Stable Isotopes in Ecosystem Studies

Abstract

The 6 values are measures of the amounts of heavy and light isotopes in a sample. Increases in these values denote increases in the amount of the heavy isotope components (Figure 1). Conversely, decreases in values denote decreases in the heavy isotope content, and a reciprocal increase in the light isotope component. Standard reference materials are carbon in the PeeDee limestone, nitrogen gas. in the atmosphere, and sulfur from the Cafiyon Diablo meteorite. The precision of the measurements is typically-0.2% o or better, and current analysis costs typically range from $30-100 per sample through commercial firms. Many reactions alter the ratio of heavy to light isotopes, or "fractionate" stable isotopes, but.the degree of fractionation is typically quite small. Even very large changes of 100°/o o (10%) between reactants and products involve only minute absolute changes of 0.04%, 0.11%, and 0.44% for the heavy isotopes of nitrogen, carbon, and sulfur, respectively (Figure 1). A mass spectrometer is required for accurate detection of these small differences and gaseous samples are required for the isotopic determinations. Sample preparation differs from many. other ecological measurements in that quantitative yields (a complete conversion of sample to gas) are required. When yields are not quantitative, isotopic fractionation between product and residual materials may result in a false apparent isotopic composition of the samples. Many combustion schemes have been developed to quantitatively break down diverse molecules into the simple gases most suitable for mass spectrometry. Most laboratories currently employ a high temperature sealed tube combustion to convert carbon and nitrogen compounds to COz and Nz (70). N2 can also be prepared from Kjeldahl digestions or ammonia (70). Sulfur-containing materials are typically con~,erted to sulfates and sulfides, which are in turn converted quantitatively to SOz (30, 41, 122). Pure CO2, Nz, and SO2 are separated from one another and from water using various cold traps that allow differential volatilization and trapping under high vacuum conditions. A pure gas is then introduced into a dual or triple collector isotope ratio mass spectrometer, and its isotopic composition measured relative to a known standard.

Forest fire increases mercury accumulation by fishes via food web restructuring and increased mercury inputs

“…The nitrogen stable isotope ratio (␦ 15 N) is commonly used as a continuous measure of trophic position (32,33). Baseline ␦ 15 N, measured in long-lived sphaeriid clams, increased from 0.15‰ before the fire to 2.58‰ in 2001.…”

Quantitative Microbial Ecology through Stable Isotope Probing

“…This work advances a quantitative approach to stable isotope probing in order to elucidate taxon-specific processes that drive element cycling in intact communities, bringing to microbial ecology the power of stable isotopes to quantify rates of element fluxes into and through organisms (65,66). Like Chip-SIP (48,67), qSIP provides a means to quantify the ecology of organisms about which we know little more than the genetic fragment used to identify their unique place on the tree of life.…”

Isotopic variation among Amazonian floodplain woody plants and implications for food-web research

“…Stable isotope analysis has become a primary tool in food-web studies to depict production sources supporting consumers and to analyze food-web structure (Boecklen et al 2011, Layman et al 2012. Use of stable isotopes in food-web research relies on natural variation in the abundance of stable isotopes within components of the ecosystem (Peterson & Fry 1987, Fry 2006.…”