times of animals vs. plants.

Expectation: Non N-fixing aquatic plants will have similar ¹³C and ¹⁵N values since they all grow in the same environment.

Reality: Wrong! Look at the wide range of values for both algae and macrophytes on the plots below.

The figure above shows a simplified version of the typical foodweb found at relatively pristine marsh sites (e.g., U3, 3A-15) where algae is the dominant base of the foodweb; data are normalized to ¹⁵N and ¹³C of mosquitofish. Only major species are shown (of the ~ 100 species that have been collected and analyzed multiple times).

In general, the data show the expected increase in the ¹⁵N and ¹³C of biota up the food chain, from algae and detritus (at the base of the foodweb) to Florida gar (at the top). Note that algae (e.g., periphyton and epiphyton have very different ¹⁵N and ¹³C values than macrophytes (e.g, sawgrass, lilypads, cattails, etc).

The figure on the left shows the 15N values of biota (normalized to mosquitofish), grouped into trophic categories based on the gut contents data. Note the very large range of 15N values for plants.

The schematic on the right shows some of the processes that affect the isotopic compositions of DIC and DIN in aquatic systems, and as a result, the 13C and 15N of the plants and algae that grow there.

Processes that affect plant values (especially aquatic plants)

Expectation: Fish show increases in ¹⁵N and ¹³C with increasing size because of the increasing trophic level of the prey.

Reality: Wrong! Note the total lack of consistent ¹⁵N and ¹³C correlations with length in largemouth bass from different sites and dates (above, sites denoted by letters). This lack of correlation is caused by the spatial variability in ¹⁵N and ¹³C at the base of the food chain (see the spatial variability data above)

Expectation: Samples from a few sites and dates would be sufficient to characterize Everglades foodwebs.

Reality: Wrong! Over 6000 samples and 6 years later, we are now narrowing in on how the ecosystem works.

Morale: don’t attempt to characterize foodwebs in large, dynamic, marsh ecosystems unless you have your own isotope lab (as we do) or have LOTS of money for analyses.

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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