Wind Resuspension Research Articles

Spatial and temporal variations in inhalable CuZnPb aerosols within the Mexico City pollution plume

We report on the CuPbZn content of PM10 and PM2.5 samples collected from three sites (urban T0, suburban T1 and rural T2) during the Mexico City MILAGRO campaign of March 2006. Daytime city centre concentrations of summation operator CuZnPb(PM10) were much higher (T0 > 450 ng m(-3)) than at the suburban site (T1 < 200 ng m(-3)). Rural site (T2) summation operator CuZnPb(PM10) concentrations exceeded 50 ng m(-3) when influenced by the megacity plume but dropped to 10 ng m(-3) during clean northerly winds. Nocturnal metal concentrations more than doubled at T0, as pollutants became trapped in the nightly inversion layer, but decreased at the rural site. Transient spikes in concentrations of different metals, e.g. a "copper event" at T0 (CuPM10 281 ng m(-3)) and "zinc event" at T1 (ZnPM10 1481 ng m(-3)) on the night of March 7-8, demonstrate how industrial pollution sources produce localised chemical inhomogeneities in the city atmosphere. Most metal aerosols are <2.5 microm and SEM study demonstrates the dominance of Fe, Ti, Ba, Cu, Pb and Zn (and lesser Sn, Mo, Sb, W, Ni, V, As, Bi) in metalliferous particles that have shapes including spherical condensates, efflorescent CuZnClS particles, cindery Zn, and Cu wire. Metal aerosol concentrations do not change in concert with PM10 mass, which is more influenced by wind resuspension than industrial emissions. Metalliferous particles can induce cell damage, and PM composition is probably more important than PM mass, with respect to negative health effects, so that better monitoring and control of industrial emissions would likely produce significant improvements in air quality.

The effects of riverine discharge on temperature, salinity, suspended sediment and chlorophyll a in a Mississippi delta estuary measured using a flow-through system

The impact of diverted Mississippi River water on temperature, salinity, total suspended sediment (TSS) and chlorophyll a were monitored in the Breton Sound estuary from September 7, 2000, to August 28, 2002. Twenty-seven transects were carried out using a flow-through system to continuously measure temperature, salinity, turbidity and fluorescence, and discrete water samples were taken at 16 locations for calibration. Discharge from the river diversion structure ranged from 0 to 213 m 3/s, with several large pulses of water released during the spring of 2001 and 2002. There was a strong seasonal temperature signal in the estuary, with summer highs of >30 °C and winter lows of <12 °C. Incoming river water was usually cooler than estuarine waters, with temperatures as low as 6 °C, but generally equilibrated to the rest of the estuary within several kilometers. Salinity in the upper estuary was fresh throughout the study, and increased up to 14 and 30 PSU along Western and Eastern routes, respectfully, with distance from the diversion. Discharge from the river diversion greatly affected salinity, with the large spring pulses often causing the entire estuary to become fresh for a short period of time (<1 month). There was also a temporal lag of about two weeks between discharge and salinity in the lower estuary. River water entering the estuary had TSS concentrations ranging from 40 to 252 mg/L, with an average of 118 mg/L. Sediment from the river diversion reached about 10–15 km into the estuary during spring pulses. There were highly fluctuating TSS concentrations at the Gulf end of the estuary during winter and spring, due to wind resuspension. Chlorophyll a concentrations were highest in mid-estuary during summer and fall low discharge, and lowest during winter and spring high discharge. Chlorophyll a levels were generally less than 10 μg/L in the upper estuary, with concentrations rising in the mid-estuary generally to 20–30 μg/L during late summer and fall, and decreasing in the lower estuary. There were several periods of less than a month with unusually high chlorophyll a concentrations, ranging from 38 to >60 μg/L, that occurred during late summer and early winter.

Why biomanipulation can be effective in peaty lakes

The effects of fish stock reduction (biomanipulation) was studied in an 85 ha shallow peaty turbid lake. The lake cleared in a 4-week period in April–May 2004, which demonstrated that biomanipulation can be effective in peaty lakes. We demonstrated that it is possible to reduce the fish stock to <25 kg ha−1 benthivorous fish and <15 kg ha−1 planktivorous fish, sufficiently low to switch the lake from a turbid to a clear state. Knowledge of lake morphology, fish stock, fish behaviour, and a variety of fishing methods was necessary to achieve this goal. It is expected that continuation of fisheries to remove young of the year planktivorous species is needed for several years, until macrophytes provide sufficient cover for zooplankton and can compete with phytoplankton. Cladocerans developed strongly after fish removal. The clearing of the lake coincided with a sudden decrease of filamentous cyanobacteria and suspended detritus, and a strong increase of Bosmina. We assume that Bosmina was able to reduce filamentous prokaryotes and detritus. After the disappearance of the cyanobacteria, Bosmina disappeared too. After the clearing of the lake Daphnia dominated in zooplankton and apparently was able to keep phytoplankton levels low. In our case, wind resuspension did not prevent biomanipulation from being successful. No correlation between windspeed and turbidity was found, neither in an 85 ha nor in a 230 ha shallow peaty lake. Regression analysis showed that on average 50% of the amount of suspended detritus can be explained by resuspension by fish and 50% by phytoplankton decomposition. The main goal of this biomanipulation experiment, clear water and increased submerged plant cover in a shallow peaty lake, was reached.

Major-ion bulk deposition around an active volcano (Mt. Etna, Italy)

Bulk atmospheric deposition of major cations (Na, K, Ca, Mg) and anions (Cl, F, SO4) were measured at 15 sites around an active volcano, Mount Etna, from 2001 to 2003. Their composition indicates several natural sources, among which deposition of plume-derived volcanogenic gas compounds is prevalent for F, Cl and S. Plume-derived acidic compounds are also responsible for the prevailing acidic composition of the samples collected on the summit of the volcano (pH in the 2.45–5.57 range). Cation species have complex origin, including deposition of plume volcanogenic ash and aerosols and soil-dust wind re-suspension of either volcanic or carbonate sedimentary rocks. Variation of the deposition rates during the March 2001–March 2003 period, coupled with previous measurements from 1997 to 2000 (Appl Geochem 16:985–1000, 2001), were compared with the variation of SO2 flux, volcanic activity and rainfall. The deposition rate was mainly controlled by rainfall. Commonly, about 0.1–0.9% of HF, HCl and SO2 emitted by the summit crater's plume were deposited around the volcano. We estimate that ∼2 Gg of volcanogenic sulphur were deposited over the Etnean area during the 2002–2003 flank eruption, at an average rate of ∼24 Mg day−1 which is two orders of magnitude higher than that typical of quiescent degassing phases.

Spatial and temporal variations of the inherent and apparent optical properties in a shallow coastal lake

Apparent and inherent optical properties in the coastal lagoon of Fogliano were measured in three seasonal surveys in 2002. Irradiance data from in situ measurements of ultraviolet and visible wavebands permitted to estimate the related attenuation coefficients. Laboratory extinction measurements on filtered (0.22 μm) and unfiltered water samples were also performed. The integrated approach between in situ and laboratory measurements allowed the determination of the role of the suspended and dissolved matter in the attenuation and extinction of ultraviolet and visible radiation within the water column. As noted, the impact of the suspended and dissolved matter on the lake optical quality was influenced by wind resuspension of particulate matter: the relative role of dissolved matter in the absorption of UV and visible radiation was prevailing at low wind velocity conditions (less than 2.2 m s −1), while at high wind velocities (3.9 m s −1), particulate matter resuspension strongly influenced the attenuation and the extinction measurements. The extinction in the analyzed wavelengths of filtered and unfiltered water samples and the in situ irradiance measurements allowed us to define new optical parameters and important correlations with limnological and classical optical measurements. By sampling at high spatial resolution (18 stations in 4 km 2), it was possible to evidence a spatial gradient of the optical and limnological properties, these distributions showed a consistent pattern in all three surveys, and were important for the characterization of the chromophoric dissolved organic matter that was estimated with the spectral slope of the extinction curve spectra. A relatively higher spectral slope was found in the southern basin with respect to the northern, where the maximum values of the attenuation coefficients and limnological parameters were found. These results suggest different sources of dissolved organic matter and/or a different level of photobleaching.

Internal Phosphorus Loading in Shallow Lakes: Importance and Control

Decreasing the algal biomass and increasing transparency in shallow, unstratified lakes is usually more difficult than for deep, stratified lakes. Eutrophic unstratified lakes (or shallow, stratified lakes susceptible to metalimnion erosion) have typically responded slowly to reduced external nutrient loading, usually because of longevity of internal loading. That is because sediment-released nutrients (especially phosphorus) readily enter the trophogenic zone of shallow lakes during the growing season and result in high lake concentrations. In stratified lakes, metalimnia may serve as barriers to phosphorus transport into the trophogenic zone. Although the whole water column in shallow lakes is usually aerobic, several mechanisms can combine to produce relatively high sediment phosphorus release rates in these lakes. These include: 1) wind resuspension and bioturbation, combined with high pH or low Fe/P ratio that maintains high P solubility, 2) periodic anoxia and reducing conditions promoted by calm, warm weather, and 3) macro-phyte senescence. Attempts to reduce algal biomass by controlling internal phosphorus loading have often been effective.

The use of sedimentary algal pigments to infer historic algal communities in Lake Apopka, Florida

The primary producer community of Lake Apopka, a large (125 km2), shallow (mean depth, 1.7 m), polymictic Florida lake, shifted from macrophyte dominance to phytoplankton dominance in the 1940s. Today, frequent wind resuspension of highly organic, unconsolidated sediments supports a meroplanktonic community that is predominantly diatoms, but during calm periods the algal community is dominated by planktonic cyanobacteria. Sedimentary algal pigments (chlorophyll derivatives and carotenoids) and chemical proxies for nutrient enrichment (polyphosphate, total phosphorus and biogenic silica) in three sediment cores were used to investigate historic changes in primary producers. Sediments were separated into three stratigraphic zones using multivariate statistical techniques. Stratigraphic zonation was established in each core although sediment deposition at one site was insufficient to adequately resolve temporal changes. These results show the importance of selecting suitable sites for paleolimnological studies. The oldest zone represents macrophyte-derived sediments, and the two overlying zones represent phytoplankton-derived sediments deposited since the 1940s. Algal pigments in the most recent sediment zone show little degradation, which might be due to the presence of viable meroplankton in the sediment. After the initial primary producer shift from macrophytes to phytoplankton, the lake experienced a short period of cyanobacterial dominance followed by a period of benthic diatom abundance before being replaced by the present algal community consisting of cyanobacteria and meroplanktonic diatoms. Chlorophyll derivatives and carotenoids were highly correlated with total phosphorus. Historic trends inferred from the data include algal and cyanobacterial productivity that increased with increased phosphorus loading. The study demonstrates that valid paleolimnological proxies for historic eutrophication are available in loosely consolidated sediments of shallow, subtropical lakes.

Age‐0 Black Crappie Abundance and Size in Relation to Zooplankton Density, Stock Abundance, and Water Clarity in Three Florida Lakes

AbstractCrappies Pomoxis spp. are popular sport fishes but are often difficult to manage due to highly variable recruitment. To assess the factors influencing recruitment, we investigated the size and abundance of age‐0 black crappie P. nigromaculatus in relation to zooplankton density, adult stock abundance, and water clarity in three productive Florida lakes. Age‐0 black crappies were collected at Lakes Wauberg, Lochloosa, and Tarpon by use of surface and bottom trawls during the spring, summer, and fall of 2000 and 2001. Zooplankton abundance was measured concurrently with trawl sampling. Early‐summer juvenile abundance was positively related to stock abundance during the previous fall among lakes, indicating a relationship between stock size and juvenile black crappie abundance. Early‐summer juvenile abundance was positively related to juvenile abundance in the fall (i.e., at the end of the first summer), suggesting that year‐class strength was set by early summer in all lakes. Lake Lochloosa had the highest crustacean zooplankton abundance in both years, but this lake had the smallest age‐0 black crappie size and intermediate age‐0 fish abundance. Thus, we found no relationship between crustacean zooplankton density and black crappie size across lakes. Reduced Secchi depths due to high chlorophyll‐a values and wind resuspension of sediments may have reduced feeding efficiency for age‐0 black crappies at Lake Lochloosa relative to the other lakes. Age‐0 black crappie abundance and size during fall were not predicted solely by food availability, stock abundance, or water clarity, suggesting that these factors interacted to influence recruitment through the first year of life.

Fish facilitate wave resuspension of sediment

The resuspension of sediment by waves or currents is a major cause of turbidity in many shallow waters. Here we show that bottom‐feeding fish greatly facilitate this process by reducing the erosion resistance of the sediment. We use a fish‐removal experiment from a large Dutch lake to show that the absence of fish makes sediment resistant to wind effects, and we use controlled lab experiments to demonstrate that the effect of fish can be explained by the characteristic hole‐punching behavior used in the search for food. Undisturbed sediment in the laboratory consolidated rapidly. The critical water velocity needed for resuspension roughly doubled in 2 weeks. However, the acquired erosion resistance could be virtually eliminated by holes corresponding to only 2% of the sediment area. An analysis of a model based on our experimental results suggested that benthivorous fish may prevent sediment consolidation that, in the absence of fish, would sufficiently stabilize the sediment during a quiet period to resist wave resuspension during subsequent windy periods. Our results imply that, even in exposed shallow lakes that are turbid because of wind resuspension, a temporary reduction of fish stock may work as "shock therapy"to restore an alternative stable clear‐water state.

Lake age and water level affect the turbidity of floodplain lakes along the lower Rhine

SUMMARY 1. We sampled a set of 93 lakes situated in the floodplains of the lower River Rhine in search for morphometric and other factors that explain their variation in clarity.2. Lakes with a drop in summer water level were less turbid at the time of sampling, mainly because of a lower concentration of inorganic suspended solids (ISS).3. We also found that older lakes were more turbid than younger lakes and that this was largely because of an increase in phytoplankton.4. Water clarity was positively related to lake depth and the presence of vegetation.5. Model calculations indicated that the underwater light climate was strongly affected by chlorophyll and ISS, the latter being the dominant factor affecting Secchi depth. Dissolved organic carbon (DOC) was less important.6. The high concentration of ISS suggests that intensive resuspension occurs in most of the lakes. Using a simple wave model, and assuming that vegetation protects sediments against resuspension, we could eliminate wind resuspension as an important process in 90% of the lakes, leaving resuspension by benthivorous fish as probably the most important factor determining transparency.7. Chlorophyll a concentration showed a strong positive correlation to ISS concentration, suggesting that resuspension may also have a positive effect on phytoplankton biomass in these lakes.8. In conclusion, in‐lake processes, rather than river dynamics, seem to be driving the turbidity of floodplain lakes along the lower River Rhine.

Importance of temporal and spatial scales in applying biological and physical process knowledge in coastal management, an example for the Ems estuary

Managing intertidal mud flats requires a knowledge about the physical and biological structure of these systems, their physical and ecological roles and the development and influence of socio-economic activities on these systems. In this paper it is shown how, against the background of the strong natural variation, the effects of dredging and anthropogenic eutrophication can be simplified, represented and applied for management purposes. Freshwater (nutrient) discharges differently but linearly affect the annual primary production in the three (Lower, Middle and Dollard) estuarine reaches. The differences in effect along the estuarine axis are primarily caused by the turbidity gradient. Channel dredging affects the slope of the logarithmic turbidity gradient between the turbidity maximum with relatively stable concentrations in suspended matter, and the North Sea near the barrier islands. The direct consequence of a varying slope of the gradient is that the most seaward situated reaches of the estuarine system are more strongly influenced by channel dredging than the upstream reaches. The relative effects of both channel dredging and eutrophication become more manifest in the lower reaches of the estuary than in the upstream parts. Based on the correlations found, the expectation is that mean annual river discharges lower than ca. 35 m 3 s −1 will turn the entire estuarine system to full heterotrophy. Results from experiments studying stabilization and destabilization by organisms are helpful to conceptual development, but are not helpful to any decision maker because the spatial scale of these systems usually does not fit the needs of an environmental manager who usually has to consider entire systems. Considering the aspect of scales results in the conclusion that at the process level (primary production process) the time scale in the Ems estuary is mainly determined by the seasonal cycle and much less by factors like wind and tide. From the management perspective and considering seasonality, the longer term effects of both anthropogenic eutrophication and channel dredging can be best judged based on an annual time scale. Weather conditions and human activities both impact either large parts of the estuary (wind resuspension, eutrophication) or the entire area (dredging), something that is due to the long tidal travelling distance in comparison to the length of the estuary. This results in the necessity to consider large areas when judging the environmental impact of the above-mentioned parameters. The available information about the socio-economic subsystem and the natural subsystem can be integrated in instruments like management knowledge systems or decision support systems to help us to structure discussions in prioritizing between policy options.

Dust resuspension without saltation

Wind resuspension (or entrainment) provides a source of dust and contaminants for the atmosphere. Conventional wind erosion models parameterize dust resuspension flux with a threshold velocity or with a horizontal abrasion flux; in the absence of abrasion the models assume dust flux is transient only. Our experiments with an uncrusted, fine material at relative humidities exceeding 40% show a long-term steady dust flux in the absence of abrasion, which fits the approximate form: F(d) = 3.6(u*)(3), where F(d) is the dust flux (in mug/m(2) s), and u* is the friction velocity (in m/s). These fluxes are generally too small to be significant sources of dust in most models of dust emission. However, they provide a potential route to transport contaminants into the atmosphere. In addition, dust release is substantial during the initial transient phase. Comparison with field data suggests that the particle friction Reynolds number may prove a better parameter than u* for correlating fluxes and understanding the potential for abrasion.

Resuspension of coarse fuel hot particles in the Chernobyl area

Measurements of resuspended aerosol in the Chernobyl 30-km exclusion zone have shown coarse fuel hot particles in the activity range 1–12 Bq 137 Cs per particle. The particles were sampled with newly designed rotating arm impactors which simultaneously collect during the same experiment three samples with fuel particles in the size ranges larger than 3 μm, larger than 6 μm and larger than 9 μm in geometric diameter. The radionuclide ratios, determined after γ-spectrometry, were in good agreement with the theoretical calculations for the radionuclide-composition of the Chernobyl Nuclear Power Plant at the moment of the accident and the measured hot particles in soil in the early years after the accident. The number concentrations of airborne hot particles were derived from digital autoradiography. For wind resuspension, maximal concentrations of 2.6 coarse hot particles per 1000 m 3 and during agricultural activities 36 coarse hot particles per 1000 m 3 were measured. The geometric diameter of single hot particles was estimated to be between 6 and 12 μm.

Algal toxins?Initiators of avian botulism?

An outbreak of avian botulism in Whitewater Lake, Manitoba, Canada was associated with reducing sediments. But any linkage between sediments and botulism was only indirect; Clostridium botulinum was not observed in the sediments. The source of the C. botulinum was unclear but carcasses that overwintered appeared to perpetuate the outbreak. The algal toxins anatoxin-a and microcystin-LR were present (17≤mg/L) when many birds were moulting and unable to fly, likely making them more sensitive to botulism. The sediment anoxia released phosphorus into lakewater so that concentrations increased from about 73 to 470 mg/L and enhanced growth of Microcystis and Aphanizomenon. Wind resuspension of sediments resulted in areas with more algal biomass and associated algal toxins. © 2000 John Wiley & Sons, Inc. Environ Toxicol 15: 558–567, 2000

Predictions on the effect of common carp (Cyprinus carpio) exclusion on water quality, zooplankton, and submergent macrophytes in a Great Lakes wetland

We conducted a study to examine the relationship between common carp (Cyprinus carpio) exclusion, water quality, zooplankton, and submergent macrophytes. Twelve 50-m2 in situ experimental enclosures were installed in degraded Cootes Paradise Marsh during the carp spawning period in 1995. Enclosures were stocked with two or three carp of similar size, ranging from 13 to 59 cm and in total biomass from 23 to 2100 kg/ha. Turbidity, total phosphorus, and total ammonia concentrations increased predictably with total carp biomass in the enclosures. Although carp had no direct effect on zooplankton community structure, increased turbidity and nutrient load associated with carp activity resulted in reduced total zooplankton biomass. We developed a relationship between species richness and water turbidity for 19 wetlands in the Great Lakes basin which indicated that above an apparent threshold of 20 NTU, there were less than five species of submergent plants, while a more diverse community existed in less turbid systems. We predict that water turbidity in Cootes Paradise Marsh may not be reduced below this threshold value of 20 NTU following carp exclusion. We emphasize the need to consider other factors that may contribute to increases in water turbidity and nutrient concentrations, including wind resuspension and substrate characteristics.

A conceptual ecological model to aid restoration of Cootes Paradise Marsh, a degraded coastal wetland of Lake Ontario, Canada

An ecological model is derived from recent studies, based on 60 years of empirical observations and experimental data, that conceptualizes how Cootes Paradise Marsh was transformed from a lush emergent marsh with considerable ecological diversity in all trophic levels, to one that is currently turbid, devoid of vegetation, and dominated by a few exotic plant and fish species. This conceptual model contains 17 key components that interact and contribute to the overall unhealthy state of the marsh. The most influential component is high water level which caused the initial loss of emergent vegetation in the 1940s and 1950s. In the absence of plants to attenuate sediment and assimilate nutrients, the marsh became turbid and windswept, and this led to the disappearance of submergent vegetation over the next two decades. Currently, high water turbidity is being maintained by wind re-suspension, high sediment loading from the watershed during the summer, high algal biomass resulting from excessive nutrient loads from sewage effluent and surface runoff, and the feeding and spawning activities of a very large population of common carp ( Cyprinus carpio). Due to vegetation loss, the substrate has become mostly loose sediment that is no longer suitable for the diverse assemblage of aquatic insect larvae that lived on the plants and detrital material in the 1940s. Benthic grazers have been kept in low abundances due to predation by benthivorous carp; consequently, epiphytic algae have proliferated and further contribute to light limitation of macrophytes. High nutrient loadings contribute to high diurnal fluxes in dissolved oxygen levels that tend to select against less tolerant organisms such as insect larvae (other than chironomids) and piscivores (northern pike and largemouth bass). Without piscivores in the marsh, the planktivores have become dominant and have virtually eliminated all of the large herbivorous zooplankton (e.g., Daphnia), except for a few pockets in the marsh inlets close to residual macrophyte beds. Because of the dominance of small-bodied inefficient grazers (rotifers and small cladocerans), algal biomass is high, and the community has a large proportion of heterotrophic forms that tolerate low light environments. This ecological model suggests that the current turbid un-vegetated state of Cootes Paradise may be very stable. It will persist as long as water levels remain unfavorable for natural re-colonization by the emergent flora, and/or water turbidities remain sufficiently high to suppress the growth of submergent vegetation. Using this conceptual model, I developed a model of how Cootes Paradise Marsh may have functioned as a healthy marsh prior to the 1940s, and use these models as a basis to explore a number of restoration and management options and discuss their implications on the aquatic foodweb.

Determination of Wind Re-suspension Rate of Radionuclides and Assessment of Radioactivity in the Atmospheric Air in Bryansk Region

The wind re-suspension rate was estimated by comparing the calculated and measured values of 137Cs concentrations and depositions in the town of Novozybkov. For the purpose a physical-mathematical model was used for the atmospheric dispersion and transport of radionuclides due to their re-suspension from the underlying surfaces and re-deposition. The data of measurements performed by SPA 'Typhoon' in 1992-1993 were also used. The estimates obtained enabled an investigation of the dependence of wind re-suspension rate on wind velocity and direction to be carried out. Based on the obtained estimates of the wind re-suspension rate, the fields of near-surface concentrations of 137Cs in an area 40 x 40 km2 in the environs of Novozybkov were calculated for various wind velocities and directions.

Internal Phosphorus Loading in Shallow Lakes: Importance and Control

ABSTRACT Decreasing the algal biomass and increasing transparency in shallow, unstratified lakes is usually more difficult than for deep, stratified lakes. Eutrophic unstratified lakes (or shallow, stratified lakes susceptible to metalimnion erosion) have typically responded slowly to reduced external nutrient loading, usually because of longevity of internal loading. That is because sediment-released nutrients (especially phosphorus) readily enter the trophogenic zone of shallow lakes during the growing season and result in high lake concentrations. In stratified lakes, metalimnia may serve as barriers to phosphorus transport into the trophogenic zone. Although the whole water column in shallow lakes is usually aerobic, several mechanisms can combine to produce relatively high sediment phosphorus release rates in these lakes. These include: 1) wind resuspension and bioturbation, combined with high pH or low Fe/P ratio that maintains high P solubility, 2) periodic anoxia and reducing conditions promoted by calm, warm weather, and 3) macrophyte senescence. Attempts to reduce algal biomass by controlling internal phosphorus loading have often been effective.

Primary production and light dynamics in an upper Mississippi River backwater

AbstractIn nutrient‐rich systems, phytoplankton production is frequently light‐limited and light attenuation can become a critical factor controlling the rates of production, especially in shallow systems subject to wind resuspension of sediments. This study examined the relation between light availability and primary production in Lake Onalaska, a shallow, nutrient‐rich impoundment of the Upper Mississippi River. Continuous water quality monitors recorded dissolved oxygen, temperature and light flux at a network of sites from July to September 1990 and these data were used to estimate the primary production, light availability and light attenuation coefficients. The gross primary production averaged 2·2gCm−2 day−1 and was due almost entirely to phytoplankton. Production was strongly light‐limited, with most of the light attenuation due to non‐algal components, possibly related to wind resuspension in this shallow system. Regression analyses revealed significant effects of wind speed and direction on light attenuation.

Analysis and Prognosis of Radiation Exposure Following the Accident at the Siberian Chemical Combine Tomsk-7

On April 6 1993 at 12.58 at the radiochemical plant of the Siberian Chemical Combinat (Tomsk-7), a set-up for treatment of the uranium solution prior to extraction was damaged and part of the activity was released into the environment. The release occurred through the collapsed wall and then standard ventilation system. The paper presents results obtained by Roshydromet divisions in assessing the magnitude of the accident and predicting its consequences. The meteorological data suggested that the most probable direction of the radioactive trail was north-east, towards the village of Georgievka. The preliminary sampling allowed the radionuclide composition of the release to be idenfified as 103 Ru, 106 Ru, 95 Zr, 95 Nb and 239 Pu. Based on the results of the aerial gamma survey conducted by specialists of Roshydromet and «Berezovgeologiya», an area of increased gamma background (> 10 μR/hour) was outlined. In addition, measurements were made across the radioactive trail using equipment on vehicles. The activity release was estimated on the basis of the theoretical relationship between gamma dose and isotopic activity. By calculations, 720 Ci of activity was released into the environment. The criterium of correctness was the condition of minimum deviation of the calculated gamma field from the measured one. For calculation of 239 Pu depositions, the relationship 239 Pu/ 95 Zr = (1-3.5) × 10 −3 was used. Based on data of depositions and hydrological predictions, estimates were made for possible contamination of the Samus-ka and Tom' rivers due to the surface run-off of radionuclides from the catchment area. For the village of Georgievka, respiration individual dose was calculated which may result from wind resuspension and uplift of radioactive dust containing 239 Pu. The paper also includes results of the actual aerial spectrometric gamma survey which confirm the preliminary estimates of the activity release