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Dreissenid mussels can alter nutrient cycling and algal productivity in many freshwater ecosystems. But their effects on sedimentary phosphorus dynamics remain largely undefined. Here, we report evidence that dreissenids affect the concentrations of five sedimentary phosphorus fractions and total phosphorus. During our study, zebra mussels were still common and coexisted with quagga mussels in many parts of the basin. The relative abundances varied across the basin, which we characterized as five west-to-east alternating zones where zebra mussels dominated zones I (coastal) and III, quagga mussels dominated zones II and IV, and few dreissenids were present in zone V. The phosphorus fractions exhibited variation concordant with and therefore potentially influenced by dreissenids. Concentrations of all fractions and TP were consistently greater in sediments where quagga mussels dominated than in sediments where zebra mussels dominated. The responses to the absence versus presence of dreissenids were mixed, with Res-P being significantly affected, NaCl-Pi and HCl-Pi being moderately affected, and NaOH-Pi being least affected. Although such dreissenid effects were somewhat altered by in-lake biogeochemical cycling and transfer, we found that elevated levels of NaCl-Pi in dreissenid-present sediments, especially in quagga-dominated sediments, could be linked to recent eutrophication and harmful algal blooms in the basin.

Surface sediments and three sediment cores from the western basin and one sediment core from the Sandusky basin were analyzed to document spatial and temporal changes in five phosphorus fractions and total phosphorus (TP). The areal distributions of the bioavailable fractions NaCl-Pi, NaBD-Pi, and NaOH-Pi and the refractory organic fraction Res-P were broadly consistent and contrasted with those of the detrital fraction HCl-Pi which showed that high concentrations occurred mostly in high-energy littoral zones and low concentrations largely in profundal depositional areas. The contrasting distributions were induced by interactions among tributary inputs, wave action, circulation, and biogeochemical cycling and transfer in the basin. As revealed by the Sandusky basin sediment record, the detrital fraction HCl-Pi was dominant (70% of TP) during European settlement and decreased rapidly by 28.0% in the early 1910s due largely to impoundments of the Maumee and Sandusky Rivers. While HCl-Pi has ever since remained relatively constant, NaCl-Pi, NaBD-Pi, and NaOH-Pi increased significantly between 1950 and 1970 in the two basins. However, the post-regulation sediment records differed considerably among these coring sites. There was a marked increase of TP in two cores, corresponding to recent return of eutrophication and massive harmful algal blooms but contrasting with a relatively constant, low loading into the lake. This signified the role of internal loading as derived partialy from legacy pollution. Furthermore, NaCl-Pi has increased progressively throughout all the records. We conclude that the increased levels of NaCl-Pi in surface sediments may have altered the internal loading and contributed to the resurgences of harmful algal blooms in Lake Erie.

Concentrations of the major and trace metals varied considerably in the western basin of Lake Erie, ranging from 0.9 to 25.3 mg/g for aluminum, from 2.9 to 36.5 mg/g for iron, from 6.4 to 74.8 mg/g for calcium, from 1.2 to 13.5 μg/g for cobalt, from 2.8 to 61.6 μg/g for copper, from 2.7 to 83.0 μg/g for lead, from 0.1 to 2.9 μg/g for cadmium, and from 7.1 to 127.3 μg/g for strontium. Distinct patterns of sediment metal variability allowed the identification of two major fluvial sources and some active in-lake biogeochemical processes. The inputs of Sr were largely from the Maumee River, the inputs of Cu, Pb, Cd, and Co were dominated by the Detroit River, and the inputs of Fe and Al were roughly evenly from the two rivers. The removal of Sr and Ca from the water column was mainly through coprecipitation with calcite. In contrast, the transfer of Cu, Pb, Cd, and Co was largely attributed to the removal of fine sediment particles from the Detroit River mouth and adjacent nearshore areas and the deposition of the metals scavenged by settling organic materials in the basin's central deeper areas. The mobility of the trace metals was different during the in-lake mass transfer, with Co being the most mobile and Cd being the least mobile. Furthermore, the trace metal mobility differences have decreased significantly during the past half-century due to a substantial increase in organic matter from eutrophication in the basin.

Concentrations of aluminum, arsenic, barium, beryllium, cadmium, calcium, chromium, cobalt, copper, iron, lead, magnesium, manganese, molybdenum, nickel, potassium, selenium, sodium, tin, titanium, vanadium, and zinc were measured in a surface sediment core from the Sandusky basin of Lake Erie to detail the history of hydrological and environmental changes back to 1800. The results from hierarchical cluster and principal component analyses revealed four elemental groups. All the trace elements clustering with aluminum, iron, and manganese in Group I were enriched due to increased inputs from anthropogenic sources. The two conservative elements sodium and potassium clustering in Group II showed patterns of changes like those of water-level fluctuations. The two carbonate elements calcium and magnesium clustering in Group III showed intriguing but complex carbonate biogeochemistry associated with biogenic production, organic acid-induced dissolution and dilution by organic and aluminosilicate materials. The terrigenous element titanium in Group IV experienced two stages of depletion from increased organic fluxes in the 1820s and 1950s. Following the enactments of stringent regulations in the early 1970s, many of these elemental inputs have reduced considerably. But the concurrent reductions in the Sandusky basin were much slower than previously thought. Large increases in inputs from local storages (internal loading) were required to account for the slow reductions. The increased internal loading was caused by augmented organic materials from accelerated eutrophication which facilitated the transfer, transport, and cycling of many trace metals. This work has implications in ongoing research efforts to tackle the eutrophication problem because the complex ecosystem including the internal loading has changed considerably over the past two centuries.

The use of stable isotopes in studies of watershed biogeochemical processes has increased greatly throughout the last several decades. Much of the sulfur cycling research has addressed the influence of changes in atmospheric acid deposition on sulfur dynamics in temperate ecosystems. Little is known about sulfur cycling in dryland ecosystems such as those in the American Southwest. To identify the sources and assess the cycling of sulfur in dryland ecosystems, chemical and isotopic compositions of water were measured on samples collected from the Pecos River (New Mexico, USA) during a reconnaissance survey in spring 2010. Based on the chemical and isotopic results, the Pecos River is readily divided into an upper basin of 6,000 km 2 above Santa Rosa Lake and a lower basin of 44,000 km 2 above Red Bluff Reservoir in western Texas. The upper basin contains river water with low concentrations of chloride (3 mg/L) and sulfate (13 mg/L), low values of δD (− 87‰) and δ 18 O (− 12.3‰), and low δ 34 S (− 4.3‰) and δ 18 O values (2.6‰) of dissolved sulfate (δ 34 S SO4 and δ 18 O SO4 ). Three different sources contributing to the pool of dissolved sulfate are identified, namely the oxidation of sulfide minerals, the soil processing of atmospheric sulfate, and the dissolution of ancient evaporites. The relative contributions of the three different sulfate sources change from reach to reach. Sulfate from evaporite dissolution primarily of Permian age dominates in the lower reaches while sulfate from sulfide oxidation dominates in the upper part of the Pecos River. Despite significant lithologic variations across the lower basin, δ 34 S SO4 values of river water are quite constant, with an average value of 11.8‰. In contrast, a 5‰ decrease was observed in δ 18 O SO4 values of river water between upstream and downstream reaches of the lower Pecos River, indicating that 63% of the dissolved sulfate had been recycled. Surprisingly, most of the sulfur cycling observed occurs in two small irrigation districts (the Fort Sumner Irrigation District and the Carlsbad Irrigation District), whereas there is only a minimal decrease (0.7‰) in δ 18 O SO4 in the largest irrigation district (the Pecos Valley Artesian Conservancy District). This study implies that the influence of land use activities on sulfur cycling may be more profound than previously thought.

Stable isotopes (δ18O and δD) are useful tracers for investigating hydrologic and climatic variability on a variety of temporal and spatial scales. Since the early isotopic studies on mountainous glaciers in the late 1960s, a great deal of information has been generated on the isotopic composition of rainfall, snow, ice, surface waters, and lake carbonate sediments across the Tibetan Plateau. However, measurements of δ18O and δD values of lake water are scarce. Here we present a new dataset of δ18O and δD values of lake waters collected from 27 lakes across the plateau during a reconnaissance survey in summer 2009. δ18O and δD values of lake water range from −19.9 to 6.6‰ and from −153 to −16‰, respectively. The average values of δ18O and δD are −6.4 and −72‰, considerably greater than those of precipitation observed in this region. The derived Tibetan lake water line, δD = 5.2δ18O − 38.9, is significantly different from the global meteoric water line. Most of the lakes, including some freshwater lakes, contain water with negative values of d-excess (d). There is a negative correlation between d and total dissolved solids (TDS). Each of these findings indicates that evaporation-induced isotopic enrichment prevails in Tibetan lakes. Moreover, we develop an isotope modeling scheme to calculate E/P ratios for Tibetan lakes, using a combination of existing isotopic fractionation equations and the Rayleigh distillation model. We use the intersection of the local evaporation line and GMWL as a first approximation of δ18O and δD values of lake water inputs to infer an E/P ratio for each lake. Our modeling calculations reveal that although variable from lake to lake, the water budget across the plateau is positive, with an average E/P of 0.52. This is in good agreement with other observational and model data that show varying degrees of increases in lake size from satellite imagery and significant decreases in lake salinity in many lakes on the plateau over the last several decades. Together with the new isotopic dataset, the proposed modeling framework can be used to examine and quantify past changes in a lake’s hydrologic balance from the isotopic record of downcore carbonate sediments in the region.

The Pecos River, situated in eastern New Mexico and western Texas, receives snowmelt from winter storms in the headwater region of the southern Rocky Mountains and runoff from warm-season monsoonal rainfall in the lower valley. The isotopic composition of the two water sources differs from each other due to their different geographical origins in the Pacific North and the Gulf of Mexico. To better assess the physical features of the Pecos River, oxygen and hydrogen isotopic compositions (δ 18 O and δD), major ion concentrations, and other physical variables (e.g., water temperature and electrical conductivity) were measured on water samples collected from the main stem and its selected tributaries during a growing season (March, May, and July) in 2005. The results of this work indicate that stream water from the Pecos River contains a relatively large magnitude of variations in δ 18 O and δD, with δ 18 O ranging from − 8.9‰ to 3.6‰ and δD from − 64.5‰ to 1.6‰. The average value of δ 18 O is around − 3‰, which is significantly larger than that of the snowmelt but almost identical to that of the Mexican monsoonal rainfall. On the other hand, the average value of δD is around − 30‰, which is significantly larger than that of the snowmelt but lower than that of the Mexican monsoonal rainfall. Application of a dual isotope index, deuterium excess (d-excess), allows us to assess the relative contribution of various hydrologic components and processes that shape the stream hydrology of the Pecos River. The river water from the upper valley is characterized by relatively low values of δ 18 O and δD and relatively high values of d-excess ( d = 10‰), documenting the isotopic fingerprint of the snowmelt. The middle basin is topographically gentle and its water quality has been severely affected by anthropogenic disturbances (e.g., water impoundments and diversions). As a result, chemical and isotopic composition of the water from the middle basin is highly variable, depending on its time, location, and degree of disturbances. Both δ 18 O and δD increase significantly from upstream to downstream and from cold to warm seasons because of evaporation-induced isotopic enrichments. The average δ 18 O value of the heavy-isotope-enriched waters from the middle basin is identical to that of the waters from the lower valley. In contrast, d-excess of the waters from the middle basin usually is negative, and substantially lower than that of the waters from the lower valley. Using a simple d-excess based hydrologic model, we estimated that there was up to 85% of stream flow which was derived from local freshwater sources (mainly from the Mexican monsoonal rainfall) in the lower valley and that there was up to 33% of stream water which was lost through evaporation occurring in stream channels and fields of the middle basin. Additionally, the correlation of d-excess and electrical conductivity further highlights the role of evaporative enrichments in regulating stream chemistry and isotope hydrology. This study demonstrates the usefulness of combined isotopic and geochemical data, especially the applicability of d-excess, for watershed baseline assessments.

The Pecos River, situated in eastern New Mexico and western Texas, receives water from a drainage area of 91 000 km 2 . There are primarily two major water inputs, namely snowmelt from winter storms in the headwater region of the southern Rocky Mountains and runoff from warm-season monsoonal rainfall in the lower valley. The Pecos River suffers from high levels of total dissolved solids (TDS >5000 mg L −1 ) under normal flow conditions. This not only poses serious problems for agricultural irrigation and safe drinking water supply, but also results in a permanent loss of biodiversity. This study examines changes in stream flow and water chemistry of the Pecos River over the last 70 a to better understand the long-term variability in stream salinity and the role of agricultural practices in salt transfer. A TDS record from the lower Pecos River near Langtry (Texas) back to 1935 was extracted to show a distinct pattern of decadal variability similar to the Pacific Decadal Oscillation (PDO), in which stream salinity is overall above average when the PDO is in positive (warm) phase and below average when the PDO is in negative (cold) phase. This is due to: (1) the dissolved salts contributed to the river are largely from dissolution of NaCl and CaSO 4 -bearing minerals (e.g., halite and gypsum) in the upper basin, (2) the amount of the dissolved salts that reach the lower basin is mainly determined by the stream flow yield in the upper basin and (3) the stream flow yield from the upper basin is positively correlated with the PDO index. This further attests that large-scale climatic oscillation is the major source of long-term changes in stream flow and salinity of the Pecos River. On the other hand, there is also a strong indication that the rate of salt export has been affected by reservoir operations and water diversions for agricultural practices.

Walker Lake, a hydrologically closed, saline, and alkaline lake, is situated along the western margin of the Great Basin in Nevada of the western United States. Analyses of the magnetic susceptibility (χ), total inorganic carbon (TIC), and oxygen isotopic composition (δ 18 O) of carbonate sediments including ostracode shells (Limnocythere ceriotuberosa) from Walker Lake allow us to extend the sediment record of lake-level fluctuations back to 2700 years B.P. There are approximately five major stages over the course of the late Holocene hydrologic evolution in Walker Lake: an early lowstand (N2400 years B.P.), a lakefilling period (∼2400 to ∼1000 years B.P.), a lake-level lowering period during the Medieval Warm Period (MWP) (∼1000 to ∼600 years B.P.), a relatively wet period (∼600 to ∼100 years B.P.), and the anthropogenically induced lake-level lowering period (b100 years B.P.). The most pronounced lowstand of Walker Lake occurred at ∼2400 years B.P., as indicated by the relatively high values of δ 18 O. This is generally in agreement with the previous lower resolution paleoclimate results from Walker Lake, but contrasts with the sediment records from adjacent Pyramid Lake and Siesta Lake. The pronounced lowstand suggests that the Walker River that fills Walker Lake may have partially diverted into the Carson Sink through the Adrian paleochannel between 2700 to 1400 years B.P.

Reservoir storage reduces fluctuation in streamflow salinity, as inflow blends with stored water. This process is directly relevant to reservoir management, yet the methods for predicting outflow salinity from inflow and storage data are not adequately addressed. This study examined three simple methods for estimating outflow salinity on a monthly time step. The first method applies when inflow approximately equals outflow. The second method is applicable to changing reservoir storage. The third method utilizes a two-layer model in which evaporative concentration is assumed to occur in the top layer. The third method requires the full account of the water balance, including evaporation and percolation losses. All three methods require reservoir storage blend with inflow in a stepwise manner. Outflow salinity was then computed monthly as a moving average. These methods were applied to high and low storage periods up to two years at three large reservoirs (Elephant Butte, Amistad, and Falcon) located along the Rio Grande. All of the equations tested provided good estimates of outflow salinity with the standard errors of estimate ranging from 5 to 10%. When a matching factor (accounting for ungauged inflow and water levels) was used in the first or the second method, they provided the estimate of monthly outflow salinity just as accurately as did the two-layer model. The accuracy of prediction, especially by the third method, can be improved if the initial reservoir salt storage is assessed with greater accuracy. Although the third method is more descriptive, the first two methods are also useful when detailed water balance data are not available.

Many freshwater and coastal marine ecosystems across the world may have undergone an ecosystem regime change due to a combination of rising anthropogenic disturbances and regional climate change. Such a change in aquatic ecosystems is commonly seen as shifts in algal species. But considerably less detail is known about the eutrophication history in terms of changes in algal productivity, particularly for a large lake with a great deal of spatial variability. Here we present an analysis of trace metals (Cu, Ni, Cd and Pb) on a sediment core recovered from Lake Erie, off the Vermilion coast of northern Ohio, USA, to reconstruct the eutrophication history of the lake over the past 210 years. Following a slow eutrophication during European settlement, Lake Erie experienced a period of accelerated eutrophication, leading to an ecosystem regime transition into a eutrophic lake state in 1950. Our results suggested that the lake's biological productivity has ever since maintained fairly high even though a significant input reduction was realized from rigorous nutrient abatements that began as early as in 1969. This work underscored the role of in-lake biogeochemical cycling in nutrient dynamics of this already eutrophic lake.

Analytical solutions to Richards' equation have been derived to describe the distribution of pressure head, water content, and fluid flow for rooted, homogeneous soils with varying surface fluxes. The solutions assume that (i) the constitutive relations for the hydraulic conductivity and water content as function of the pressure head are exponential, (ii) the initial water content distribution is a steady-state distribution, and (iii) the root water uptake is a function of depth. Three simple forms of root water uptake are considered, that is, uniform, stepwise, and exponential functional forms. The lower boundary of the rooted soil profile studied is a water table, while at the upper boundary time-dependent surface fluxes are specified, either infiltration or evaporation. Application of the Kirchhoff transformation allows us to linearize Richards' equation and derive exact solutions. The steady-state solution is given in a closed form and the transient solution has the form of an infinite series. The solutions are used to simulate the hydraulic behavior of the rooted soils under different conditions of root uptake and surface flux. The restricted assumptions for the solutions may limit the applicability, but the solutions are relatively flexible and easy to implement compared to other analytical and numerical schemes. The analytical solutions provide a reliable and convenient means for evaluating the accuracy of various numerical schemes, which usually require sophisticated algorithms to overcome convergence and mass balance problems.

[1] Here we show an analysis of river flow and water chemistry data from eleven gauging stations along the Pecos River in eastern New Mexico and western Texas, with time spanning 1959–2002. Analysis of spatial relationship between the long-term average flow and total dissolved solids (TDS) concentration allows us to illuminate four major processes controlling river chemistry, namely saline water addition, evaporative concentration with salt gain or loss, dilution with salt gain or loss, and salt storage. Of the 10 river reaches studied, six reaches exhibit the process dominated by evaporative concentration or freshwater dilution with little change in salt load. Four reaches show considerable salt gains or losses that are induced by surface-ground water interactions. This analysis suggests that the evaporative concentration and freshwater dilution are the prevailing mechanisms, but local processes (e.g., variations in hydrologic flowpath and lithologic formation) also play an important role in regulating the hydrochemistry of the Pecos River.

[1] The hydrochemistry has been examined using the major element composition of river water at 12 gauging stations along the Rio Grande. As the Rio Grande Basin consists of two watersheds that have different hydrologic and climatic regimes, two chloride concentration records from the El Paso and Falcon Dam gauging stations have been extracted to reflect long-term variability in river chemistry of the upper and lower basins over the last 50–70 years. Both records contain decadal variability in chloride concentration but are different in nature. The chloride concentration record from the upper basin displays a distinct pattern of decadal variability similar to the Pacific Decadal Oscillation (PDO). This indicates the chloride concentration at El Paso is largely determined by the amount of stream discharge of the upper basin that is associated with the PDO. Conversely, there is no such pattern of decadal variability in the chloride concentration record from the lower basin though several of the chloride concentration maxima coincide with minima in the PDO index. Instead, the chloride concentration record from the lower basin contains a progressively increasing trend of chloride concentration from 1970 to 1990, suggesting that anthropogenic disturbances (e.g., dam constructions and increased irrigation demands) may also play a role in intervening long-term changes in river chemistry.

[1] Measurements of the oxygen isotopic composition (δ18O) of the total inorganic carbon (TIC) fraction from cored sediments of Walker Lake, Nevada, were conducted at an average resolution of ∼3 years per sample over the last 1200 years. On the basis of radiocarbon analysis on the total organic carbon (TOC) fraction, a δ18O time series was created to reconstruct changes in hydrologic conditions back to AD 800. The timings of variations in the TIC δ18O record are generally consistent with the tree ring-based Sacramento River flow record spanning AD 869 to 1977, indicating that Walker Lake δ18O contains information about past changes in at least regional hydrologic conditions. Comparison with the δ18O record from Pyramid Lake sediments indicates that both basins have recorded five century-scale oscillations in regional hydrologic conditions since AD 800. Several of these changes in hydrologic conditions appear synchronous with century-scale California Current water temperature changes derived from analysis of sediment cores from the Santa Barbara Basin also attesting to the regional extent of these climatic fluctuations. Nearly synchronous oscillations in the Sierra wetness and the California Current suggest that regional changes in atmospheric circulation may have played an important role in century-scale climate variability over the last millennium.