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Is Food Web Structure a Main Control on Mercury Concentrations in Fish in the
Everglades?
Carol Kendall and Bryan E. Bemis
U. S. Geological Survey, Menlo Park, CA
Joel Trexler
Florida International University, Miami, FL
Ted Lange
Florida Fish and Wildlife Conservation Commission, Eustis, FL
Q. Jerry Stober
U. S. Environmental Protection Agency, Athens, GA
Many lake studies have found good correlations of the mercury (Hg)
concentrations in fish and the relative trophic positions of fish as determined by
analyses of gut contents and/or stable isotopes. But is this true in dynamic marsh
ecosystems such as the Everglades? Based on the extensive REMAP assessment
of the Everglades ecosystem, Stober et al. (2001) concluded that spatial
(especially N-S) differences in “food web structure” are a likely cause of spatial
differences in Hg concentrations in mosquitofish. Food web structure is a general
term that encompasses differences in food chain length, food web complexity, and
food web base. The extensive isotope and gut contents datasets assembled by
several agencies can be used to evaluate this hypothesis.
Over 5000 algae, macrophyte, invertebrate, and fish samples were collected at 15
sites studied by the USGS ACME (Aquatic Cycling of Mercury in the
Environment) team during multiple trips 1995-1999. These samples were
analyzed for 13C and 15N to determine temporal and spatial differences in food
web structure; a smaller subset was also analyzed for 34S. Samples consisted of
both composites and individuals. Most samples were analyzed in bulk; however,
muscle tissue was analyzed for most large fish. This large dataset was used to
calculate the differences in 15N and 13C between all possible consumer-diet
pairs. The difference in 15N between consumer and diet (15N) is a proxy for
the length of the food chain between the species at a particular site and date, and
the difference in 13C (13C) reflects differences in the base of the food webs of
the species. An evaluation was then made of how often the 15N and 13C
values differed between types of sites. There were no statistically significant
differences in 15N values between high and low nutrient sites, suggesting that
there was no evidence for shorter food chains at high nutrient sites, as found by
Stober et al. (2001). However, there are statistically significant differences in
13C values between low and high nutrient sites. One likely explanation for the
13C differences is a difference in the dominant base of the food webs at such
sites. Food webs at low nutrient sites appear to be more algal dominated while
macrophyte debris appears to be a significant contribution to the base of the food
webs at high nutrient sites.
Samples of periphyton, mosquitofish, and sediment samples collected during
September 1996 at about 100 REMAP marsh sites were analyzed for 13C, 15N,
and 34S. The 15N calculated for periphyton and mosquitofish pairs (n = 68)
showed no significant correlation with region (Above Alligator Alley, Between
Alligator Alley and Tamiami Trail, Below Tamiami Trail) (p = 0.67) or latitude (p
= 0.20), and no correlation with mosquitofish Hg levels (p = 0.55), suggesting a
lack of evidence for spatial differences in food chain length. However, the 13C
values showed strong spatial patterns, including statistically significant
differences between data from different regions (p = 0.04) and latitudes (p < 0.01)
(fig. 1).
Figure 1. Graph showing significant
correlation of 13C with latitude (n =
68; p < 0.01; R2adj = 0.13).
One likely explanation for the patterns is that they reflect spatial differences in the
relative importance of algae versus macrophyte debris to the mosquitofish food
web. In general, the spatial patterns of 13C values are very similar to spatial
patterns interpreted by Stober et al. (2001) as differences in food web structure.
Hence, two very different sets of samples and data from the same regional
assessment agree that there are similar spatial differences in some aspect of the
food web structure, but disagree about whether this difference is mostly due to
trophic variations or food base differences. An evaluation of the gut contents of
mosquitofish collected in 1996 and 1999 at the same REMAP sites, indicated that
these data did not support the hypothesis that trophic position could be used to
explain spatial differences in Hg concentrations (Stober et al., 2001). We are
currently re-evaluating the isotope data in conjunction with the gut contents data
to see if this will provide new insights into causes of spatial variations in food
web characteristics.
This work was made possible by contracts from and collaborations with the US
Geological Survey, US Environmental Protection Agency, and Florida Fish and
Wildlife Conservation Commission.
References:
Stober, Q.J., K. Thornton, R. Jones, J. Richards, C. Ivey, R. Welch, M. Madden,
J. Trexler, E. Gaiser, D. Scheit, and S. Rathbun, 2001. South Florida Ecosystem
Assessment, EPA Report 904-R-01-003.
Carol, Kendall, U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo
Park, CA, 94025, Phone: 650-329-4576, Fax: 650-329-5590, [email protected],