Spatial and Temporal Patterns in Isotopic Composition of Aquatic Organisms at
Everglades Nutrient Removal Project Sites
Scott D. Wankel, and Carol Kendall
U. S. Geological Survey, Menlo Park, CA
Peter J. Rawlik
South Florida Water Management District, West Palm Beach, FL
The Everglades Nutrient Removal Project (ENR) began in 1994 with the goal of
reducing the phosphorus (P) load downstream of the Everglades Agricultural
Area. The effects of different vegetative habitats in treatment cells on P loads
were evaluated. Cell 3 is dominated by emergent macrophytes such as cattails and
water hyacinths, while cell 4 is dominated by an assemblage of submerged
macrophytes and periphyton. During some sampling periods, there appeared to be
significant differences in the mercury (Hg) concentrations of some aquatic
organisms from the different cells, perhaps as a consequence of differences in
food web structure (Hurley et al., 1999). We have analyzed sets of biota collected
in 1997-98 from several locations in the four different cells of the ENR for 15N
and 13C to (1) better understand the biogeochemical controls on the isotopic
composition of Everglades foodwebs, and (2) assess whether there is any
evidence for differences in food webs at the different sites that might explain
differences in Hg concentrations.
Statistically significant differences in the isotopic composition (both 13C and
15N) of muscle tissue from mosquitofish, least killifish, bluefin killifish, sailfin
molly and grass shrimp in Cell 3 between June 1997 and June 1998 suggest a shift
in the isotopic composition of the diets of these fish. Samples collected in June
1997, January 1998, and June 1998 show increasing 15N values and decreasing
13C values. For example, the 15N of least killifish increase from +11.3 to
+13.2‰ while the 13C values decrease from -25 to -29‰. This swing in the
isotopic composition of these fish can be explained either by a change in diet or
by a shift in the isotopic composition of their food source. The close tracking of
the isotopic compositions of omnivorous fish (mosquitofish, killifish) with
herbivorous fish (sailfin molly) over time suggests changes in the isotopic
compositions of the algae. Since the isotopic composition of algal material is
controlled largely by the composition of the inorganic nutrients within the water
column, the isotopic composition of the fish reflects changes in the
biogeochemistry operating in the water column at the time of algal growth.
The 15N and 13C values for mosquitofish, sailfin molly, and shrimp collected in
June 1998 from Cell 3 and Cell 4 show significantly different averages and
distributions (fig. 1). The data from Cell 3 plot in distinct clusters whereas data
from Cell 4 show a striking alignment along a negative slope. It is not clear
whether these differences reflect differences in diet or just differences in the
isotopic composition of plants at the base of the food webs. At both sites, sailfin
mollys have the lowest 15N and highest 13C values, shrimp have intermediate
values, and mosquitofish have the highest 15N and lowest 13C values. Hence,
there appears to be no major differences in their relative trophic positions in June
1998. Therefore, it is likely that the differences in 13C and 15N of organisms
between Cell 3 and Cell 4 reflect differences in the isotopic composition of the
diets between cells. Aquatic plants from Cell 4 show a much greater range of
13C values than plants from Cell 3, but a similar range of 15N values. The larger
range of 13C values in Cell 4 may be related to the higher amounts of open water
and periphyton growth in Cell 4 compared to Cell 3. Increased light penetration
may cause larger changes in the 13C of dissolved inorganic carbon (DIC) in the
water column by allowing more light penetration and therefore more benthic
photosynthesis. Higher rates of benthic photosynthesis would lead to higher 13C
values as seen in the organisms sampled in Cell 4.
Figure 1. 13C and 15N values of selected biota collected from Cell 3 and Cell 4
in June 1998.
The variations in isotopic composition over both time and space within the ENR
emphasize the importance of localized biogeochemistry for interpretation of
stable isotope data. Fluctuations in water depth (hydroperiod) and plant cover
over time in the ENR may explain a significant portion of the variability of 13C
and 15N of primary consumers (mosquitofish, killifish, sailfin mollies).
Understanding the mechanisms for the foodweb base shift, whether it is a shift in
diet or isotopic composition of the diet (or both?), is critical to tracing
contaminant bioaccumulation within these Everglades foodwebs.
References
Hurley, J.P., Cleckner, L.B., and Gorski, P., 1999. Everglades Nutrient Removal
Project small fish bioaccumulation study – final report to the SFWMD (contract
PC C-8691-0300).
Scott Wankel, U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo
Park, CA, 94025, Phone: 650-329-4303, Fax: 650-329-5590, sdwankel@usgs.gov