Particle Mixing Rates of Freshwater Bivalves: Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae)

Sediment mixing by freshwater suspension feeding bivalves Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae) was studied by adding illite clay particles with adsorbed ^I37Cs as a submillimeter thick layer to the surface of silt clay sediments contained in clear rectangular cells maintained in a temperature regulated aquarium. A Nal gamma detector scanned the sediment column in each cell at 0.2 cm intervals five times over 22 days and recorded changes in ^I37Cs activity over time with depth in cells containing 3 A. grandis, 4 S. striatinum, and a control cell containing no bivalves. Sediment mixing by these organisms was diffusional. The diffusion coefficient in the control cell was 0.02 cm²/yr, consistent with molecular diffusion of¹³⁷Cs tracer. Whole cell biodiffusion coefficients (D_b)for A. grandis and S. striatinum were 0.81–2.11 cm²/yr and 0.53 cm²/yr, respectively. Adjusting to equal population densities, the 11-27× higher sediment mixing rate of A. grandis was likely due primarily to its larger size. When D_bfor similar sized organisms was compared, S. striatinum was found to mix sediments at about the same rate as the marine bivalve Nucula proxima but at a 5× lower rate than the freshwater amphipod Diporeia sp. A. grandis mixes sediments 5–14× more slowly than the similar sized conveyor belt deposit feeding marine bivalve, Yoldia limatula.. While deposit feeding organisms are the dominant sediment mixers in the Great Lakes, suspension feeding bivalves can be locally significant.     

Stable Carbon and Oxygen Isotope Records From Lake Erie Sediment Cores: Mollusc Aragonite 4600 BP–200 BP

We present the first δ¹⁸O and δ¹³C data from mollusc aragonite from Lake Erie for the 4.6–0.2 ka (4600 BP to 200 BP ¹⁴C yrs) time interval and describe single and composite species isotope trends. Composite species δ¹⁸O data show an almost 2.0%o increase from 3.3 to 3.0 ka followed by a nearly 2.5%o decrease at 2.8 ka. Oxygen isotope values then fluctuate by < l‰ until 0.2 ka. This trend in oxygen values is also evident in single species analyses of Sphaerium striatinurn. The most dramatic changes in isotope values, which occur from 3.3–2.8 ka, may reflect a pattern of water level changes in Lake Erie which occurred during the Nipissing flood and its lower water aftermath. Carbon isotope data show progressively more ¹³C enriched values from 4.6 ka (averaging–6.5‰ PDB) to the present (–0.57‰). This trend may reflect the dilution of isotopically light CO₂ from the oxidation of organic matter due to rising lake levels. The short-term increase in δ¹⁸O values is coupled with a corresponding decrease in δ¹³C values. A similar pattern for Lake Erie prior to 10.5 ka was also associated with a lowering of water levels in the lake.     

Inflow of surface and groundwater to Lake Ladoga based on stable isotope (2H, 18O) composition

Stable isotope (deuterium – ²H and oxygen-18 – ¹⁸O) surveys of water in Lake Ladoga and several rivers and small lakes within its watershed were conducted during 2012–2018. Over these 6 years, specific spatial and time differences in water isotope composition of Ladoga and small water bodies and streams were found. These differences depended firstly, on wetland type and the lakes distribution on the watershed and secondly, on groundwater discharge and type of lithological substrate (geological sediments or crystalline rocks). Besides landscape, climate (air temperature, precipitation, evapotranspiration) is a main physical factor which controls all scales of the stable isotope variations. In modern times, the sub-latitudinal gradient of climate but also presence of permafrost in the past effects the stable isotope variation. The isotopic composition divides the tributaries of Ladoga into three groups with a wide range of δ¹⁸O seasonal variations, and a significant atmospheric component. The isotopic composition of Ladoga water is generally stable ranging from δ¹⁸O = ?11.7 to ?9.5‰ (average δ¹⁸O = ?10.6‰), δ²H = ?84 to ?74‰ (average δ²H = ?78‰). The effect of evaporation is clearly seen due to accumulation of deuterium and oxygen-18 and a shift to the right from the Local Meteoric Water Line of δ²H versus δ¹⁸O diagram. In Lake Ladoga there is a local deviation of δ¹⁸O which can be explained by subaqueous discharge of groundwater from Riphean sediments.     

Variability in fish and water hydrogen and oxygen stable isotope values in the nearshore region of a large water body

Incorporating organismal hydrogen (δ²H or δD) and oxygen (δ¹⁸O) stable isotope ratios into aquatic food web research can help elucidate relative reliance on shoreline inputs and/or feeding at different trophic levels; however, their interpretation is complicated by the fact that aquatic organisms derive hydrogen and oxygen from both their diet and the ambient water in which they reside. We examined spatial and seasonal patterns in δ²H and δ¹⁸O of field-collected water and small-bodied fishes. Samples were gathered from nearshore Lake Michigan, a dynamic region of a large, increasingly oligotrophic freshwater lake. We examined seasonal and spatial δ²H and δ¹⁸O values of surface and bottom water, and compared spatial δ²H and δ¹⁸O values of young-of-year yellow perch (Perca flavescens) and seasonal δ²H and δ¹⁸O values of round goby (Neogobius melanostomus). Nearshore δ²H and δ¹⁸O water values were more variable than previously-described offshore values. Variation in nearshore water δ²H and δ¹⁸O values was likely related to differential precipitation and/or discharges from tributaries, and there may be a surface dilution effect which led to lower δ¹⁸O values in nearshore surface waters than bottom waters. In our study, fish tissue δ²H and δ¹⁸O also varies more spatially than seasonally, and some findings suggest that fish tissue δ²H may reflect feeding at higher trophic levels. Though characteristics of the study system affect their interpretation, we suggest that δ²H and δ¹⁸O can be another tool to assess food web structure, with δ²H in particular having potential to resolve questions when δ¹³C or δ¹⁵N are inconclusive.     

Hydrogeochemistry,Isotopic Composition and Water Age in the Hydrologic System of a Large Catchment within a Plain Humid Environment (Argentine Pampas): Quequén Grande River,Argentina

The Quequén Grande River (QGR) is a large catchment (10 000 km²) in the Pampa Plain in Argentina. From November 2004 to April 2013, a hydrochemical and stable isotopes monitoring program was conducted, which included three sampling stations of monthly composite precipitation, weekly samples in two sites along the river and several groundwater samples. A standard data interpretation was initially performed applying standard statistics, Piper diagrams and δ¹⁸O versus δ²H diagrams. The time evolution of the values of δ¹⁸O in precipitation and streamwater were also determined. The integration of hydrogeochemistry and stable isotopes data indicates the existence of three main components of streamflow: (i) baseflow characterized by electrical conductivity (EC) from 1200 to 1800 µs/cm and an isotope composition quite constant around δ¹⁸O ?5.3‰ and δ²H ?33.8‰. Water age for groundwater contribution is typically around 30 to 40 years using chlorofluorocarbons; (ii) direct runoff composed of channel interception and overland flow, which is of low EC in the order of 50 to 100 µs/cm, and a highly variable isotopic composition; and (iii) translatory flow (pre‐event water that is stored within the subsoil) with an intermediate EC and isotopic composition close to that of the weighted average composition of precipitation. The hydrochemical and stable isotopic data allow the differentiation between baseflow and direct runoff. In addition to this, chlorofluorocarbon dating is a useful tool in assessing the dominance of baseflow in a stream. The data lead to a conceptual model in which an intermediate flow system, with mean residence time (MRT) of around 35 years, discharges into the drainage network. A regional flow system (MRT > 50 years) discharges to the ocean. It is concluded that in this large plain catchment streamflow separation, only two components can be applied in: (i) short storm precipitation events having a high sampling frequency and (ii) during long dry periods when pre‐event soil water is not released. Copyright © 2016 John Wiley & Sons, Ltd.     

Chemical and Isotopic Tracer Evaluation of Water Mixing and Evaporation in a Dammed Texas River During Drought

The interaction between drought and river regulation is monitored to better understand river flow mixing, evaporation and surface‐groundwater exchange in changing regional climates and in increasingly regulated waterways. This study compared Brazos River stable isotope (δ¹⁸O and δD) and electrical conductivity values with reservoir, creek and aquifer samples in the Brazos watershed, the largest watershed in Texas. The combination of tributaries, rainfall and the Brazos River Alluvium Aquifer, on the one hand, and the Lake Whitney reservoir, on the other hand, represent endmembers of dilute run‐off water and evaporated saline water, respectively. A simple isotope mixing model that uses monthly river discharge, Lake Whitney discharge, historical monthly precipitation δ¹⁸O and pan evaporation accurately reconstructs river δ¹⁸O (±0.5‰ on average). Data and isotope balance modelling support continued evaporation of ¹⁸O‐enriched Lake Whitney water as it flows downstream, although the most evaporation took place in Lake Whitney. The difference between river and precipitation δ¹⁸O, or Δ¹⁸O_RIV‐_PPT, here a measurement of degree of evaporation, ranged from ?0.1‰ for a small creek, to 1.7‰ for the Brazos River, to at least 2.7‰ in Lake Whitney. This study indicates that drought in regulated rivers may enhance reservoir discharge dominance in river flows during peak drought conditions when combined run‐off and baseflow dominance would be expected in a similar undammed river. Copyright © 2016 John Wiley & Sons, Ltd.     

Stable Oxygen and Carbon Isotopic Compositions of Lates stappersii Otoliths from Lake Tanganyika,East Africa

In this study, we measured growth trends in oxygen and carbon isotopic ratios in whole sagittal otoliths from three adult centropomid fish (Lates stappersii) from each of three sub-basins of Lake Tanganyika, East Africa. Sampling density was 20 to 50 samples per otolith. Both δ¹⁸O and δ¹³C values increase with age. The δ¹⁸O data suggest that otoliths were precipitated near the expected equilibrium with the ambient environment (ca. +3.5‰) and support a migration pattern from surface waters during larval stages to deeper waters (40 to 80 m) for mature fish. Relatively high δ¹⁸O values in the southern sub-basin are consistent with cooler temperatures in the region during seasonal upwelling. The δ13C increase from otolith core to edge is large (up to 4‰) and is interpreted as due to ontogenetic changes in diet and contributions from a decrease in the proportion of respired CO₂ incorporated into otolith carbonate as metabolic rates of the fish dropped with maturity. The data seem to successfully reveal life strategy and migration patterns of L. stappersii, document regional differences in lake conditions, and provide a record of temperature within the water column during which the fish lived. Higher resolution studies and analyses of historical samples could be used to constrain modern and past growth patterns, and to reconstruct past temperature gradients and productivity patterns in the lake.     

Evaluation of the origin of nitrate influencing the Klju? groundwater source, Serbia

This paper describes the use of the dual isotope method involving δ(15)N and δ(18)O measurements of dissolved nitrates to assess the origin and fate of groundwater nitrate at the Klju? groundwater source, Serbia. A sampling campaign was conducted in September 2007 during flow conditions obtaining groundwater from observation wells and river water fed by a shallow aquifer hosted in alluvial (sandy-gravel) sediments. Nitrate isotope ratios ranged from +5.3 to +16.9‰ and δ(18)O(NO(3)) values varied from -2.3 to +5.0‰. Two major contamination sources were identified with isotopic compositions characteristic for nitrate derived from nitrification of soil organic nitrogen (+5.3 to +7.8‰ for δ(15)N) resulting in nitrate concentrations of 33.6 and 78.8 mg/L and nitrate derived from animal wastes or human sewage, e.g. via septic systems, yielding δ(15)N values of +9.9 to +11.9‰ and elevated nitrate concentrations of 31.2-245.8 mg/L. The occurrence of nitrification and denitrification was also revealed based on concentration and isotope data for dissolved nitrate.     

Temporal trends of phytoplankton and zooplankton stable isotope composition in tropical Lake Malawi

Stable isotope ratios of three seston size classes (20–100 μm, 2–20 μm, and 0.2–2 μm) and zooplankton species were analyzed to determine the plankton food web structure of Lake Malawi. Over an annual cycle, seston δ¹³C varied between ?20.41‰ and ?27.43‰ with a mean value of ?24.27‰ ± 1.2 while δ¹³C values for zooplankton fluctuated between ?22‰ and ?25‰ with a mean of ?23.84‰ ± 0.77. Seston δ¹³C fluctuations appeared to be related to changes in physical and meteorological conditions in the lake that ultimately control nutrient availability. The highest seston δ¹³C values observed during the rainy and mixed seasons likely result from high phytolankton growth rates. δ¹⁵N of plankton was temporally variable, suggesting short term changes in N cycling dynamics that control the supply of N to phytoplankton. Very low seston δ¹⁵N values recorded during the mixing season suggest excess NO₃^? availability resulting from upwelling and vertical mixing. In contrast to expectations the calanoid Tropodiaptomus cunningtoni appeared to feed at a trophic level higher than that of all other zooplankton species, including the cyclopoid, Mesocyclops aequatorialis aequatorialis. δ¹⁵N values indicate that zooplankton were nearly 2 trophic levels above seston in the early stratified season. This implies that adult zooplankton could be utilizing forms of food other than phytoplankton during this period, such as nauplii or protozoans. This extra step in the food web, and the trophic positions of large zooplankton species, may alter estimates of food web efficiency and potential fish production for Lake Malawi.     

Hydrochemical Processes and Environmental Isotopic Study of Groundwater in Kuwait

Abstract

The present paper studies the hydrochemical processes and the isotopic characteristics of the main aquifers, the Kuwait Group and Dammam aquifers in the State of Kuwait. The water chemical types are dominantly NaCl and Na₂SO₄ in the Kuwait Group aquifer and Na₂SO₄, CaSO₄, and NaCl in the Dammam Limestone aquifer. The groundwater of the Kuwait Group aquifer is supersaturated with respect to quartz and calcite, and near saturation with respect to aragonite, but under-saturated with respect to anhydrite, dolomite, gypsum, and halite. While the groundwater of the Dammam aquifer is strongly under-saturated with respect to anhydrite, halite, and gypsum and is supersaturated with respect to quartz, dolomite, aragonite, and calcite. The saturation indices of the calcite and dolomite are increased in the direction of flow from southwest toward north-northeast. Al-Rawdhatain and Umm Al-Aish groundwater samples lie near the meteoric water line. They represent the effect of recharge by rainfall during pluvial period. The brackish palaeo-groundwater of the Dammam aquifer has low values of δ¹⁸O (?4.5 per mil) and δD (?35 per mil), which reveal that these waters are related to the palaeo-water from the cool wet period in the eastern Arabia 11,000 to 60,000 years B.P. The investigated brackish groundwater samples showed that the deuterium excess values are much lower than that of the Mediterranean areas. This may suggest that the evaporation was a prevailing process before the infiltration to the aquifers.     

Silicon Isotopic Fractionation in Lake Tanganyika and Its Main Tributaries

Silicon isotopic measurements in Lake Tanganyika were performed using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in dry plasma condition. Isotopic signatures are reported for dissolved ortho-silicic acid [Si(OH)₄] collected during a 1-year-long surface waters survey in the southern basin along with several of the major tributaries. Deep-water Si isotopic profiles from a north-south transect cruise conducted in July 2002 are also described. The nutrient-like shape dissolved Si profiles and the isotopic disequilibrium between surface (δ²⁹Si = 0.87±0.08 ‰) and deep waters (0.61 ± 0.05 ‰) suggest the occurrence of biological isotopic discrimination induced by diatoms biomineralisation in a fresh water system. Short-term surface water Si isotopic and diatom biomass variations obtained during the 1-year bi-weekly monitoring (2002–2003) in the south confirms this biological effect. Five epilimnion biogenic opal samples also were analyzed. Their signature (δ²⁹Si of 0.28 ± 0.12‰) compared to those of surrounding waters are consistent with the diatom isotopic fractionation effect measured on marine tropical diatoms. This demonstrates the species and temperature independent character of the silicon isotope fractionations by diatoms. River signatures present variable dissolved Si concentrations which were positively correlated to δ²⁹Si values in the range of previously published world river data. Because of its fast response to climate variability, nutrient dynamics, and limnological changes, δ²⁹Si in siliceous organisms should be very useful in studying environmental changes and particularly the recent decline of diatom Si utilization in Lake Tanganyika.     

Large variation in periphyton δ13C and δ15N values in the upper Great Lakes: Correlates and implications

Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) are frequently used to examine food web structure. Despite periphyton's importance to lake food webs, little is known about spatial variation of periphyton δ¹³C and δ¹⁵N values in the Great Lakes. We present periphyton δ¹³C and δ¹⁵N values from 28 sites the upper Great Lakes, including Lake Superior, the north shore of Lake Michigan, and Green Bay. We also examined variation in periphyton isotope values relative to several water quality parameters (TP, TN, TKN, NO₃⁻, K_d) as well as periphyton C:N. There was a large range in both periphyton δ¹³C (range = 13.5‰) and δ¹⁵N (range = 10.2‰) among sites. Periphyton in more eutrophic sites had more depleted δ¹³C and more enriched δ¹⁵N compared to more oligotrophic sites. Our finding of high variability in periphyton isotope values in the Upper Great Lakes has implications for stable isotope-based reconstructions of food web structure.     

Otolith microchemistry and isotopic composition as potential indicators of fish movement between the Illinois River drainage and Lake Michigan

Naturally occurring chemical markers in otoliths offer a potential but untested means to identify source environment for fishes in the upper Illinois River system and Lake Michigan, including individuals that may breach or circumvent electrical barriers in the Chicago Sanitary and Ship Canal or be transferred via bait buckets between these formerly isolated drainages. The objectives of this study were to determine whether water and fish otolith stable isotopic and elemental compositions differ among Lake Michigan, the upper Illinois River, and three tributaries of the upper Illinois River (Fox, Des Plaines and DuPage Rivers) and to determine whether otolith isotopic and elemental signatures could be used to identify the water body from which individual fish were collected. Water and fish otolith samples were obtained from each site during 2007 and analyzed for δ¹⁸O and a suite of trace element concentrations; otoliths also were analyzed for δ¹³C. Otolith δ¹³C values for Lake Michigan fish were distinct from individuals collected in the Illinois River and tributaries. Fish collected in the Fox and Des Plaines Rivers could be distinguished from one another and from fish captured in the Illinois and DuPage Rivers using otolith Sr:Ca and Ba:Ca ratios. Otolith isotopic and elemental compositions may enable identification of source environment for fishes that move or are transferred between the Illinois River drainage and Lake Michigan; however, temporal variation in otolith chemical signatures should be assessed.     

Diet Breadth and Variability in Sander spp. Inferred from Stable Isotopes

We used stable isotopes of carbon and nitrogen to evaluate trophic similarity between sauger Sander canadensis and walleye S. vitreus in three Missouri River impoundments characterized by unique differences in riverine habitat. Mean δ¹⁵N was similar for sauger and walleye in each reservoir ranging from 15.7 to 17.8‰ for sauger and 15.2 to 17.7‰ for walleye. However, mean δ¹³C was greater for sauger (?24‰) than for walleye (?25‰) in Lake Oahe (lacustrine habitat), where rainbow smelt Osmerus mordax is an important prey species for walleye. Variation in δ¹⁵N and δ¹³C values was similar between walleye and sauger in Lewis and Clark Lake (riverine habitat), but was greater for sauger than for walleye in Lake Oahe, implying that in pelagic environments, sauger exhibit a larger diet breadth and lower diet consistency compared with walleyes. Isotope analyses support observations from traditional gut content studies that diet overlap between sauger and walleye varies with environmental conditions and is more similar in riverine food webs than in large lakes and impoundments. Copyright © 2015 John Wiley & Sons, Ltd.     

Temporal and spatial variations of runoff composition revealed by isotopic signals in Nianchu River catchment,Tibet

Understanding the hydrology of glaciated catchments is an important step in assessing the vulnerability of water resources to a changing climate. Based on multi-isotopes of water (²H, ¹⁸O and ³H) and dissolved radon (²²²Rn), the temporal and spatial variabilities of major hydrological processes along the main flow and tributaries in the Nianchu River catchment were examined and the isotopic response to climate variation was identified. Geographic variation in changes of isotopic composition that differ from other rivers in the Nianchu River catchment was apparent. Along the direction of runoff, river δ¹⁸O exhibited more depletion, which was closely related to water mixing and groundwater discharge. End-member mixing analysis using isotopic tracers suggested that annual recharge from summer rainfall and glacial meltwater maintained the surface water resources (their respective contributions rate were 65.9% and 26.5%); groundwater had a significant contribution on runoff in the dry season (about 46.6%). Summer rainfall and meltwater rapidly infiltrated through a series of faults and fissures and were, stored in underground reservoirs and released to runoff in the dry season, thereby ensuring rapid circulation and renewal of water resources (annual renewal proportion was about 40%). It was concluded that rainfall infiltration, meltwater and groundwater storage play important roles in the hydrology of this alpine-cold catchment. Similar to a general alpine-cold catchment, the stable isotopes (δ²H, δ¹⁸O) of river runoff will gradually be enriched, while groundwater reserves will increase in the Nianchu River catchment as a result of climate warming and an acceleration of glacial-melting.     

Evaluation of δD and δ18O as natural markers of invertebrate source environment and dispersal in the middle Mississippi River‐floodplain ecosystem

Movement of invertebrates among large rivers, tributaries, and floodplain lakes or dispersal of adult aquatic insects from riverine or floodplain habitats may provide important subsidies to food webs in receiving habitats. Intensive sampling at habitat interfaces and artificial labelling are two approaches to assess freshwater invertebrate dispersal, but these are difficult to implement at a landscape scale. Natural chemical tracers have been used to track dispersal of fishes and marine invertebrates, but the potential applicability of stable isotope ratios as natural tracers of invertebrate dispersal in freshwater environments has not been assessed. We evaluated stable hydrogen and oxygen isotopes (δD and δ¹⁸O) as natural markers of source environment and dispersal of macroinvertebrates in the middle Mississippi River, tributaries and floodplain wetlands. Water and invertebrates were collected from 12 sites during 2007–2008. Water δD and δ¹⁸O differed among the river, its tributaries, and floodplain wetlands and were strongly correlated with invertebrate δD and δ¹⁸O. Variability in invertebrate δ¹⁸O rendered it ineffective as an indicator of invertebrate source environment. Mean δD of Mississippi River invertebrates differed from δD of invertebrates from floodplain wetlands; δD distinguished invertebrates from these two environments with >80% accuracy. Neither δD nor δ¹⁸O of aquatic insects changed following emergence from their natal site. Preservation method (ethanol or freezing) did not affect invertebrate δD or δ¹⁸O. Invertebrate δD may be a useful natural tracer of natal environment and dispersal in the Mississippi River‐floodplain ecosystem and other freshwater systems where spatial variation in water δD is present. Copyright © 2010 John Wiley & Sons, Ltd.     

Stable carbon and nitrogen isotope analysis of seston in a regulated Rocky Mountain river,USA

The δ¹³C and δ¹⁵N of seston, organic matter sources (riparian vegetation, algae, macrophytes, plankton) and an invertebrate detritivore were examined in a regulated Rocky Mountain river (fourth order). The isotopic composition of seston varied among sites and seasons for all size fractions; among-site variation was weakest for ultra-fine particulate organic matter (< 53 μm). Comminution of seston to ultra-fine size partially obscured isotopic differences among sites. Multiple isotope signatures of seston were distinct among sites at different distances from a dam (0.5, 5.3, 8.7 km) and between the river and a third order tributary. Seston (> 53 μm) from 0.5 km below the dam was relatively ¹³C-depleted (?30 to ?28°), reflecting the contribution of reservoir-derived plankton; seston from the site 8.7 km downstream was relatively ¹³C-enriched (?18 to ?23°), reflecting autochthonous inputs. Tributary seston had an intermediate δ¹³C value (?22 to ?26°), reflecting allochthonous inputs (conifer litter) and macrophyte (Elodea) detrital inputs. δ¹⁵N values of seston were generally similar among sites (4–8°) but were more depleted for the tributary (1–4°). Depleted δ¹⁵N values (0–2.5°) for seston from below the dam in late summer corresponded to a blue-green algae bloom in the reservoir. Isotopic shifts of organic matter incubated for 60–90 days in situ were small (< 2°) and varied in direction depending on isotope and litter type. The isotopic composition of stonefly larvae, Pteronarcys californica, varied among sites, and closely matched that of the local seston (especially FPOM), suggesting that with respect to detritus origin, larvae were opportunistic foragers. The origin of organic matter is influenced by flow regulation, resulting in compressed isotopic gradients.     

新疆鄯善县水环境氢氧同位素特征及其指示作用

根据鄯善县氢氧稳定同位素资料及氘盈余（d）值,分析了地表与地下水体的δD、δ18O和d值的分布规律,并得到地下水的主要补给来源及其与地表水的相互作用关系;地下水的δD在-87.88‰～-65.97‰间,δ18O在-9.4‰～-12.37‰间;地表水的δ18O在-10.08‰～-13.63‰间,d在14.12‰～32.19‰间。结果表明：鄯善县地下水和地表水同源于大气降水,且经历了较强的蒸发作用;地下水与地表水之间的水力联系较弱,深层地下水主要接受河水在洪积扇区的入渗补给,浅层地下水主要接受河流引水灌溉入渗;不同深度地下水之间的水力联系较为密切,为统一的地下水系统。     

UNDERSTANDING AND OVERCOMING BASELINE ISOTOPIC VARIABILITY IN RUNNING WATERS

Natural abundances of stable isotopes in lotic food webs yield valuable information about sources of organic matter for consumers and trophic structure. However, interpretation of isotopic information can be challenging in the face of variability in organisms at the base of food webs. Unionid and dreissenid mussels, commonly used as baseline organisms in lakes, are uncommon in many river settings and can have variable diets, thus making them unsuitable as a universal baseline for many river food web studies and often forcing reliance on more common benthic insects for this purpose. Turnover rates of body carbon and nitrogen in insects are relatively rapid (1 to 50 days half‐life). These rapid turnover rates in primary consumers can result in considerable temporal variability in δ¹³C that rivals that of algae (>10‰ range within a site). This suggests that using primary consumers as a surrogate baseline for algae may not circumvent the problem of temporal variability and the resultant mismatch of sources with longer‐lived, slow‐growing secondary and tertiary consumers. There are several strategies for reducing the influence of these confounding factors when bivalves with a known diet are not present. These include sampling over large spatial scales and correlating δ¹³C of consumers with the source of interest (e.g. benthic algae), sampling baseline organisms multiple times in the weeks preceding sampling of larger consumers (particularly in response to large changes in discharge) and using algal‐detrital separation methods and multiple tracers as much as possible. Incorporating some of these recommendations and further exploring variability at the base of the food web will potentially provide greater insights into consumer–resource coupling in running waters and more robust conclusions about food web structure and energy flow in these dynamic systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Establishing spatial trends in water chemistry and stable isotopes (δ15N and δ13C) in the Elwha River prior to dam removal and salmon recolonization

Two high‐head dams on the Elwha River in Washington State (USA) have changed the migratory patterns of resident and anadromous fish, limiting Pacific salmon to the lower 7.9 km of a river that historically supported large Pacific salmon runs. To document the effects of the dams prior to their removal, we measured carbon and nitrogen stable isotope ratios of primary producers, benthic macroinvertebrates, and fish, and water chemistry above, between and below the dams. We found that δ¹⁵N was significantly higher in fish, stoneflies, black flies, periphyton and macroalgae where salmon still have access. Fish and chloroperlid stoneflies were enriched in δ¹³C, but the values were more variable than in δ¹⁵N. For some taxa, there were also differences between the two river sections that lack salmon, suggesting that factors other than marine‐derived nutrients are structuring longitudinal isotopic profiles. Consistent with trophic theory, macroalgae had the lowest δ¹⁵N, followed by periphyton, macroinvertebrates and fish, with a range of 6.9, 6.2 and 7.7‰ below, between, and above the dams, respectively. Water chemistry analyses confirmed earlier reports that the river is oligotrophic. Phosphorous levels in the Elwha were lower than those found in other regional rivers, with significant differences among regulated, unregulated and reference sections. The removal of these dams, among the largest of such projects ever attempted, is expected to facilitate the return of salmon and their marine‐derived nutrients (MDN) throughout the watershed, possibly altering the food web structure, nutrient levels and stable isotope values that we documented. Published in 2010 by John Wiley & Sons, Ltd.