Stable Isotope Applications for Studying Food Web Dynamics and Nutrient Sources I
Presiding: C Kendall, U.S. Geological Survey; G Cabana, University of Quebec at Trois Rivieres
H44A-01 15:30h
Environmental Factors That Control the Expression of Stable Isotope Fractionation During Nitrate Utilization by Phytoplankton
Kinetic isotope fractionation of the stable isotopes 15N and 14N during nitrate utilization by phytoplankton is a central process in determining the stable nitrogen isotope composition of organic matter within aquatic ecosystems. Many important applications of stable isotope research rely on knowledge of this fractionation factor in order to correctly interpret 15N/14N measurements, such as determination of trophic level within food webs, detection of the source of nitrogen to an ecosystem, and estimates of the extent of nitrate utilization in an environment. Variations in the isotope fractionation factor can alter both the organic and inorganic components of the nitrogen cycle and serve to complicate the interpretation of stable isotope measurements. I will summarize recent findings that resolve the mechanism for nitrate isotope fractionation by phytoplankton and illustrate how environmental parameters that affect growth rates can cause a reduction in the expression of the intrinsic fractionation factor. I will also show evidence that combining knowledge of these parameters with an understanding of the physical supply of nitrate to an ecosystem allows for the fractionation factor to be accurately incorporated into interpretations of 15N/14N within aquatic environments.
H44A-02 15:45h
Examining the Utility of Stable Hydrogen Isotopes in Aquatic Food-Web Ecology
The utility of stable hydrogen isotopes (dD) in hydrology and terrestrial ecology is well understood, but it has not been sufficiently examined in the field of aquatic ecology. Here, we present initial results from: (1) the Colorado River (AZ), Fossil Creek (AZ) and Devil's Hole (NV), where we examined the usefulness of dD to distinguish between allochthonous and autochthonous inputs to aquatic food webs, and (2) from the Sopochnaya River, Russia, where we tested the ability of dD to discern between anadromous and freshwater steelhead trout. In general, aquatic inputs (-320 to -168 per mil) were much more depleted than terrestrial inputs (-166 to -105 per mil). Macroinvertebrates displayed dD values similar to presumed food sources (e.g., baetid mayflies ranged from -299 to -222 per mil). In some cases, mixing models suggested that dD was a better predictor of food-source origin than d13C. As expected, dD values for anadromous trout (-121 to -103 per mil) were more enriched than those of freshwater residents (-161 to -123 per mil), and strong correlations existed between dD, d34S, and d13C. Methodological considerations (e.g., exchangeable hydrogen) and certain assumptions (e.g., importance of food vs. water on tissue dD) will be discussed.
H44A-03 16:00h
Linking Food Webs and Biogeochemical Processes in Wetlands: Insights From Sulfur Isotopes
To better understand the transfer of nutrients into prairie wetland food webs we have investigated the cycling of S (via S isotope systematics and geochemistry) in a prairie wetland landscape by characterizing sources (ground water, interstitial water, surface water) and processes in a small catchment comprised of four wetlands in eastern South Dakota. We focused on S to derive process information that is not generally available from carbon isotopes alone. The wetlands chosen for study spanned a considerable range in SO4 concentration (0.1-13.6 mM), which corresponded with landscape position. Ground water Δ34SSO4 values remained relatively constant (mean = -13.2 per mil) through time. However, Δ34SSO4 values of wetland surface waters ranged from -2.9 to -30.0 per mil (CDT) and were negatively correlated with SO4 concentrations (p<0.05). The isotopic variability of surface water SO4 resulted from mixing with re-oxidized sulfides associated with recently flushed wetland soils. The Δ34S signatures of wetland primary (Gastropoda: Stagnicola elodes) and secondary (Odonata: Anax sp.) consumers were significantly related to surface water Δ34SSO4 values (p<0.05) suggesting that food web components were responding to changes in the isotopic composition of the S source. Both primary and secondary consumer Δ34S signatures differed between wetlands (ANOVA, p<0.05). These data illustrate the complexity of S cycling in prairie wetlands and the influence of wetland hydrologic and biogeochemical processes on prairie wetland food webs. Additionally, this work has demonstrated that sulfur isotopes can provide unique source and process information that cannot be derived from traditional carbon and nitrogen isotope studies.
H44A-04 16:15h
Using Stable Isotopes to Link Nutrient Sources in the Everglades and Biological Sinks in Florida Bay: A Biogeochemical Approach to Evaluate Ecosystem Response to Changing Nutrient Regimes
Anthropogenic influences in South Florida have led to deterioration of its two major ecosystems, the Everglades wetlands and the Florida Bay estuary. Consequently, the Comprehensive Everglades Restoration Plan has been proposed to restore the Everglades ecosystem; however, restoration efforts will likely exert new ecological changes in the Everglades and ultimately Florida Bay. The success of the Florida Everglades restoration depends on our understanding and ability to predict how regional changes in the distribution and composition of dissolved organic and inorganic nutrients will direct the downstream biogeochemical dynamics of Florida Bay. While the transport of freshwater and nutrients to Florida Bay have been studied, much work remains to directly link nutrient dynamics in Florida Bay to nutrient sources in the Everglades. Our study uses stable C and N isotopic measurements of chemical and biological materials from the Everglades and Florida Bay as part of a multi-proxy approach to link nutrient sources in the Everglades to biological sinks in Florida Bay. Isotopic analyses of dissolved and particulate species of water, aquatic vegetation and sedimentary organic matter show that the watersheds within the Everglades are chemically distinct and that these signatures are also reflected in the bay. A large east-west gradient in both carbon and nitrogen (as much as 10‰ for Δ15N POM) reflect differing nutrient sources for each region of Florida Bay and is strongly correlated with upstream sources in the Everglades. Isotopic signatures also reflect seasonal relationships associated with wet and dry periods. High C and N measurements of DOM and POM measurements suggest significant influence from waste water in Canal C-111 in eastern Florida Bay, particularly during the dry season. These observations show that nutrients from the Everglades watersheds enter Florida Bay and are important in controlling biogeochemical processes in the bay. This study proves that stable isotopic measurements are important tools in determining nutrient source and biogeochemical processing in these ecosystems and can be used to evaluate future ecological responses to hydrologic restoration.
H44A-05 16:30h
Isotopic Perspectives on the Foundation of Estuarine-Dependent Fish Biomass: Macrophytes Versus Microphytes
Considerable trophic heterogeneity exists among the tidal rivers on Florida's west coast. Spring-fed estuaries tend to be oligotrophic, and urbanized surface-fed estuaries may be hypereutrophic. We compared wet- and dry-season primary producer sources of fish biomass among four rivers with differing trophic states. Dry-season 15-N distributions for vascular plants, microalgae and estuarine fish reflected river trophic status. Within rivers, 15-N values in vascular plants were regulated by extent of immersion in river water. All subaerial plants had 13-C values indicative of a common atmospheric CO2 source, whereas subaqueous microalgae had 13-C values that changed seasonally. In all rivers except the spring-fed Weeki Wachee, recycling of nitrogen within the sediments (i.e., denitrification) appeared to provide some of the nutrients used by benthic microalgae (BMA) during the dry season; the BMA were N-enriched relative to phytoplankton (POM) from the same waters. Most consumers appeared to be dependent on BMA during the dry season. Among Myakka and Peace River consumers, there was a pronounced shift from dry-season dependence on BMA to dependence on POM during the wet season. This contrasted with the oligotrophic Weeki Wachee River, where consumers remained dependent on BMA during both seasons. In the hypereutrophic Alafia River, only a partial shift to POM occurred, suggesting that the wet-season POM in the Alafia was less available to consumers. During the wet season, the chlorophyll maximum in the Alafia River shifts downstream to dredged locations that are prone to benthic hypoxia. We suggest that wet-season benthic hypoxia interferes with the transfer of sedimented POM biomass to higher trophic levels via surface-deposit-feeding macroinvertebrates.
H44A-06 16:45h
Isotopic Evidence for Chemosynthetic Contributions to the Subterranean Food Web of the Upper Floridan Aquifer
The possibility that subterranean life in the Upper Floridan aquifer can proliferate through a microbial food web based on chemosynthesis was examined using multiple isotopic measurements (13C, 14C, and 15N) on the tissues of obligate aquifer-dwelling (troglobitic) macrofauna of the Upper Floridan aquifer. Specimens that were captured at the mouth of springs along streams had Δ13C values that reflected terrestrial vegetation as a primary food resource. In contrast, highly depleted Δ13C values were measured in the tissues of some specimens which came from areas that were distant from any natural direct conduits to the land's surface. The highly depleted Δ13C signature exhibited by these subterranean organisms was well beyond those observed in biota supported by terrestrial photosynthetic production, but were consistent with microbial chemosynthetic pathways. Evidence for chemosynthesis was also provided by differences in Δ14C values. Thus, multiple sources of energy appear to be fueling the subterranean food web of the aquifer.