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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.

No abstract

A combination of stable isotopic measurements was used to study food web structure of Georges Bank, an important northwestern Atlantic fishing ground. Particulate, invertebrate, and fish samples were analyzed to test δ13C, δ15N, and δ34S measurements as trophic‐level indicators in offshore systems. Neither sulfur nor carbon measurements proved valuable. Sulfur isotopic compositions showed little change with trophic level, and an apparent diversity of phytoplankton carbon isotopic inputs at the base of the food web complicated use of δ13C to estimate trophic position. Nitrogen isotopic distributions were, however, robust measures of trophic position and showed four broad trophic levels; unsampled large top carnivores may represent a fifth trophic level.

Using natural-abundance C/C ratios as tracers, carbon turnover rates were determined for postlarval brown shrimp, Penaeus aztecus, in five laboratory growth experiments. Although tissue turnover in adult animals generally occurs during maintenance metabolism and is a function of time, turnover for young postlarval shrimp was accelerated during growth, and was primarily a function of weight gained rather than time. Metabolic loss of tissue carbon during growth was usually approximated by the function, Fraction lost=1-(initial weight/final weight). For shrimp that switch diets in the sea, model calculations show that this high turnover rate coupled with a four-fold weight increase suffices for shrimp to achieve a close isotopic resemblance of 1‰ or less (δC units) to the new diet.In accordance with these predictive calculations, shrimp which had increased in weight by a factor of four or more in the culture experiments showed essentially constant isotopic values reflecting their new diets. For these larger animals, the average animal-diet difference varied across three diets from-0.9 to +11‰, and the δC range among individuals was ≦1.4‰ in each experiment.