North Lake Research Articles

Left-lateral transpressive deformation and its tectonic implications, Sveconorwegian orogen, Baltic Shield, southwestern Sweden

The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vänern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north. Structural studies in three areas north of lake Vänern (Värmlandsnäs, Kil and Torsby-Charlottenberg) suggest that the MZ displays coeval, left-lateral, strike-slip and reverse, dip-slip senses of shear characteristic of a transpressive tectonic regime active under upper greenschist-facies conditions. In the northernmost Torsby-Charlottenberg area, at relatively shallow crustal levels, the MZ splays out to form two left-lateral, contractional strike-slip duplexes which together define a positive flower structure. Regionally, the MZ defines the western flank of an east-verging thrust system which formed in the eastern part of the Sveconorwegian orogen. The structures within this thrust system preceded the MZ, contain a subordinate, left-lateral strike-slip component of movement and were rotated into the Sveconorwegian Frontal Deformation Zone (SFDZ) in the frontal part of the orogen. The MZ is inferred to have formed in an extrusive (dispersive) tectonic environment which developed in connection with a slightly oblique collision and crustal shortening in a WNW-ESE direction. The initial stages of this collision produced the east-verging thrust system and shortening was absorbed, at this stage, by crustal thickening. Late, possibly out-of-sequence thrusting with a right-lateral, strike-slip sense of shear along the SFDZ marked the waning stages of crustal shortening in the Sveconorwegian orogen.

Potential methane emission from north‐temperate lakes following ice melt

Methane, a radiatively active “greenhouse” gas, is emitted from lakes to the atmosphere throughout the open‐water season. However, annual lake CH4 emissions calculated solely from open‐water measurements that exclude the time of spring ice melt may substantially underestimate the lake CH4 source strength. We estimated potential spring CH4 emission at the time of ice melt for 19 lakes in northern Minnesota and Wisconsin. Lakes ranged in area from 2.7 to 57,300 ha and varied in littoral zone sediment type. Regression analyses indicated that lake area explained 38% of the variance in potential CH4 emission for relatively undisturbed lakes; as lake area increases potential CH4 emission per unit area decreases. Inclusion of a second term accounting for the presence or absence of soft organic‐rich littoral‐zone sediments explained 83% of the variance in potential spring CH4 emission. Total estimated spring CH4 emission for 1993 for all Minnesota lakes north of 45° with areas ≥4 ha was 1.5 × 108 mol CH4 assuming a 1 : 1 ratio of soft littoral sediment to hard littoral sediment lakes. Emission estimates ranged from 5.3 × 107 mol assuming no lakes have soft organic‐rich littoral sediments to 4.5 × 108 mol assuming all lakes have soft organic‐rich littoral sediments. This spring CH4 pulse may make up as much as 40% of the CH4 annually emitted to the atmosphere by small lakes.

Recent Fire History of the Northern Great Plains

-The fire history of the northern Great Plains has been largely reconstructed from the historical record. To clarify this history, segments of 1-m cores from four lakes in North and South Dakota and northeastern Montana were examined for charcoal fragments. The cores spanned a time interval of 172 to 380 yr. Samples integrating 5-10 yr of deposition were taken from 2-cm sections along each core and sieved. Charcoal concentrations ranged from 6-14,799 fragments/gram of sediment, and mean charcoal abundance was negatively correlated with longitude. Results suggest periods of increased fire from A.D. 1700-1740 and A.D. 1850-1900. These peaks were broadly synchronous across the region and ranged in duration from 20-40 yr. Postsettlement patterns of charcoal deposition were highly variable but generally much lower than presettlement intervals.

Influence of Simulated Burning of Soil-Litter From Low Sagebrush, Squirreltail, Cheatgrass, and Medusahead on Water-Soluble Anions and Cations

We evaluated the influence of temperature and heating time on water-soluble anions and cations of soil-litters of low sagebrush (Artemisia arbuscula ssp. longicaulis), squirreltail (Elymus elymoides), cheatgrass (Bromus tectorum), and medusahead (Taeniatherum caputmeduase ssp. asperum). Soil-litters were collected from volcanic tablelands north of Honey Lake, California. These high clay, montmorillonitic, soils have rarely experienced wildfires. Soil-litter samples, in a 50 mt crucible, were placed in a preheated muffle furnace using a time-temperature matrix of 150, 250, 350, and 450°C at 1, 5,and 15 min. High performance ion-exchange chromatography was used to quantify water-soluble cations and anions. For most measured solutes, there was either a significant (P less than or equal to 0.05) burn time × temperature or a significant burn time × temperature × soil-litter type interaction. As compared to pre-burn values, heating of soil generally increased the concentration of the water-soluble solutes measured; nitrate was the exception. Maximum solute values were generally obtained at a temperature of 350°C at 5 and 15 min heating time. A temperature of 450°C at 15 min heating time resulted in the significant reduction of solute concentration below the maximum values. Nitrate was significantly higher in squirreltail soil-litter than the other soil-litters when the burn time was 1 min. Heating, in general, fostered an increase in water-soluble ammonium. For most solutes, heat-induced increases were greatest for the low sagebrush soil-litter.

The abundance of 15N in N2O in aquatic ecosystems with emphasis on denitrification

AbstractUsing soil-water systems under laboratory conditions several experiments were carried out to elucidate the possible factors governing the variations of δ15N in dissolved N2O in aquatic systems. Under anoxic conditions, the δ15N value in N2O during the initial reduction step from NO3− to N2 was as low as –27‰ relative to that of the initial nitrate (NO3−). The δ15 N value of N2O then increased to that of NO3 − as denitrification progressed until a stationary concentration of N2O was established. The effects of NO3− concentrations on the reduction of N2O to N2 during the denitrification process were considered important to the variations in δ15N (N2O)observed in nature. Based on data for δ15N (N2O) in the eastern tropical North Pacific and Lake Kizaki, Japan, several possibilities for the use of δ15N as a source indicator of N2O derived from microbial oxidation-reduction reactions are examined.

Efficacy of Praziquantel in Natural Populations of Mallards Infected with Avian Schistosomes

In 1991, mallards (Anas platyrhynchos) on North and South Lake Leelanau in Leelanau Co., Michigan were trapped, examined for natural infections of avian schistosomes, and treated with praziquantel. Prevalence of infection from the 113 birds recaptured in 1992 was compared with that of the previous year on these 2 inland lakes after some of the birds were treated. Infections were determined by examining diluted fecal samples for hatched schistosome miracidia. Approximately 15% of the 366 birds captured the first year homed the second and made up about 27% of the resident mallard population. Of the birds treated in 1991, only 1.8% showed an infection in 1992 compared with 14.6% for previously untreated birds. These data indicate that praziquantel is an effective therapeutic agent for reducing natural infections of the parasite in mallards. Furthermore, yearly treatment of mallards at specific sites may not be necessary for effective control of swimmer's itch on North and South Lake Leelanau.

Absolute gravity measurements at Nordic geodetic laboratories in Fennoscandia and Spitsbergen 1991–1993

Two dolerite dykes occurring in semi-penetratively deformed Transscandinavian Igneous Belt (TIB) granitoids, east of the traditional ‘Protogine Zone’ have a composite UPb baddeleyite age of 1568+30−8 Ma. Since the dolerites are deformed themselves, this age sets a maximum time constraint for the ductile deformation in the easternmost part of the Sveconorwegian orogen, northeast of Lake Vänern, west of the Sveconorwegian Frontal Deformation Zone (SFDZ; Wahlgren et al., 1994).The new age agrees, within the error limits, with previously reported but less precise UPb and SmNd ages for petrographically and geochemically indistinguishable dolerites in the orthogneisses west of the traditional ‘Protogine Zone’, and together provides abundant evidence that these dolerites belong to the same generation of mafic magmatism. Therefore, the dolerites straddle the boundary between the orthogneisses and the semi-penetratively deformed TIB, i.e. the traditional ‘Protogine Zone’. Apart from intracrustal deformation, the area north of Lake Vänern from the SFDZ in the east to the Mylonite Zone in the west has constituted a coherent segment of crust from the time of the TIB intrusions (∼ 1850-1650 Ma).The dolerites in this study have intruded as dykes into isotropic host rocks. The present dip and concordance to the schistosity in the surrounding rocks are due to subsequent progressive deformation in the eastern part of the Sveconorwegian orogen. It is suggested that the structural geometry and concordant relationships between the dolerites and the orthogneisses west of the traditional ‘Protogine Zone’ are also mainly a consequence of this subsequent progressive deformation that post-dates the dolerite emplacement.

Quality standard for the identification and treatment of persons coinfected with human immunodeficiency virus and Mycobacterium tuberculosis.

Journal Article Quality Standard for the Identification and Treatment of Persons Coinfected with Human Immunodeficiency Virus and Mycobacterium tuberculosis Get access John P. Phair, John P. Phair Reprints or correspondence: Dr. John P. Phair, Northwestern University Medical School, 680 North Lake Shore Drive, Suite 1106, Chicago, Illinois 60611-4402. Search for other works by this author on: Oxford Academic PubMed Google Scholar Peter A. Gross, Peter A. Gross Search for other works by this author on: Oxford Academic PubMed Google Scholar Jonathan E. Kaplan, Jonathan E. Kaplan Search for other works by this author on: Oxford Academic PubMed Google Scholar Henry Masur, Henry Masur Search for other works by this author on: Oxford Academic PubMed Google Scholar King K. Holmes, King K. Holmes Search for other works by this author on: Oxford Academic PubMed Google Scholar Catherine M. Wilfert, Catherine M. Wilfert Search for other works by this author on: Oxford Academic PubMed Google Scholar William J. Martone, William J. Martone Search for other works by this author on: Oxford Academic PubMed Google Scholar Kenneth G. Castro Kenneth G. Castro Search for other works by this author on: Oxford Academic PubMed Google Scholar Clinical Infectious Diseases, Volume 21, Issue Supplement_1, August 1995, Pages S130–S131, https://doi.org/10.1093/clinids/21.Supplement_1.S130 Published: 01 August 1995

PROVIDING WORKPLACE LITERACY: INSTITUTIONAL COLLABORATION WITH BUSINESS AND INDUSTRY

This study attempts to provide both businesses and institutions of higher education with a descriptive analysis of the programs of five companies that have used community colleges in their basic skills programs. Modified case studies were used to obtain data collected through individual interviews with representatives from selected community colleges and companies. The five companies represented include Texas Instruments Defense Systems Corporation and SGS‐Thomson Microelectronics (electronics companies), Abbott Laboratories (a pharmaceutical company), J & E Die Casting (a small die‐casting firm), and Company X, a semiconductor company that requested anonymity. The community colleges included were Richland College, Brookhaven College, and North Lake College, all members of the Dallas County Community College District, Dallas County, Texas. Numerous factors contributed to the success or demise of the programs studied. Elements that served as powerful assets when adequately supported were detrimental when n...

Measurement of Microbial Biomass and Activity in Landfill Soils

Two complementary techniques, which have been widely used to provide a general measure of microbial biomass or microbial activity in natural soils, were evaluated for their applicability to soils from the Mallard North and Mallard Lake Landfills, DuPage County, Illinois, U.S.A. Included were: (1) a potassium sulphate extraction technique with quantification of organic carbon for measurement of microbial biomass; and (2) an arginine ammonification technique for microbial activity. Four profiles consisting of replaced soils were sampled for this study; units included topsoil (mixed mollisol A and B horizons), compacted clay cover (local calcareous Wisconsinan age glacial till), and mixed soil/refuse samples. Internally consistent results across the four profiles and good correlations with other independent indicators of microbial activity (moisture, organic matter content, nitrogen, and phosphorus) suggest that, even though these techniques were developed mainly for natural mineral soils, they are also applicable to disturbed landfill soils.

The role of microorganisms in elemental mercury formation in natural waters

Gas evasion of elemental Hg (Hg°) from the open ocean plays a prominent role in the global mercury cycle. Elemental Hg is formed primarily by reduction of ionic Hg in the mixed layer of aquatic systems. By culturing phytoplankton in defined media, and by incubating natural seawater and freshwater samples, we have demonstrated that Hg° is produced by microorganisms, with formation rates (0.5 to 10% d -1 ) similar to those estimated from mass balance studies. Our results also suggest that <3 μm microorganisms are the primary Hg reducers in natural waters. Eucaryotic phytoplankton are capable of reducing ionic Hg to Hg° but the rate of reduction is insufficient to account for the observed reduction rates found in incubated field samples. Bacteria are thus the more likely Hg reducers. In seawater, cyanobacteria such as Synecococcus may account for much of the mercury reduction, while in the eutrophic, polluted Upper Mystic Lake north of Boston other procaryotic microorganisms are contributing to the overall Hg reductive capacity of the medium. By reducing ionic Hg, microorganisms play a pivotal role in the aquatic biogeochemistry of Hg, not only by enabling evasion to the atmosphere, but by directly decreasing the amount of ionic Hg available for methylation.

Planktonic and microbial contributions to phosphorus release from fresh and air-dried sediments

There was a fivefold increase in the concentration of P (dissolved reactive phosphorus) after intact sediment cores from North Lake, Western Australia, were air-dried and reflooded. Effects of plankton and microbial biomass on phosphorus release were investigated. Freshly killed plankton released P before drying, but less P was released when plankton biomass was air-dried. Bacteria were important contributors of P during drying and rewetting, and their contributions occurred through two processes: (I) when sufficient moisture and oxygen were available, bacteria rapidly removed soluble P from the water and incorporated it into the particulate phase, the amount of P taken up being positively correlated with bacterial respiration; and (2) upon drying, bacterially stored P was partly returned to the water, the release increasing with increasing bacterial uptake. The bacterial contribution of P upon drying was further investigated by sterilizing air-dried and wet sediments with a low dose of gamma irradiation (10 kGy). It was thus possible to discriminate among P contributed from the native (initial) microbial biomass (Pi) before drying, P released from the increased (developed) microbial biomass (Pii) during drying, and P stored in bacteria that had survived air-drying (Ps). It was estimated that air-drying killed about 76% of the microbial biomass. At relatively low external P concentrations (&lt;1000 μg L-1), the increased P brought about by air-drying was mainly derived from killed microbial biomass.

Tree and Tundra Cover Anomalies in the Subarctic Forest-tundra of Northwestern Canada

Factors that may account for subarctic vegetation patterns that are not reconcilable with synoptic climate were sought by reference to data on local climate, soils, parent materials, vegetation, and topography. Where the forest-tundra lies climatically northward, the vegetation transition zone is relatively narrow, and the soils are nutrient-rich, fine-textured loamy Cryosols derived from sedimentary rocks. Where the forest-tundra lies south of its climatic potential, the zone is wide, and the soils are relatively dry, nutrient-poor loamy sand and sandy loam Brunisols derived from crystalline rocks. Seven regional anomalies in landscape cover of tree and upland tundra vegetation are discussed with reference to topoclimate, edaphic controls, and bioclimatic feedback. Prominent anomalies include the Thelon River tree oasis, the great breadth of the southeastern forest-tundra, and the southward plunge of the forest-tundra north of Great Slave Lake. Elevation differences as small as 50 m are shown to affect tree cover. Key words: bioclimate, edaphic, fire, spruce, topoclimate, tree line, tundra

Modern sedimentation rate dating and environmental implication of Nushan Lake

为探讨本世纪以来女山湖生态环境的演化历史,以¹³⁷Cs和²¹⁰pb放射性同位素为测试手段,分析该湖近期沉积物中¹³⁷Cs和²¹⁰Pb的含量分布,并根据其沉降特征和衰减规律,对沉积物做了年代定位,分段推导出平均沉积速率,建立柱状沉积年代序列。在此基础上,利用沉积记录,对沉积速率的变化进行环境解释,探讨了本世纪以来湖泊环境的演化过程。结果表明,1929~1938年沉积速率达4.8mm/a,是淮河流域洪涝使泥沙倒灌入湖的结果,1958年以后沉积速率增强则直接受控于水利工程的拦截作用。;Nushan Lake, located in the Tanlu Fault zone, Jiashan County, Anhui Province, covers an area of 100 km² with the deepest depth of 3m. According to the spot investigation, lacustrine mud in the northern part is over 8m in depth. The terrestrial deposit input is mainly from the Chihe River of the south. Occasionally, the mud-sand input is carried by the Huaihe River in the north of Nushan Lake. The short streams around the lake basin also participate in the lake sedimentation. A chronologic succession of a core taken from Nushan Lake has been buUt to reveal the evolutionary history of ecological environments based on determining sedimentation rates of the core with radinuclides ²¹⁰Pb and ¹³⁷Cs datings. Then, combining with some environmental paramenters such as χ_LF, χ_M, Gralasize, Pigments, TOC, the environmental implication of the lake in the 20's century is discussed. It is indicated that the major factors to influence the sedimentation rate of Nushan Lake axe (1) mainly ia-flow from Chihe River; (2) large amount of input mud-sand from Huaihe River; and (3) human activities in this century. Four stages can be divided for the environmental change of Nushan Lake in this century:Stage I (depth 19.5~14.5cm, 1904~1929):The sedimentation rate is 2.0nim/a. It is stable for the lake sedimental environment. Stage Ⅱ(depth 4.5~10.0cm, 1929~1938):Thesedimentation rate is 2.0mm/a. Debris deposits inflow of Huaihe drainage led to the unstable lake sedimentary environment. It is abundant for the terristrial organic matter. Stage Ⅲ (depth 10.0~5.0cm, 1938~1966):The sedimentation rate is 1.8m/a. It has experienced the lasting 4-year-arid-event in 1941~1944, which leads to less mud-sand and lower sedimentation rate. Stage Ⅳ (depth 5.0~1.5cm, 1966~1985):The sedimentation rate is 1.8m/a. A series of water-conservancy project in the Huaihe drainage play an important role, which has reduced the influence of natural factors and makes lake sedimentary environment stable. Stage Ⅴ (depth 1.5~0cm, 1985~1991):The sedimentation rate is 3.0m/a. The water-conservancy project of Nushan Lake in 1980's leads to the obvious increasing of sedimentation rate.

Measurement of microbial biomass and activity in landfill soils

Two complementary techniques, which have been widely used to provide a general measure of microbial biomass or microbial activity in natural soils, were evaluated for their applicability to soils from the Mallard North and Mallard Lake Landfills, DuPage County, Illinois, U.S.A. Included were: (1) a potassium sulphate extraction technique with quantification of organic carbon for measurement of microbial biomass; and (2) an arginine ammonification technique for microbial activity. Four profiles consisting of replaced soils were sampled for this study; units included topsoil (mixed mollisol A and B horizons), compacted clay cover (local calcareous Wisconsinan age glacial till), and mixed soil/refuse samples. Internally consistent results across the four profiles and good correlations with other independent indicators of microbial activity (moisture, organic matter content, nitrogen, and phosphorus) suggest that, even though these techniques were developed mainly for natural mineral soils, they are also applicable to disturbed landfill soils.

Three‐dimensional crustal structure of the Mendocino Triple Junction region from local earthquake travel times

The large‐scale, three‐dimensional geometry of the Mendocino Triple Junction at Cape Mendocino, California, was investigated by inverting nearly 19,000 P wave arrival times from over 1400 local earthquakes to estimate the three‐dimensional velocity structure and hypocentral parameters. A velocity grid 175 km (N‐S) by 125 km (E‐W) centered near Garberville, California, was constructed with 25 km horizontal and 5 fan vertical node spacing. The model was well resolved near Cape Mendocino, where the earthquakes and stations are concentrated. At about 40.6° N latitude a high‐velocity gradient between 6.5 and 7.5 km/s dips gently to the south and east from about 15 km depth near the coast. Relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted Gorda Plate. Therefore the depth to the top of the Gorda Plate near Cape Mendocino is interpreted to be ∼15 km. The Gorda Plate appears intact and dipping ∼8° eastward due to subduction and flexing downward 6°–12° to the south. Both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40.3° N latitude and maintains a linear trend 15° south of east to at least 123° W longitude. The top of a large low‐velocity region at 20–30 km depth extends about 50 km N‐S and 75 km E‐W (roughly between Garberville and Covelo) and is located above and south of the southern edge of the Gorda Plate. We interpret this low velocity area to be locally thickened crust (8–10 km) due to either local compressional forces associated with north‐south compression caused by the northward impingement of the rigid Pacific Plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the Cascadia Subduction Zone. South of Cape Mendocino and southwest of the Garberville fault, high velocities indicative of oceanic crust are detected at 15 km depth. We interpret this evidence as tectonic underplating of the Pacific oceanic crust beneath the coastal Franciscan Complex, similar to the situation seismically imaged in central coastal California (Page and Brocher, 1994). This geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening, while deep‐seated mantle upwelling is confined to an area centered about 75 km north of Clear Lake (Benz et al, 1992; Zandt et al., 1992).

Potential hazards associated with polycyclic aromatic hydrocarbons in weathered crude oil from Kuwait's oil lakes

Abstract Levels of phenanthrene, fluoranthene, pyrene, and benz(a)pyrene in weathered crude oil from Kuwait's north (Raudhatain#l and Sabriya#5) and central (Magwa#l and Magwa#11) oil lakes were determined. The average levels of phenanthrene showed the highest values in all lakes (35.65–88.50 mg/kg). The average benz(a)pyrene levels ranged between 1.19 and 10.48 mg/kg. The average levels of fluoranthene ranged between 3.85 and 22.7 mg/kg. The highest average level of pyrene was 27.38 mg/kg. During the period September 1992 ‐ June 1993, the average rate of change in phenanthrene levels (n=4) was negative (‐0.72 mg/kg. month) indicating a decrease in its levels; whereas the average rate of change in benz(a)pyrene levels was positive (+0.34 mg/kg.month) and exhibited the highest increase among all investigated PAH levels. Based on quantitative carcinogenic classifications, the average level of benz(a)pyrene in weathered crude oil collected in June 1993 poses approximately 3 and 26 times the carcinogenic equi...

Femur Fractures in Alpine Skiing: Classification and Mechanisms of Injury in 85 Cases

A retrospective review of 85 femur fractures sustained while alpine skiing in the north Lake Tahoe area was performed to determine incidence, severity, and mechanisms of injury. The incidence of femur fractures averaged 0.8 per 100,000 days of skiing and accounted for 0.4% of reported accidents in this series. Skiers sustaining femur fractures had a mean age of 30.2 years, significantly older than the skier mean age overall. Comminuted femoral shaft and trochanteric fractures that were the result of high-energy impact most often occurred in advanced skiers in firm or icy conditions. Firm ski conditions provided the most striking correlation to this type of fracture in the older age groups. In skeletally immature individuals (< 18 years of age), the femur fractures were mainly of the low-energy impact and minimally comminuted variety and were produced by an indirect, torsional, or bending mechanism. This injury typically occurred while skiing fast and resulted from catching the tip of a ski in wet or heavy snow. The majority of fractures (55%) occurred in the young adult population (18-45 years of age). Being predominantly advanced skiers, most of their fractures were the result of a high-energy, direct-impact fall or collision that led to severely comminuted femoral shaft and trochanteric fractures. Nineteen percent of those injured were > 45 years of age. More than two thirds of these individuals sustained fractures about the hip, typically a simple fracture in low-impact falls on hard or icy surfaces.

Delineation and analysis of the surface and subsurface structural lineament patterns in the North Lake Nasser area and its surroundings, Aswan, upper Egypt

Abstract The present work is an integrated photogeological-geophysical study which deals essentially with the delineation and analysis of the structural lineament pattern in the surroundings of the Aswan region, with special emphasis on the North Lake Nasser area to the south of Aswan, upper Egypt. This integrated study is based mainly on the utilization of the structural lineaments map of the southeastern desert of Egypt interpreted from the Landsat-I Satellite images in addition to the aerial photomosaics and aeromagnetic maps of North Lake Nasser area. Results obtained have been used in conjunction with the previously available seismological information in delineating sites of potential earthquake activity in the North Lake Nasser area.

Effects of oxygen concentration on phosphorus release from reflooded air-dried wetland sediments

Intact sediment cores from North Lake (Perth, Western Australia) were air-dried for 40 days, reflooded with lake water and incubated at 20°C. Under aerated conditions, air-drying increased phosphorus release. When aeration was stopped, air-dried sediments continued to release P into the water, but the rate was much slower than that in the early stage of the aeration. The results suggest a gradual depletion of labile phosphorus from the dried sediments. Under anaerobic conditions, the phosphate release was also markedly higher for air-dried sediments than for the 'wet' controls. The accumulation of soluble inorganic phosphorus during air-drying, owing to breakdown of organic material and a drying-induced decrease in phosphate sorption, may be the cause of release on rewetting. The results suggest that drought-induced sediment dehydration in natural wetlands may be followed by a significant increase of internal phosphorus loading under both aerobic and anaerobic conditions. Care must therefore be taken in using drawdown as a lake management technique to improve water quality.