Trophic structure within a food web is often implicated as a control on how mercury is distributed and transferred throughout an aquatic ecosystem. Methyl mercury bioaccumulates in the food web, with higher concentrations typically found in tissues of organisms that occupy higher trophic positions. Reducing mercury contamination in the ecosystem is a major focus of restoration efforts in the Everglades. Therefore, knowledge about Everglades trophic structure is critical for making management decisions about how to effectively minimize mercury concentrations in biota.

Figure 1. 15N versus 13C at site U3 during September 1997. The 15N:13C slope for invertebrates and fish is +1.3 (n = 12; p = 0.02; R2adj = 0.37). Symbols are median values. [larger image]

The nitrogen (

N) and carbon (

C) isotopic composition of tissues is a tracer of diet that can be used in many aquatic ecosystems to distinguish the relative trophic positions of organisms. We use measured

N and

Within this massive data set, we focus on several sites with multiple collection periods during 1996-1998 that have a sufficient number of organisms to investigate spatial and temporal differences in food web structure. The organisms analyzed for tissue

N and

C represent a broad range of trophic positions within the food web, from primary producers to top-tier predator fish.

N and

Figure 2. 15N versus 13C at site F1 during September 1997. The 15N:13C slope for invertebrates and fish is –1.5 (n = 6; p = 0.03; R2adj = 0.66). Symbols are median values. [larger image]

We use

N versus

C plots to identify relative trophic positions of biota within the food web for each site and collection date. Based on laboratory and field studies, the expected pattern of trophic enrichment is increasing

N and

The magnitude of

N and

C ranges varies among sites and collection dates. This is not entirely surprising, given the large range of consumer-diet isotopic fractionations reported for different species (for example, McCutchan, 1999). However, the statistically significant, yet different,

N:

C slopes for the biota analyzed in each site-date group suggest that processes influencing consumerdiet isotopic fractionations within a food web are spatially and temporally unique. The

N:

N:

C slopes are typically positive, as expected (fig. 1). However, rare negative slopes (for example, site F1, September 1997) suggest the unlikely possibility that consumers are relatively depleted in the heavier isotope of carbon than their diets (fig. 2). Figure 3 shows location of sampling sites U3 and F1, in Water Conservation Area 2. Although this pattern has been observed for individual species in controlled growth experiments (for example, DeNiro and Epstein, 1978), an alternative explanation for the multiple-species pattern plotted here is that the biota isotopes reflect the presence of multiple food web bases with distinct

C and

Figure 3. Location of sampling sites U3 and F1, in Water Conservation Area 2. [larger image]

References

DeNiro, M.J. and Epstein, S., 1978. Influence of diet on the distribution of carbon isotopes in animals. Geochimica et Cosmochimica Acta, 42: 495-506.

McCutchan, J.H., Jr., 1999. Carbon Sources for Macroinvertebrates in St. Vrain Creek, Colorado. Ph.D. Thesis, University of Colorado, Boulder, 300 pp.

Contact: Bryan, Bemis, U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA, 94025, Phone: 650- 329-5603, Fax: 650-329-5590, bebemis@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

Back to Project Homepage