Isotopic Views of Food Web Structure in the Florida

Everglades

Introduction

Nearly one million acres of the Everglades are under a health

advisory that discourages the human consumption of largemouth

bass and several other fish because of high mercury contents.

Food web structure (base of food web, number of trophic steps)

plays a potentially critical role in determining the patterns of

mercury contamination of the Everglades ecosystem.

Methylmercury (MeHg) is present in low concentrations in water,

yet after entering the base of the food web it biomagnifies to toxic

concentrations in organisms that occupy higher trophic positions

(like bass). One of the main research questions under investigation

by a multi-agency task force in the Everglades is how MeHg

bioaccumulates up the food chain in this complex aquatic

ecosystem. Understanding variations in food web structure may

help explain mercury patterns in the Everglades and ultimately

lead to more effective restoration of Everglades ecosystems.

Approach

The primary focus of this study is to determine the trophic

structure of aquatic biota in the Everglades ecosystem by analyzing

tissue samples for nitrogen and carbon isotopes. 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 two sampling periods

(September 1997 and January 1998) when a sufficient number and

variety of aquatic organisms were collected to show the general

food web structure (Figure 1).

Isotopic Clues to Relative Trophic Position

Analysis of the stable nitrogen and carbon isotopic

compositions (δ

15N and δ13C values, respectively) of organisms in

a food web provides information about trophic relationships, or

“who eats whom”. This method is based on the observation that

selective metabolism of the lighter isotopes of these elements (

14N

and

12C) during food assimilation and waste excretion causes

animals to become enriched in the heavier isotopes (

15N and13C)

relative to their diets (McCutchan et al., 2003).

The δ

15N and δ13C values of tissues are integrated measures of

diet assimilated over time, with consumers typically enriched in

the heavier isotopes of nitrogen by about 3-4‰ and carbon by 0.8-

1.2‰ (i.e., higher

15N/14N and13C/12C) relative to their diet

(McCutchan et al., 2003). This expected stepwise isotopic

increase through the food chain (Figure 2A) has been used to

reconstruct relative trophic positions of organisms and estimate

mercury bioaccumulation rates in fish (Cabana and Rasmussen,

1994). Most studies using this approach have focused on

applications in northern temperate lakes, whereas relatively few

studies have applied these isotopic methods to complex,

subtropical wetlands like the Everglades.

Synoptic View of Everglades Food Webs

By comparing δ

15N and δ13C values for individual organisms,

we can gain valuable information about food web structure.

Figure 2B shows “average” isotopic relationships among

organisms collected from selected marsh sites across the

Everglades. For this comparison, isotopic values of organisms

were normalized to the isotopic composition of mosquitofish at

U.S. Department of the Interior Fact Sheet FS 2004-3138

U.S. Geological Survey November 2004

Figure 1. Satellite image of South Florida, showing biota collection

sites (image from South Florida Water Management District). The

inset map shows the location of the satellite image.
each collection site to eliminate inter-site biogeochemical

differences. As a check on the isotope-based reconstructions, we

use color-coded symbols on the isotopic plots to suggest general

trophic groups of the organisms analyzed:

The definitions of these groups are modified slightly from those of

Loftus et al. (1998), which were based on stomach content data

obtained in the Shark Slough region of Everglades National Park,

south of our study area. Stomach contents analysis provides direct

information about an organism’s recent foraging preferences.

However, unlike isotopic analysis, this method does not

distinguish between what an organism ingests and what it

assimilates (metabolizes) to make new tissue.

In general, organisms occupying relatively higher trophic

positions have correspondingly higher (less negative) δ

15N values,

as expected according to Figure 2A. For example, invertebrates

that eat plants and detritus (or other organisms with that diet)

typically have the lowest δ

15N and δ13C values of the fauna

sampled. The δ

15N values of fish noticeably increase as the

relative proportion of plants and algae in their diets decreases. For

example, sailfin molly (an herbivore) has relatively low δ

15N

values and Florida gar (a carnivore) has the highest, whereas

mosquitofish (an omnivore) has intermediate values. The

invertebrates generally have δ

13C values similar to those measured

for particulate organic matter (POM) and detritus, which suggests

that macrophytes cannot be a major food source to these marsh

food webs. A notable exception is crayfish, which has δ

13C values

more consistent with a macrophyte food source.

Differences in Food Webs Among Sites

The food webs fall into two general categories, depending on

whether or not δ

15N values can be used to distinguish among

relative trophic positions. An organism’s diet typically consists of

multiple food items of potentially different trophic positions, so

there is some expected overlap in δ

15N. However, for sites 3A-15,

U3, L35B, and F1, organisms occupying relatively higher trophic

positions have higher δ

15N values (Figure 3). For these sites, δ15N

can potentially be used to investigate MeHg bioaccumulation

pathways through the food web. Interestingly, site F1 shows an

apparent reversal of expected δ

13C patterns, whereby lower δ13C

values are found for higher trophic positions (Figure 3). The

reasons for this are unclear, but it is possible that multiple bases of

the food web with different δ

13C values (e.g., plants and detritus)

could contribute to this pattern. This could also result from

seasonal variations in the isotopic composition of dissolved

inorganic carbon and nitrogen in the water column, which affect

the δ

13C and δ15N values of plants at the base of the food web.

In contrast with the other sites discussed so far, sites L67 and

Cell 3 do not show distinct δ

15N values among different trophic

groups (Figure 4). For example, at both of these sites, sailfin

molly (an herbivorous fish) has the same δ

15N value as that of

largemouth bass (a top predator). Does this result mean that the

two fish had eaten the same diet (i.e., only plants or large fish)?

Obviously, that conclusion is unrealistic.

A more likely explanation for the poor separation of trophic

groups by δ

15N is that mobile, longer-lived fish like bass (i.e.,

those occupying the upper trophic positions) migrate as water

levels change in the Everglades. As they forage in regions with

different biogeochemical reactions, their tissues begin to acquire

the isotopic values of their new diet. Thus, the relatively low δ

15N

values of largemouth bass, Florida gar, warmouth, bowfin, and

chain pickerel at L67 and Cell 3 (compared with those expected

based on the more herbivorous biota) probably reflect isotopic

labeling in other local food webs. For the dates collected, these

Figure 2. Isotopic views of food webs in the Everglades: (A) Expected increase in δ

15N and δ13C of tissue with trophic level, based on field

and laboratory studies (McCutchan et al., 2003). (B) Average biota isotopic compositions from selected marsh sites across the

Everglades. Values are relative to mosquitofish. Error bars indicate one standard deviation. Organisms occupying relatively higher

trophic positions have higher δ

15N values, as predicted.

Mostly Carnivorous Fish (fish and decapods)

Mostly Omnivorous Fish (<25% plants & algae)

Herbivorous-Omnivorous Fish (>25% plants & algae)

Invertebrates

Plants & Algae

Particulate Organic Matter (POM) & Detritus

0

3

6

9

12

0 0.8 1.6 2.4 3.2

Relativ

e

δ

15N (‰)

Relative δ

13C (‰)

-10

-8

-6

-4

-2

0

2

4

-4 -2 0 2 4

δ15

N (relativ

e to mo

s

q

uitofis

h)

δ

13C (relative to mosquitofish)

Least Killifish

Mosquitofish

Shrimp

Bluefin

Killifish

Crayfish

Florida Gar

Sailfin Molly

Warmouth

Golden

Topminnow

Periphyton

Epiphyton

Chironomid

Largemouth

Bass

Damselfly

Naiad

Water

Boatman

Beetle

Zooplankton

Spider

Bladderwort

Dragonfly

Naiad

POM & Detritus

Macrophytes

A B

Plants & Detritus

Herbivores

Omnivores

Carnivores
sites appear not to be amenable to isotopic reconstruction of food

web structure.

The Influence of Local Biogeochemistry

In addition to providing clues about food web structure, this

dataset also provides valuable information about biogeochemical

processes in the Everglades environment. Differences in the δ

15N

and δ

13C ranges of organisms among sites (Figures 3-4) suggest

that isotopic values largely reflect local biogeochemical

differences in the water column or sediments. Such processes

include denitrification, which increases the δ

15N value of dissolved

nitrate, and methane production and subsequent oxidation, which

decrease the δ

13C value of dissolved CO2. The isotopic

compositions of aquatic plants and detritus appear to integrate

these isotopic signatures, and the patterns are reflected in the δ

15N

and δ

13C values of higher-level consumers in the aquatic food web.

Because there is considerable spatial variability in nutrient sources

and biogeochemical processes across the Everglades, it is difficult

to determine the trophic relationships in many locations. The large

δ

15N variation in organisms throughout the food web at a given site

makes δ

15N values more useful than δ13C for establishing relative

trophic positions. In contrast, the smaller trophic influence on δ

13C

Figure 3. Collection sites for which δ

15N differentiates organisms occupying different trophic positions. Symbols are averages, with the

number of organisms analyzed shown in parentheses. 3A-15 (January 1998); U3 (September 1997); L35B (January 1998); F1

(September 1997).

0

2

4

6

8

10

12

14

16

-38 -36 -34 -32 -30 -28 -26 -24 -22

δ

15N (‰)

δ

13C (‰)

Least Killifish (15)

Mosquitofish (14) Shrimp (9)

Bluefin Killifish (5)

Lake Chubsucker (2)

Florida Gar (4)

Sailfin Molly (4)

Gator Flea (5)

Warmouth (1)

Golden

Topminnow (7)

Crayfish (5)

Filam. Mat (6)

Hydrilla (2)

Periphyton (1)

Spotted Sunfish (4)

Epiphyton

(7)

Bladderwort (7)

Snail (1)

POM (11)

Cichlid (3)

Pygmy Sunfish (1)

Large Boatman (5)

Water Boatman (5)

Beetle (5)

Worm (5)

Spider (1)

Pickeralweed (7)

0

2

4

6

8

10

12

14

16

-38 -36 -34 -32 -30 -28 -26 -24 -22

δ

15N (‰)

δ

13C (‰)

Seminole

Killifish (1)

Largemouth

Bass (6)

Redear Sunfish (6)

Bladderwort (1)

Mosquitofish (5)

Golden Topminnow (3)

Least Killifish (2)

Shrimp (6)

Bluefin Killifish (1)

Bluegill (9)

Gator Flea (1)

Periphyton (4)

Damselfly

Naiad (1)

Spotted Sunfish (5)

Snail (1)

POM (1)

Spider (1)

Large Boatman (1)

0

2

4

6

8

10

12

14

16

-38 -36 -34 -32 -30 -28 -26 -24 -22

δ

15N (‰)

δ

13C (‰)

Redear Sunfish (5)

Mosquitofish (19)

Shrimp (14)

Bluefin Killifish (1)

Lake Chubsucker (11)

Florida Gar (5)

Largemouth Bass (11)

Sailfin Molly (2)

Damselfly Naiad (1)

Warmouth (11)

Bowfin (13)

Golden Shiner (4)

Dragonfly

Naiad (1)

Crayfish (1)

Filam. Mat (3)

Cattail (6)

Yellow Bullhead (2)

Sawgrass (8)

Pickerelweed (7)

Chain Pickerel (1)

Spotted Sunfish (14)

Epiphyton (1)

Black Crappie (2)

Bladderwort

(2)

Brown Bullhead (1)

Bluegill (5)

POM (5)

[δ

13C = -37.4‰

δ

15N = -3.8‰]

Spider (1)

Spotted Tilapia (3)

Seminole

Killifish (1)

Swamp

Darter (1)

Eel Grass (9)

Duck Potato (10)

0

2

4

6

8

10

12

14

16

-38 -36 -34 -32 -30 -28 -26 -24 -22

δ

15N (‰)

δ

13C (‰)

Cattail (1)

Periphyton (1)

Mosquitofish (5)

Golden

Topminnow (1)

Least

Killifish (4)

Shrimp (5)

Bluefin

Killifish (1)

Filam. Mat (1)

Gator Flea (1)

POM (1)

3A-15 U3

L35B F1
values (Figure 2A) and the large range of biota δ

13C values caused

by various environmental effects mean that the different trophic

positions within the food web do not separate significantly by δ

13C.

Instead, δ

13C appears to better reflect food web base (e.g., algae or

detritus).

Food Web Structure and Everglades

Restoration

Improving the water quality in the Everglades is a critical goal

of restoration efforts in this ecosystem. Part of this effort has

focused on monitoring mercury concentrations in sport fish and

other organisms, with the goal of understanding the response of the

ecosystem to changes in water levels, contaminant loads, and other

factors. To provide the context for these measurements, we must

address how mercury accumulates to toxic levels in higher trophic

positions within the food web.

Nitrogen isotopes of organisms can discriminate among

relative trophic positions at some Everglades sites (and therefore

describe food web structure and bioaccumulation pathways), but

not at others. Spatial differences in biogeochemical reactions

across the Everglades complicate our ability to use isotopes for

this purpose. More importantly, monitoring chemical indicators in

organisms collected at a given site (e.g., isotopes, mercury

concentration) may result in misleading conclusions about what

those indicators mean if one assumes that the organisms do not

move. Longer-lived organisms that migrate seasonally and feed in

different regions of the Everglades likely reflect a weighted

average of chemical “labeling”. The implications of this effect are

that monitoring mercury concentrations in large sport fish like

largemouth bass may not be an accurate measure of mercury

responses to restoration efforts at that site. Monitoring shorterlived species that do not migrate into and out of a given collection

site may be a more effective approach toward assessing the success

of Everglades restoration.

Bryan E. Bemis and Carol Kendall

U.S. Geological Survey

345 Middlefield Road

Menlo Park, CA 94025

References

Cabana, G. and Rasmussen, J.B., 1994. Modelling food chain

structure and contaminant bioaccumulation using stable nitrogen

isotopes. Nature, 372: 255-273.

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.

McCutchan, J.H., Jr., Lewis, W.M., Jr., Kendall, C. and McGrath,

C.C., 2003. Variation in trophic shift for stable isotope ratios of

carbon, nitrogen, and sulfur. OIKOS, 102: 378-390.

The data on which this Fact Sheet is based are available at the

website http://wwwrcamnl.wr.usgs.gov/isoig/projects/Everglades.

Related information is available at the website

http://sofia.usgs.gov/people/kendall.html.

For more information about isotopic studies in the

Everglades, please contact:

Carol Kendall

U.S. Geological Survey

345 Middlefield Road

Menlo Park, CA 94025

Phone: (650) 329-4576

Email: ckendall@usgs.gov

Figure 4. Sites for which δ

15N does not help distinguish organisms occupying different relative trophic positions. Symbols are averages,

with the number of organisms analyzed shown in parentheses. L67 (January 1998); Cell 3 (January 1998).

0

2

4

6

8

10

12

14

16

-38 -36 -34 -32 -30 -28 -26 -24 -22

δ

15N (‰)

δ

13C (‰)

Redear

Sunfish (8)

Mosquitofish (20)

Golden

Topminnow (3)

Least

Killifish (8)

Shrimp (11)

Bluefin

Killifish (8)

Lake

Chubsucker (9)

Florida

Gar (6)

Largemouth

Bass (15)

Sailfin

Molly (5)

Damselfly Naiad (2)

Spotted Sunfish (4)

Warmouth

(7)

Bowfin

Go (6) lden Shiner (11)

Dragonfly Naiad (1)

Crayfish (3)

Gator

Flea (2)

Cattail (7)

Lily Pad (8)

Water Lettuce (7)

Chain

Pickerel (4)

Bluegill (2)

Periphyton (1)

B Hydrilla (5) ladderwort (2)

POM (4)

Tilapia (2)

Flagfish (2)

Hyacinth (3)

0

2

4

6

8

10

12

14

16

-38 -36 -34 -32 -30 -28 -26 -24 -22

δ

15N (‰)

δ

13C (‰)

Least Killifish (9)

Mosquitofish (15)

Shrimp (15)

Bluefin Killifish (4)

Lake Chubsucker (2)

Florida Gar (2)

Sailfin

Molly (4)

Gator

Flea (11)

Bowfin (3)

Golden Shiner (9)

Cattail (25)

Filam. Mat (5)

Hydrilla (1)

Damselfly Naiad (4)

Epiphyton

(7)

Water

Lettuce (11)

Snail (4)

Bluegill (6)

Brown Bullhead (2)

Largemouth

Bass (2)

Bladderwort (7)

Redear Sunfish (4)

Scuds

(1)

Sawgrass (7)

Spikerush (6)

Dragonfly

Naiad (9)

Crayfish (10)

[δ

13C = -20.3‰

δ

15N = 12.7‰]

Blue Tilapia (5)

May

Fly (1)

Spider (1)

Duckweed (1)

L67 Cell 3