Isotopic Evidence for Spatial and Temporal Changes in Everglades Food Web
Structure
Bryan E. Bemis, Carol Kendall, and Scott D. Wankel
U.S. Geological Survey, Menlo Park, California, USA
Ted Lange
Florida Fish and Wildlife Conservation Commission, Eustis, Florida, USA
David P. Krabbenhoft
U.S. Geological Survey, Madison, Wisconsin, USA
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.
The nitrogen (15N) and carbon (13C) 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 15N and 13C values of
Everglades biota to investigate spatial and temporal variability in food web
structure. Plants, invertebrates, and fish were collected from 16 well-studied
ACME (Aquatic Cycling of Mercury in the Everglades) sites during 1995-1999 as
part of a collaboration between the USGS and the Florida Fish and Wildlife
Conservation Commission (FFWCC). These collections provided more than 350
site-date sampling groups.
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 15N and 13C represent a broad range of trophic positions
within the food web, from primary producers to top-tier predator fish. 15N and
13C values of primary producers reflect those of the water chemistry, modified
by isotopic fractionations associated with nutrient uptake and growth. The
isotopic compositions of invertebrates and fish are integrated measures of diet,
with an expected relative enrichment of the heavier isotope (i.e., higher 15N/14N
and 13C/12C) at each trophic level.
We use 15N versus 13C 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 15N and
13C toward higher trophic positions (e.g., site U3, September 1997) (fig. 1). The
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.
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.
magnitude of 15N and 13C 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 (e.g., McCutchan, 1999). However,
the statistically significant, yet different,15N:13C slopes for the biota analyzed
in each site-date group suggest that processes influencing consumer-diet isotopic
fractionations within a food web are spatially and temporally unique. The
15N:13C slope differences likely indicate spatial and temporal differences in the
food web base and/or complexity of trophic interactions.
15N:13C slopes are typically positive, as expected (fig. 1). However, rare
negative slopes (e.g., site F1, September 1997) suggest the unlikely possibility
that consumers are relatively depleted in the heavier isotope of carbon than their
diets (fig. 2). Although this pattern has been observed for individual species in
controlled growth experiments (e.g., 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 13C and 15N
values. For example, if mosquitofish, bluefin killifish, least killifish, and shrimp
are components of one food chain at site F1 and golden topminnow and gator flea
utilize another food web base (fig. 2), then this could explain the appearance of a
single, “reversed” trophic hierarchy in carbon isotope space.
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.
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