projects > application of stable isotope techniques to identifying foodweb structure, contaminant sources, and biochemical reactions in the everglades > abstract
Effects of Seasonal and Spatial Differences in Food Webs on Mercury Concentrations in Fish in the EvergladesAbstract from the 2002 Spring AGU Meeting.
A clear understanding of the aquatic food web is essential for determining the entry points and subsequent biomagnification
pathways of contaminants such as methyl-mercury (MeHg) in the Everglades. Anthropogenic changes in nutrients can
significantly affect the entry points of MeHg by changing food web structure from one dominated by algal productivity to one
dominated by macrophytes and associated microbial activity. These changes in the base of the food web can also influence the
distribution of animals within the ecosystem, and subsequently the bioaccumulation of MeHg up the food chain.
As part of several collaborations with local and other federal agencies, more than 7000 Everglades samples were collected in
1995-99, and analysed for d13C and d15N. Many organisms were also analysed for d34S, gut contents, total Hg, and MeHg.
Carbon isotopes effectively distinguish between two main types of food webs: ones where algae is the dominant base of the
food web, which are characteristic of relatively pristine marsh sites with long hydroperiods, and ones where macrophyte
debris appears to be a significant source of nutrients, which are apparently characteristic of shorter hydroperiod sites, and
nutrient-impacted marshes and canals.
Many organisms show significant (5-12%) spatial and temporal differences in d13C and d15N values across the Everglades. These
differences may reflect site and season-specific differences in the relative importance of algae vs. macrophyte debris to
the food web. However, there is a lack of evidence that these sites otherwise differ in food chain length (as determined by
d15N values). This conclusion is generally supported by gut contents and mercury data. Furthermore, there are no
statistically significant differences between the Delta d15N (predator-algae) values at pristine marsh, nutrient-impacted
marsh, or canal sites.
The main conclusions from this preliminary comparison of gut contents, stable isotope, and Hg data are: (1) there is little
evidence for spatial variations in food chain length - hence, this does not appear to be the dominant explanation for spatial
variations in Hg in predators, (2) the poor correlation of d15N and Hg for many organisms, reflective of the heterogeneous
and dynamic nature of the ecosystem, makes it difficult to account for changes in Hg with trophic position, and (3) seasonal
and spatial variations in hydrology and nutrient conditions, which are often reflected in changes in the base of the food
web, appear to be the dominant controls on the isotopic compositions of organisms in the Everglades. Hence, biota isotopes
provide a tool for monitoring how future ecosystem changes affect the distribution of algae vs. macrophyte-dominated food
webs across the Everglades.
1ckendall@usgs.gov, U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA 94025 United States
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U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology This page is: http://sofia.usgs.gov /projects/int_geochem_foodweb/abstract-agu02.html Comments and suggestions? Contact: Heather Henkel - Webmaster Last updated: 11 October, 2002 @ 09:30 PM (KP) |