Laura Sigg: Investigating the Speciation, Bioavailability and Ecotoxicology of Trace Metals in Natural Waters

Laura Sigg is an internationally renowned expert in the aquatic geochemistry and ecotoxicology of trace metals. During her outstanding career as a researcher at Eawag, the Swiss Federal Institute of Aquatic Science and Technology, and adjunct professor at the Department of Environmental Systems Science of ETH Zurich, she pioneered the development of new theoretical concepts as well as practical methods for the assessment of the speciation, fate and effect of trace metals in the hydrosphere (Fig. 1). During his stay in Toulouse as invited professor some years ago, George Luther III proposed us to edit a special issue of Aquatic Geochemistry to honor our colleague Laura Sigg, who serves as a long-term associate editor of this journal. We were very happy to take up this challenge and invited people to submit original manuscripts to a special issue focusing on the fields of speciation, bioavailability and ecotoxicology of trace metals in natural waters. After being educated in chemistry with Paul W. Schindler at the University of Bern, where she learned the precision and rigor of experimental work and how to assess and synthetize ideas, Laura Sigg moved to Zurich and worked for her PhD with Werner Stumm. She started the first part of her career with ground-breaking research on thermodynamic models for adsorption processes at the solid–water interface and published two highly influential papers which became classics in this field (Stumm et al. 1980; Sigg and Stumm 1981)

Retention of Sediments and Nutrients in the Iron Gate I Reservoir on the Danube River

This work addresses an intensively debated question in biogeochemical research: "Are large dams affecting global nutrient cycles?” It has been postulated that the largest impoundments on the Lower Danube River, the Iron Gates Reservoirs, act as a major sink for silica (Si) in the form of settling diatoms, for phosphorus (P) and to a lesser extent for nitrogen (N). This retention of P and N in the reservoir would represent a positive contribution to the nutrient reduction in the Danube River. Based on a 9-month monitoring scheme in 2001, we quantified the nutrient and the sediment retention capacity of the Iron Gate I Reservoir. The sediment accumulation corresponded to 5% TN (total nitrogen), 12% TP (total phosphorus) and 55% TSS (total suspended solids) of the incoming loading. A mass balance revealed that more N and P are leaving the reservoir than entering via the inflow. Based on these current results, the reservoir was temporarily acting as a small nutrient source. The nutrient accumulation in the sediments of the Iron Gate I Reservoir represents only 1% of the "missing” load of 106t N and 1.3 × 105t P defined as the difference between the estimated nutrient export from the Danube Basin and the measured flux entering the Black Sea. This result disproves the hypothesis that the largest impoundment on the Danube River, the Iron Gates Reservoir, plays a major role in N and P eliminatio

From Molecules to Ecosystems: Topics, Challenges, and Players in Environmental Chemistry

This article is meant to be appetizer and introduction to this special issue of CHIMIA on 'Chemistry at EAWAG'. Two major topics presently challenge environmental chemistry, not only at EAWAG: The human impact on global and regional biogeochemical cycles and the pollution of the environment by anthropogenic chemicals. In order to tackle the various tasks associated with these problems, environmental chemists have to build bridges between the molecular scope of laboratory experiments and the systems approach of comprehensive field and modeling studies. This requires the ability to collaborate with other disciplines and to combine their knowledge with a profound understanding of chemistry. Some consequences of these requirements for research and education in environmental chemistry are also addressed in this article

Solute transfer across the sediment surface of a eutrophic lake: I. Porewater profiles from dialysis samplers

Porewater profiles often are used to identify and quantify important biogeochemical processes occurring in lake sediments. In this study, multiple porewater profiles were obtained from two eutrophic Swiss lakes using porewater equilibrators (peepers) in order to examine spatial and seasonal trends in biogeochemical processes. Variability in profile shapes and concentrations was small on spatial scales of a few meters, but the uncertainty in calculated diffusive fluxes across the sediment surface was, on average, 35%. Focusing of Fe and Mn oxides toward the lake center resulted in systematic increases in porewater concentrations and diffusive fluxes of Fe2+ and Mn2+ with increasing water depth; these fluxes are postulated to be regulated by the pH-dependent dissolution of reduced-metal phases. Despite higher concentrations of inorganic carbon, NH 4 + , Si and P in pelagic compared to littoral sites, diffusive fluxes of these substances across the sediment surface increased only slightly or not at all with increasing water depth. Porewater profiles did reveal temporal changes in Fe2+, Mn2+, Ca2+ and Mg2+ that were an indirect result of the large, seasonal changes in seston deposition, but no clear seasonal variations were found in diffusive fluxes of nutrients across the sediment surface. The intense mineralization occurring at the sediment surface was not reflected in the porewater profiles nor in the calculated diffusive fluxes. Calculated diffusive fluxes across the sediment surface resulted from decomposition occurring primarily in the top 5-7 cm of sediment. Diffusive fluxes from this subsurface mineralization were equal to the solute release from mineralization occurring at the sediment-water interface. Buried organic matter acts as a memory of previous lake conditons; it will require at least a decade before reductions in nutrient inputs to lakes fully reduce the diffusive fluxes into the lake from the buried reservoir of organic matte

Biogenic silica accumulation in the sediments of Iron Gate I Reservoir on the Danube River

Abstract.: Damming of rivers can result in severe downstream effects such as changing sediment and nutrient fluxes that potentially affect coastal ecosystems. Closing of the Iron Gates Dams in the lower Danube River was linked to a decrease in dissolved silica flux to the Black Sea of 600,000 t yr−1. A recent study on the Iron Gate I, however, indicated a dissolved silica removal within the reservoir of only 16,000 t yr−1. Such an order of magnitude difference between actual budgets and earlier estimates is unlikely to be caused by changes in hydrological or biogeochemical conditions. In order to separate annual variations and downstream effects of damming, we analyzed the sedimentary records of biogenic silica using dated sediments. Results confirm the detailed budgets of dissolved silica. In 2001, a total biogenic silica accumulation in the sediments of the Iron Gate I Reservoir of 19,000 t Si yr−1 was determined and represents the highest retention over the past 20 years. The accumulation of biogenic silica in the Iron Gate I Reservoir was compared with data from the coastal Black Sea. Biogenic silica in the sediments of the coastal Black Sea start decreasing before Iron Gate I Dam was completed in 1971. In conclusion, construction of the largest impoundment on the Danube River, the Iron Gate I Reservoir, was not solely responsible for decreasing the silica loads downstream at the coastal Black Se

Surface Complexation and Its Impact on Geochemical Kinetics

The weathering of rocks, the formation of soils, the alteration and dissolution of sediments are a consequence of surface reac-. tions. Furthermore, many redox processes such as the oxidation of V02+, Mn2+ and Fe2+, the ncn-biotic degradation of organic substances and photosensitized processes are catalyzed by surfaces. The electric double layer theory, despite its efficiency in quantifying certain phenomena of colloid stability, has limitations because it neglects chemical speciation at the surface and does not provide information on the chemical structure of the interfacial region. The surfaces of naturally occurring solids are characterized by functional groups, e. g., OH- groups on the surface of hydrous oxides at on organic surfaces. Specific adsorption of - or interaction with - H+, OH-, metal ion s and ligands occurs through coordination at the surface; inner-sphere surface complexes can be formed. The form of occurrence of the individual compounds (speciation) needs to be known in order to understand their reactivity; especially the geometry of the coordination shell of surface sites or of reactants at surfaces is a prerequisite for interpreting reaction rates occuring at the particle-water interface. Some case studies on the oxidation of Mn2+ and V02+ and on the dissolution of hydrous oxides and silicates are presented. In each case, the kinetics of the processes and how it is affected by solution variables such as H+ and ligands (such as oxalate and other di- ar hydroxy-carboxylates) are explained by simple mechanistic models that involve the coordination at the mineral-solution interface. Simple rate laws are derived illustrating the rates\u27 dependence on the concentration (activity) of surface species

Methane in the Danube Delta: The importance of spatial patterns and diel cycles for atmospheric emission estimates

Methane (CH4) is one of the substantial greenhouse gases in our atmosphere and its concentration has increased by ~ 4 % over the last decade. Although sources driving these increases are not well constrained, one potential contribution comes from wetlands, which are usually intertwined with rivers, channels and lakes, creating a considerable need to acquire higher resolution data to facilitate 5 modelling and predictions. Here we took a fully contained sensor set-up to obtain measurements of CO2, CH4, O2 and auxiliary parameters, installed on a houseboat for accessibility, to assess and analyse surface water concentrations within the Danube Delta, Romania. Over 3 seasons, we transected a ~ 400 km route with concentration mapping and additional stations for monitoring diel cycles. Overall, the delta was a source for CH4 throughout all seasons, with concentrations ranging between 0.113–15.6 μmol L−1. The dataset was split into three different subsystems; lakes, rivers and channels, with channels 10 showing the highest variability. We found large to extreme diel cycles in both the lakes and channels, with concentrations varying by an order of magnitude between these two systems. The observed strong diel cycle within the lake suggests daily vertical stratification allowing for macrophytes to create a temporal oxycline due to lack of light and movement between the stems as previously suggested. While throughout the day, there was a consistent overall surface concentration of CH4 at around 0.4 μmol L−1, there was a clear linear trend with an O2:CH4 molar ratio of −50:1 during the phase of nocturnal convection 15 with the two water stratified bodies mixing during the night. Daily spot sampling techniques and neglecting such diel cycles reducing the estimated average methane concentrations by 25 % and increase by 3.3 % for channels and lakes, respectively. On an individual lake basis, spot sampling can potentially incur an uncertainty range of a factor of 4.5. Analyses also included a ‘hot spot’, with a 10-fold stronger methane increase (4–15.6 μmol L−1) overnight compared to the lake, with an almost immediate and extreme decrease in CH4 following sunrise. Calculated diffusive CH4 fluxes for the overall delta yielded an 20 average of 49 ± 61 μmol m−2 h−1 corresponding to an extrapolated annual flux of 0.43 ± 0.53 μmol m−2 yr−1. Our data illustrate the importance of collecting information on diel cycles in different habitats to improve the emission estimates from wetland systems

Nutrient transfer from soil to surface waters: Differences between nitrate and phosphate

Abstract.: Nitrate (NO3 -) and soluble reactive phosphorus (SRP), the two major dissolved N and P species available to aquatic biota, respond differently to varying water discharge rates (Q) in agricultural drainage pipes and rivers (Fig. 1): SRP concentrations are positively related to Q, whereas NO3 - concentrations decrease with increasing discharge rates. In addition, NO3-N concentrations exceed (in mass units) SRP concentrations up to 700-fold even though the liquid manure applied to agricultural fields has a N:P ratio equal to only about 5. Preferential flow of rainwater across the soil column and different affinities of the two nutrients for the soil matrix explain these differences in behaviour and mobility: i. Concentrations of substances that have a high sorption affinity for the soil matrix (such as SRP) tend to increase in drainage pipes and streams as water discharge increases. ii. Concentrations of species that are not retarded by sorption processes (such as NO3 -) and, hence, do not accumulate in the topsoil, tend to be negatively related to discharge rate. Differences in the availability and pool size of NO3 - and SRP in the topsoil explain the different hysteresis patterns if NO3 - and SRP concentrations are plotted versus the corresponding discharge rate during precipitation events (Fig. 2

Seasonal Fluctuations of Bacterial Community Diversity in Agricultural Soil and Experimental Validation by Laboratory Disturbance Experiments

Natural fluctuations in soil microbial communities are poorly documented because of the inherent difficulty to perform a simultaneous analysis of the relative abundances of multiple populations over a long time period. Yet, it is important to understand the magnitudes of community composition variability as a function of natural influences (e.g., temperature, plant growth, or rainfall) because this forms the reference or baseline against which external disturbances (e.g., anthropogenic emissions) can be judged. Second, definition of baseline fluctuations in complex microbial communities may help to understand at which point the systems become unbalanced and cannot return to their original composition. In this paper, we examined the seasonal fluctuations in the bacterial community of an agricultural soil used for regular plant crop production by using terminal restriction fragment length polymorphism profiling (T-RFLP) of the amplified 16S ribosomal ribonucleic acid (rRNA) gene diversity. Cluster and statistical analysis of T-RFLP data showed that soil bacterial communities fluctuated very little during the seasons (similarity indices between 0.835 and 0.997) with insignificant variations in 16S rRNA gene richness and diversity indices. Despite overall insignificant fluctuations, between 8 and 30% of all terminal restriction fragments changed their relative intensity in a significant manner among consecutive time samples. To determine the magnitude of community variations induced by external factors, soil samples were subjected to either inoculation with a pure bacterial culture, addition of the herbicide mecoprop, or addition of nutrients. All treatments resulted in statistically measurable changes of T-RFLP profiles of the communities. Addition of nutrients or bacteria plus mecoprop resulted in bacteria composition, which did not return to the original profile within 14days. We propose that at less than 70% similarity in T-RFLP, the bacterial communities risk to drift apart to inherently different state

Analysis of the Major Fe Bearing Mineral Phases in Recent Lake Sediments by EXAFS Spectroscopy

Extended X-ray absorption fine structure (EXAFS) spectroscopy and chemical analyses were combined to determine the Fe bearing minerals in recent lake sediments from Baldeggersee (Switzerland). The upper section of a laminated sediment core, deposited under eutrophic conditions, was compared to the lower part from an oligotrophic period. Qualitative analysis of FeK EXAFS agreed well with chemical data: In the oligotrophic section Fe(II)-O and Fe(III)-O specieswere present, whereas a significant fraction of Fe(II)-S sulfides was strongly indicated in the eutrophic part. A statistical analysis was performed by least square fitting of normalized reference spectra. The set of reference minerals included Fe(III) oxides and Fe(II) sulfides, carbonates and phosphates. In the oligotrophic regime no satisfying fit was obtained using the set of reference spectra, indicating that siderite (FeCO3) was not present in a significant amount in these carbonate-rich sediments. Simulated EXAFS spectra for a(Cax, Fe1-x)CO3solid solution allowed reconstructing the specificfeatures of the experimental spectra, suggesting that this phase was the dominant Fe carrier in the oligotrophic section of the core. In the eutrophic part, mackinawite was positively identified and represented the dominant Fe(II) sulfide phase. This finding agreed with chemical extraction, which indicated that18-40 mol% of Fe was contained in the acid volatile iron sulfide fraction. EXAFS spectra of the eutrophic section were best fitted by considering the admixture of mackinawite and the Fe-Ca carbonate phase inferred to be predominant in the oligotrophic regim