The use of stable isotopes to solve biogeochemical problems in ecosystem analysis is increasing rapidly because stable isotope data can contribute both source-sink (tracer) and process information: The elements C, N, S, H, and all have more than one isotope, and isotopic compositions of natural materials can be measured with great precision with a mass spectrometer. Isotopic compositions change in predictable ways as elements cycle through the biosphere. These changes have been exploited by geochemists to understand the global elemental cycles. Ecologists have not until quite recently employed these techniques. The reasons for this are, first, that most ecologists do not have the background in chemistry and geochemistry to be fully aware of the possibilities for exploiting the natural variations in stable isotopic compositions, and second, that stable isotope ratio measurements require equipment not normally available to ecologists. This is unfortunate because some of the more intractable problems in ecology can be profitably addressed using stable isotope measurements. Stable isotopes are ideally suited to increase our understanding of element cycles in ecosystems. This review is written for ecologists who would like to learn more about how stable isotope analyses have been and can be used in ecosystem studies. We begin with an explanation of isotope terminology and fractionation, then summarize isotopic distributions in the C, N, and S biogeochemical cycles, and conclude with five case studies that show how stable isotope measurements can provide crucial information for ecosystem analysis. We restrict this review to studies of natural variations in C, N, and S isotopic abundances, cxcluding from consideration ~5N enrichment studies and hydrogen and oxygen isotope studies. Our focus on C, N, and S derives in part from our

Stable isotopes in ecosystem studies

The 1999 Regional Haze Rule provides a context for this review of visibility, the science that describes it, and the use of that science in regulatory guidance The scientific basis for the 1999 regulation is adequate The deciview metric that tracks progress is an imperfect but objective measure of what people see near the prevailing visual range The definition of natural visibility conditions is adequate for current planning, but it will need to be refined as visibility improves Emissions from other countries will set achievable levels above those produced by natural sources Some natural events, notably dust storms and wildfires, are episodic and cannot be represented by annual average background values or emission estimates Sulfur dioxide (SO2) emission reductions correspond with lower sulfate (SO4 2−) concentrations and visibility im-provements in the regions where these have occurred Non-road emissions have been growing more rapidly than emissions from other sources, which have remained

Visibility: Science and Regulation

Nitrate (NO3 ) concentrations in public water supplies have risen above acceptable levels in many areas of the world, largely as a result of overuse of fertilizers and contamination by human and animal waste. The World Health Organization and the U.S. Environmental Protection Agency have set a limit of 10 mg L nitrate (as N) for drinking water because nitrate poses a health risk, especially for children, who can contract methemoglobinemia (blue-baby syndrome). Nitrate in lower concentrations is non-toxic, but the risks from long-term exposure are unknown, although nitrate is a suspected carcinogen. High concentrations of nitrate in rivers, lakes, and coastal areas can cause eutrophication, often followed by fi sh-kills, due to oxygen depletion. Increased atmospheric loads of anthropogenic nitric and sulfuric acids have caused many sensitive, low-alkalinity streams in North America and Europe to become acidifi ed. Still more streams that are not yet chronically acidic could undergo acidic episodes in response to large rain storms and/or spring snowmelt, seriously damaging sensitive local ecosystems. Future climate changes may exacerbate the situation by affecting biogeochemical controls on the transport of water, nutrients, and other materials from land to freshwater ecosystems. The development of effective management practices to preserve water quality, and remediation plans for sites that are already polluted, requires the identifi cation of actual N sources and an understanding of the processes affecting local nitrate concentrations. In particular, a better understanding of hydrologic fl owpaths and solute sources is required to determine the potential impact of contaminants on water supplies. Determination of the relation between nitrate concentrations in groundwater and surface water and the quantity of nitrate introduced from a particular source is complicated by:

Tracing Anthropogenic Inputs of Nitrogen to Ecosystems

Bacterial volatiles: the smell of small organisms

The biotic ligand model (BLM) is a mechanistic approach that greatly improves our ability to generate site-specific ambient water quality criteria (AWQC)for metals in the natural environment relative to conventional relationships based only on hardness. The model is flexible; all aspects of water chemistry that affect toxicity can be included, so the BLM integrates the concept of bioavailability into AWQC--in essence the computational equivalent of water effect ratio (WER) testing. The theory of the BLM evolved from the gill surface interaction model (GSIM) and the free ion activity model (FIAM). Using an equilibrium geochemical modeling framework, the BLM incorporates the competition of the free metal ion with other naturally occurring cations (e.g., Ca2+, Na+, Mg2-, H+), togetherwith complexation by abiotic ligands [e.g., DOM (dissolved organic matter), chloride, carbonates, sulfide] for binding with the biotic ligand, the site of toxic action on the organism. On the basis of fish gill research, the biotic ligands appear to be active ion uptake pathways (e.g., Na+ transporters for copper and silver, Ca2+ transporters for zinc, cadmium, lead, and cobalt), whose geochemical characteristics (affinity = log K, capacity = Bmax) can be quantified in short-term (3-24 h) in vivo gill binding tests. In general, the greater the toxicity of a particular metal, the higher the log K. The BLM quantitatively relates short-term binding to acute toxicity, with the LA50 (lethal accumulation) being predictive of the LC50 (generally 96 h for fish, 48 h for daphnids). We critically evaluate currently available BLMs for copper, silver, zinc, and nickel and gill binding approaches for cadmium, lead, and cobalt on which BLMs could be based. Most BLMs originate from tests with fish and have been recalibrated for more sensitive daphnids by adjustment of LA50 so as to fit the results of toxicity testing. Issues of concern include the arbitrary nature of LA50 adjustments; possible mechanistic differences between daphnids and fish that may alter log K values, particularly for hardness cations (Ca2+, Mg2+); assumption of fixed biotic ligand characteristics in the face of evidence that they may change in response to acclimation and diet; difficulties in dealing with DOM and incorporating its heterogeneity into the modeling framework; and the paucity of validation exercises on natural water data sets. Important needs include characterization of biotic ligand properties at the molecular level; development of in vitro BLMs, extension of the BLM approach to a wider range of organisms, to the estuarine and marine environment, and to deal with metal mixtures; and further development of BLM frameworks to predict chronic toxicity and thereby generate chronic AWQC.

Biotic Ligand Model, a Flexible Tool for Developing Site-Specific Water Quality Guidelines for Metals

Sulfur released from any given natural or anthropogenic source carries an isotopic signature that can be used to trace its flow through the environment. Measurements of the concentration and isotopic composition of sulfur in weekly bulk precipitation samples collected over a 4-year period at a remote location in northern Ontario were recorded. The long-term isotopic data and the measurement on the production and release of dimethyl sulfide from boreal wetlands show that biogenic sources can account for up to 30 percent of the acidifying sulfur burden in the atmosphere in remote areas of Canada. The data suggest that significant biological reemission of anthropogenic sulfur is occurring. The role of this process in the continuing acidification of the environment for years to come must be a matter of concern.

https://science.sciencemag.org/content/sci/237/4819/1189.full.pdf

Biogenic Sulfur and the Acidity of Rainfall in Remote Areas of Canada

(45) Seifert, W. K., see ref 14, p 21. (46) Pym, J. G.; Ray, J. E.; Smith, G. W.; Whitehead, E. V. Anal. Chem. 1975,47, 1617. (47) Datillung, M.; Albrecht, P. Mar. Pollut. Bull. 1976, 7,13. (48) Ensminger, A,; Joly, G.; Albrecht, P. Tetrahedron Lett. 1978,18, 1575. (49) Simoneit, B. R. T.; Kaplan, I. R. Mar. Enuiron. Res. 1980, 3, 113. (50) Seifert, W. K.; Moldowan, J. M.; Smith, G. W.; Whitehead, E. V. Nature (London) 1978,271, 436. (51) Eganhouse, R. P.; Simoneit, B. R. T.; Kaplan, I. R. Enuiron. Sci. Technol. 1981, 15, 315. (52) Simoneit, B. R. T.; Mazurek, M. A.; Cahill, T. A. J. Air Pollut. Control Assoc. 1980, 30, 387. (53) Crisp, P. T.; Brenner, S.; Venkatesan, M. I.; Ruth, E.; Kaplan, I. R. Geochim. Cosmochim. Acta 1979,43,1791. (54) Swisher, R. D. J. Am. Oil Chem. SOC. 1963,40, 648. (55) Lake, J. L.; Nonvood, C.; Dimock, C.; Bowen, R. Geochim.

Deposition and chemistry of pollutant metals in lakes around the smelters at Sudbury, Ontario.

The distribution of trace metals in humic acid, fulvic acid, and the associated sediments from Lake Ontario has been determined. Thc principal metal components of the humic acids are copper (1000-1600 pg/g) and iron (1000-2000 pg/g). Metals enriched in the humic and fulvic acids compared to the entrapping sediments include Cu, Pb, Ni, and Cr, whereas the concentrations of Co, Cd, Mn, and Zn are much less in these acids. It is estimated that nearly all the Cu, about 510% of the Pb, Cr, and Ni, and <5% of the Co, Cd, and Zn in the sediments are hound to the organic matter. Evidence is presented to show that the humic and fulvic matter have acquired most of their trace metal burden in the overlying water.

Trace metals in humic and fulvic acids from Lake Ontario sediments

SINCE the mid-1950s, there has been a marked increase in levels of air pollution in the Arctic region1–3. This is apparent from the pervasive haze that has been detected over large areas of the Northern Hemisphere3–6, which can cover up to 9% of the Earth's surface. The haze is most pronounced during January to April, and is characterized by a reduction in visibility and anomalously high levels of organic and inorganic compounds of the type found in polluted environments3,6,7. Most of the present knowledge about the origin of the chemical constituents of the haze comes from studies of elemental ratios and trajectory analyses2,4,6,7. Here we report the results of an analysis of the isotopic composition of sulphur in the Arctic haze. We find that most of the sulphur in the haze comes from Europe rather than from more local anthropogenic or biogenic sources, indicating that this form of air pollution becomes distributed globally.

Origin of sulphur in Canadian Arctic haze from isotope measurements

Sulphur pollution has closely been linked to the problems of acid precipitation and lake acidification1. Although variations in sulphur isotopes can in principle be used to fingerprint the sources of sulphur in the lacustrine environemnt (see refs 2–5), no attempt has so far been made to apply the technique in the study of pollutant sulphur in lakes in an historical sense. We report here the concentrations and isotopic composition of sulphur in sediments of several softwater lakes of northern Ontario. An objective of the study has been to ascertain whether the accumulation of sulphur in the sediments is related to the flux of pollutant sulphur into the lakes.

Robert D. Coker

Papers

Biogenic Sulfur and the Acidity of Rainfall in Remote Areas of Canada

Deposition and chemistry of pollutant metals in lakes around the smelters at Sudbury, Ontario.

Trace metals in humic and fulvic acids from Lake Ontario sediments

Origin of sulphur in Canadian Arctic haze from isotope measurements

Sulphur in sediments chronicles past changes in lake acidification