Environmental fate of fungicides and other current-use pesticides in a central California estuary.

The current study documents the fate of current-use pesticides in an agriculturally-dominated central California coastal estuary by focusing on the occurrence in water, sediment and tissue of resident aquatic organisms. Three fungicides (azoxystrobin, boscalid, and pyraclostrobin), one herbicide (propyzamide) and two organophosphate insecticides (chlorpyrifos and diazinon) were detected frequently. Dissolved pesticide concentrations in the estuary corresponded to the timing of application while bed sediment pesticide concentrations correlated with the distance from potential sources. Fungicides and insecticides were detected frequently in fish and invertebrates collected near the mouth of the estuary and the contaminant profiles differed from the sediment and water collected. This is the first study to document the occurrence of many current-use pesticides, including fungicides, in tissue. Limited information is available on the uptake, accumulation and effects of current-use pesticides on non-target organisms. Additional data are needed to understand the impacts of pesticides, especially in small agriculturally-dominated estuaries.

Pyrethroid and organophosphate pesticide-associated toxicity in two coastal watersheds (California, USA).

Portions of the Santa Maria River and Oso Flaco Creek watersheds in central California, USA, are listed as impaired under section 303(d) of the Clean Water Act and require development of total maximum daily load (TMDL) allocations. These listings are for general pesticide contamination, but are largely based on historic monitoring of sediment and fish tissue samples that showed contamination by organochlorine pesticides. Recent studies have shown that toxicity in these watersheds is caused by organophosphate pesticides (water and sediment) and pyrethroid pesticides (sediment). The present study was designed to provide information on the temporal and spatial variability of toxicity associated with these pesticides to better inform the TMDL process. Ten stations were sampled in four study areas, one with urban influences, and the remaining in agriculture production areas. Water toxicity was assessed with the water flea Ceriodaphnia dubia, and sediment toxicity was assessed with the amphipod Hyalella azteca. Stations in the lower Santa Maria River had the highest incidence of toxicity, followed by stations influenced by urban inputs. Toxicity identification evaluations and chemical analysis demonstrated that the majority of the observed water toxicity was attributed to organophosphate pesticides, particularly chlorpyrifos, and that sediment toxicity was caused by mixtures of pyrethroid pesticides. The results demonstrate that both agriculture and urban land uses are contributing toxic concentrations of these pesticides to adjacent watersheds, and regional water quality regulators are now using this information to develop management objectives.

Evaluation of methods to determine causes of sediment toxicity in San Diego Bay, California, USA.

Regulation of waterbodies impaired due to sediment toxicity may require development of Total Maximum Daily Load (TMDL) allocations to reduce chemicals of concern. A key step in this process is the identification of chemicals responsible for toxicity, and sediment toxicity identification evaluation procedures (TIEs) are the primary tools used to accomplish this. Several sites in San Diego Bay (CA, USA) are listed as impaired due to sediment toxicity associated with organic chemicals and metals, and due to degraded benthic macroinvertebrate communities. Sediment was collected from one of these sites, at the confluence of Switzer Creek in San Diego Harbor. The sediment was subjected to selected whole-sediment TIE treatments to evaluate the efficacy of these procedures for identifying the causes of toxicity at Switzer Creek. Toxicity was assessed using the estuarine amphipod Eohaustorius estuarius. The results indicated that toxicity of San Diego Bay sediment was likely partly due to mixtures of pyrethroid pesticides. These experiments showed that the effectiveness of the individual TIE procedures varied by treatment. Variability was mainly due to inconsistency between results of samples subjected to various Phase II TIE procedures, including chemical analyses of samples subjected to high-pressure liquid chromatography and direct analyses of acetone extractions of carbonaceous resin. The procedures require further refinement to ensure maximum sorption and complete elution and detection of sorbed chemicals. Despite these inconsistencies, the results indicate the utility of these procedures for identifying chemicals of concern in this system.

Direct measurement of free copper in serum or plasma ultrafiltrate.

Copper is an essential element and, under conditions of overload, a toxicant. A dramatic example of copper toxicity is Wilson disease (WD), a treatable but often fatal condition that is difficult to diagnose and monitor. A method for direct measurement of free copper concentrations in serum or plasma ultrafiltrate by inductively coupled mass spectrometry was developed and validated to assist with diagnosis and monitoring WD. The method was shown to be accurate (94%-109% recoveries), linear (0.5-800 microg/dL [0.08-126 micromol/L]), and precise (coefficient of variation, < 15% over the analytic measurement range). A reference interval (0-10 microg/dL [0-1.6 micromol/L]) was determined parametrically with 137 healthy adult (20-59 years) blood donors. No statistically significant difference was observed in males (n = 69) vs females (n = 68). Free copper concentrations for patients diagnosed with WD were at least 6-fold greater than the upper limit of the reference interval before treatment but fell within the reference interval after treatment.

Evaluation of phase II toxicity identification evaluation methods for freshwater whole sediment and interstitial water.

Phase I whole sediment toxicity identification evaluation (TIE) methods have been developed to characterize the cause of toxicity as organic chemicals, metals, or ammonia. In Phase II identification treatments, resins added to whole sediment to reduce toxicity caused by metals and organics can be separated and eluted much like solid-phase extraction (SPE) columns are eluted for interstitial water. In this study, formulated reference sediments spiked with toxic concentrations of copper, fluoranthene, and nonylphenol were subjected to whole sediment and interstitial water TIE treatments to evaluate Phase I and II TIE procedures for identifying the cause of toxicity to Hyalella azteca. Phase I TIE treatments consisted of adding adsorbent resins to whole sediment, and using SPE columns to remove spiked chemicals from interstitial water. Phase II treatments consisted of eluting resins and SPE columns and the preparation and testing of eluates for toxicity and chemistry. Whole sediment resins and SPE columns significantly reduced toxicity, and the eluates from all treatments contained toxic concentrations of the spiked chemical except for interstitial water fluoranthene. Toxic unit analysis based on median lethal concentrations (LC50s) allowed for the comparison of chemical concentrations among treatments, and demonstrated that the bioavailability of some chemicals was reduced in some samples and treatments. The concentration of fluoranthene in the resin eluate closely approximated the original interstitial water concentration, but the resin eluate concentrations of copper and nonylphenol were much higher than the original interstitial water concentrations. Phase II whole sediment TIE treatments provided complementary lines of evidence to the interstitial water TIE results.

Statewide investigation of the role of pyrethroid pesticides in sediment toxicity in California's urban waterways.

A statewide investigation of urban creek sediment toxicity was conducted in California in recognition of increased incidences of toxicity linked to pyrethroid pesticides. The goals were to examine the spatial occurrence and magnitude of sediment toxicity in California urban creeks, and to examine the role of pyrethroids in toxic urban creek sediment samples. After a preliminary screening of 90 sites, 30 creeks were sampled in eight geographical regions. Sediment toxicity was assessed using 10 day bioassays with the resident amphipod Hyalella azteca. Bioassays were conducted at two test temperatures of 23 degrees C and at 15 degrees C to provide evidence of the cause of toxicity, and to more accurately reflect ambient environmental temperatures. Twenty-five of 30 samples were toxic when tested at 23 degrees C, and all 30 samples were toxic when tested at 15 degrees C. The magnitude of toxicity increased in samples tested at 15 degrees C suggesting the influence of pyrethroids, which are more toxic at colder temperatures. Pyrethroids were present in all sediment samples and were the only compounds detected at concentrations toxic to H. azteca. Bifenthrin was the pyrethroid of greatest toxicological concern, occurring in all 30 samples at concentrations up to 219 ng/g. Pyrethroid contamination of urban creeks was most severe in the Los Angeles, Central Valley, and San Diego regions, respectively. However, pyrethroids were also linked to urban creek aquatic toxicity in all regions sampled, including the less urbanized areas of the North Coast and Lake Tahoe.

Relative sensitivities of toxicity test protocols with the amphipods Eohaustorius estuarius and Ampelisca abdita.

A series of dose-response experiments was conducted to compare the relative sensitivities of toxicity test protocols using the amphipods Ampelisca abdita and Eohaustorius estuarius. A. abdita is one of the dominant infaunal species in the San Francisco Estuary, and E. estuarius is the primary sediment toxicity species used in the San Francisco Estuary Regional Monitoring Program. Experiments were conducted with a formulated sediment spiked with copper, fluoranthene, chlorpyrifos, and the three pyrethroid pesticides permethrin, bifenthrin, and cypermethrin, all chemicals of concern in this Estuary. The results showed that the protocol with A. abdita was more sensitive to fluoranthene and much more sensitive to copper, while E. estuarius was more sensitive to chlorpyrifos, and much more sensitive to the pyrethroid pesticides. These results, considered in conjunction with those from previous spiking studies [Weston, D.P., 1995. Further development of a chronic Ampelisca abdita bioassay as an indicator of sediment toxicity: summary and conclusions. In: Regional Monitoring Program for Trace Substances Annual Report. San Francisco Estuary Institute, Oakland, CA, pp 108-115; DeWitt, T.E., Swartz, R.C., Lamberson, J.O., 1989. Measuring the acute toxicity of estuarine sediments. Environ. Toxicol. Chem. 8: 1035-1048; DeWitt, T.H.E., Pinza, M.R., Niewolny, L.A., Cullinan, V.I., Gruendell, B.D., 1997. Development and evaluation of a standard marine/estuarine chronic sediment toxicity method using Leptocheirus plumulosus. Draft report prepared for the US Environmental Protection Agency, Office of Science and Technology, Washington DC, under contract DE-AC06-76RLO 1830 by Batelle Marine Science Laboratory, Battelle Memorial Institute, Pacific Northwest Division, Richland, WA], suggest that, in general, A. abdita is more sensitive to metals, E. estuarius is more sensitive to pesticides, and both protocols have roughly comparable sensitivities to hydrocarbons. The preponderance of evidence from previous field studies indicate that E. estuarius is considerably more responsive to ambient sediment samples [Bay, S.M., Gries, T.H., Anderson, B.S., Phillips, B.M., Field, J.L., Moore, D.W., Greenstein, D.J., 2005. Comparison of marine amphipod test species responsiveness to contaminated sediments. In: Conference Proceeding: Annual Meeting of the Society of Environmental Toxicology and Chemistry. SETAC, Baltimore, Maryland; Anderson, B.S., Hunt, B.M., Thompson, B., Lowe, S., Taberski, K., Carr, R.S., in press. Patterns and trends in sediment toxicity in the San Francisco estuary. Environ. Res.]. One reason for this may be that tube building behavior of A. abdita isolates this species from contaminants in pore water, and results from the current experiments partially support this hypothesis.

Characterization of changes in therapeutic ultrasound transducer performance over time using the angular spectrum method.

Strongly focused large aperture transducers used in high-intensity focused ultrasound treatments are prone to manufacturing defects and degradation. Current methods for evaluating transducer quality measure only bulk physical changes of transducers. We have determined the pressure distribution at the transducer surface, using the angular spectrum method, to detect defects of the transducer. Three therapeutic transducers were investigated. The pressure distribution at the focal plane of each transducer was measured and input into a back-projection algorithm to calculate the pressure distribution at the transducer surface. A number of scan window sizes were used for the pressure distribution measurement at the focal plane to determine the effect on the resolution of the calculated pressure distribution at the transducer surface. Results showed that one transducer might have suffered manufacturing defects. The second transducer degraded over 1 year of use with one half of the transducer suffering a partial loss of efficiency. The third transducer remained unchanged over 1 year. The scan window of 40 mm X 40 mm at the focal plane was required to identify defects 6 mm in diameter on the transducer surface. The results demonstrate that the angular spectrum method could be a useful tool for evaluating transducer quality.

Causes of water toxicity to Hyalella azteca in the New River, California, USA.

The New River (CA, USA) was created in 1905 to 1907 when the Colorado River washed out diversionary works and flowed into the Salton Basin, creating the Salton Sea. Approximately 70% of the river's current flow is agricultural wastewater from the Imperial Valley. The river is contaminated with pesticides, industrial organic chemicals, metals, nutrients, bacteria, and silt. Monitoring for the State of California Surface Water Ambient Monitoring Program has indicated persistent water column toxicity to the epibenthic amphipod Hyalella azteca. Four toxicity identification evaluations (TIEs), along with chemical analyses, were performed, and the results indicated multiple and varying causes of toxicity. The first two TIEs characterized the causes of toxicity as a combination of metals and organics, but only the second sample contained enough total copper to contribute to toxicity. The third TIE used an emerging method for characterizing and identifying toxicity caused by pyrethroid pesticides. This TIE characterized organics as the cause of toxicity, and a carboxylesterase enzyme treatment further identified the cause of toxicity as pyrethroids. The final TIE used the enzyme and Phase II procedures to identify cypermethrin as the cause of toxicity. The TIE results demonstrate the evolving causes of toxicity in the New River and should assist regulators with implementing the total maximum daily load process for pesticides, particularly pyrethroids. Further research will determine if pyrethroids and other New River contaminants are having an impact on the Salton Sea.

Spatial relationships between water quality and pesticide application rates in agricultural watersheds.

Pesticide applications to agricultural lands in California, USA, are reported to a central data base, while data on water and sediment quality are collected by a number of monitoring programs. Data from both sources are geo-referenced, allowing spatial analysis of relationships between pesticide application rates and the chemical and biological condition of water bodies. This study collected data from 12 watersheds, selected to represent a range of pesticide usage. Water quality parameters were measured during six surveys of stream sites receiving runoff from the selected watershed areas. This study had three objectives: to evaluate the usefulness of pesticide application data in selecting regional monitoring sites, to provide information for generating and testing hypotheses about pesticide fate and effects, and to determine whether in-stream nitrate concentration was a useful surrogate indicator for regional monitoring of toxic substances. Significant correlations were observed between pesticide application rates and in-stream pesticide concentrations (p < 0.05) and toxicity (p < 0.10). In-stream nitrate concentrations were not significantly correlated with either the amount of pesticides applied, in-stream pesticide concentrations, or in-stream toxicity (all p > 0.30). Neither total watershed area nor the area in which pesticide usage was reported correlated significantly with the amount of pesticides applied, in-stream pesticide concentrations, or in-stream toxicity (all p > 0.14). In-stream pesticide concentrations and effects were more closely related to the intensity of pesticide use than to the area under cultivation.

Solid-phase sediment toxicity identification evaluation in an agricultural stream.

The lower Santa Maria River watershed provides important aquatic habitat on the central California coast and is influenced heavily by agricultural runoff. As part of a recently completed water quality assessment, we conducted a series of water column and sediment toxicity tests throughout this watershed. Sediment from Orcutt Creek, a tributary that drains agricultural land, consistently was toxic to the amphipod Hyalella azteca, which is a resident genus in this river. Toxicity identification evaluations (TIEs) were conducted to determine cause(s) of toxicity. We observed no toxicity in sediment interstitial water even though concentrations of chlorpyrifos exceeded published aqueous toxicity thresholds for H. azteca. In contrast to interstitial water, bulk sediment was toxic to H. azteca. In bulk-phase sediment TIEs, the addition of 20% (by volume) coconut charcoal increased survival by 41%, implicating organic chemical(s). Addition of 5% (by volume) of the carbonaceous resin Ambersorb 563 increased survival by 88%, again suggesting toxicity due to organic chemicals. Toxicity was confirmed by isolating Ambersorb from the sediment, eluting the resin with methanol, and observing significant toxicity in control water spiked with the methanol eluate. A carboxylesterase enzyme that hydrolyzes synthetic pyrethroids was added to overlying water, and this significantly reduced toxicity to amphipods. Although the pesticides chlorpyrifos, DDT, permethrin, esfenvalerate, and fenvalerate were detected in this sediment, and their concentrations were below published toxicity thresholds for H. azteca, additivity or synergism may have occurred. The weight-of-evidence suggests toxicity of this sediment was caused by an organic contaminant, most likely a synthetic pyrethroid.

Evidence of pesticide impacts in the Santa Maria River watershed, California, USA.

The Santa Maria River provides significant freshwater and coastal habitat in a semiarid region of central California, USA. We conducted a water and sediment quality assessment consisting of chemical analyses, toxicity tests, toxicity identification evaluations, and macroinvertebrate bioassessments of samples from six stations collected during four surveys conducted between July 2002 and May 2003. Santa Maria River water samples collected downstream of Orcutt Creek (Santa Maria, Santa Barbara County, CA, USA), which conveys agriculture drain water, were acutely toxic to cladocera (Ceriodaphnia dubia), as were samples from Orcutt Creek. Toxicity identification evaluations (TIEs) suggested that toxicity to C. dubia in Orcutt Creek and the Santa Maria River was due to chlorpyrifos. Sediments from these two stations also were acutely toxic to the amphipod Hyalella azteca, a resident invertebrate. The TIEs conducted on sediment suggested that toxicity to amphipods, in part, was due to organophosphate pesticides. Concentrations of chlorpyrifos in pore water sometimes exceeded the 10-d median lethal concentration for H. azteca. Additional TIE and chemical evidence suggested sediment toxicity also partly could be due to pyrethroid pesticides. Relative to an upstream reference station, macroinvertebrate community structure was impacted in Orcutt Creek and in the Santa Maria River downstream of the Creek input. This study suggests that pesticide pollution likely is the cause of ecological damage in the Santa Maria River.

High-frequency ultrasound for monitoring changes in liver tissue during preservation.

Currently the only method to assess liver preservation injury is based on liver appearance and donor medical history. Previous work has shown that high-frequency ultrasound could detect ischemic cell death due to changes in cell morphology. In this study, we use high-frequency ultrasound integrated backscatter to assess liver damage in experimental models of liver ischemia. Ultimately, our goal is to predict organ suitability for transplantation using high-frequency imaging and spectral analysis techniques. To examine the effects of liver ischemia at different temperatures, livers from Wistar rats were surgically excised, immersed in phosphate buffer saline and stored at 4 and 20 degrees C for 24 h. To mimic organ preservation, livers were excised, flushed with University of Wisconsin (UW) solution and stored at 4 degrees C for 24 h. Preservation injury was simulated by either not flushing livers with UW solution or, before scanning, allowing livers to reach room temperature. Ultrasound images and corresponding radiofrequency data were collected over the ischemic period. No significant increase in integrated backscatter (approximately 2.5 dBr) was measured for the livers prepared using standard preservation conditions. For all other ischemia models, the integrated backscatter increased by 4-9 dBr demonstrating kinetics dependent on storage conditions. The results provide a possible framework for using high-frequency imaging to non-invasively assess liver preservation injury.

Comparison of sample preservation methods for clinical trace element analysis by inductively coupled plasma mass spectrometry.

The effects of chemical additives and storage temperatures on measurement of 16 trace elements in urine by inductively coupled plasma mass spectrometry (ICP-MS) were evaluated. A 24-hour urine specimen was supplemented with concentrations of the elements. Aliquots containing 1 of 4 chemical additives were stored at 3 different temperatures in sealed polypropylene containers. Elemental concentrations were determined by ICP-MS for the resulting samples after 1, 2, 8, and 65 days of storage. Initial element concentrations measured within 8 hours of specimen preparation were consistent with expected concentrations (except for aluminum). For most elements, preservation and storage conditions yielded consistent measured concentrations throughout the experiment. Notable exceptions were for aluminum (general rise over time) and mercury (general decrease over time). Adding boric acid and potassium pyrosulfate resulted in sample contamination; elemental contamination was concentration-dependent for both. Although little microbial contamination was observed during the experiment, refrigeration of samples is recommended to curtail bacterial growth in nonsterile specimens. In light of these results, refrigerated urine storage without the use of chemical additives is an effective preservation method for ICP-MS analysis of many trace elements.

In situ water and sediment toxicity in an agricultural watershed.

The Salinas River receives inputs from extensive farmlands before flowing into the Salinas River National Wildlife Refuge and the Monterey Bay National Marine Sanctuary (CA, USA). Previous monitoring using laboratory toxicity tests and chemical analyses identified toxic agricultural drain-water inputs in this system. Using caged daphnids (Ceriodaphnia dubia) and amphipods (Hyalella azteca), we investigated in situ toxicity at stations downstream from an agricultural drain relative to a reference station. A flow sensor indicated highly variable inputs from irrigation, and daily synoptic chemical analyses using enzyme-linked immunosorbent assay techniques demonstrated fluctuating concentrations of organophosphate pesticides. Test organism mortality in the field coincided with contaminant concentrations that exceeded chemical effect thresholds for the test species. Laboratory toxicity tests using C. dubia were comparable to results from field exposures, but tests with H. azteca were not. Laboratory exposures can be reasonable surrogates for field evaluations in this system, but they were less effective for assessing short-term temporal variability. Results from the field toxicity studies corroborated results of bioassessment surveys conducted as part of a concurrent study. Toxicity identification evaluations indicated that organophosphate pesticides caused toxicity to daphnids and that effects of suspended solids were negligible.

Ecotoxicologic impacts of agricultural drain water in the Salinas River, California, USA.

The Salinas River is the largest of the three rivers that drain into the Monterey Bay National Marine Sanctuary in central California (USA). Large areas of this watershed are cultivated year-round in row crops, and previous laboratory studies have demonstrated that acute toxicity of agricultural drain water to Ceriodaphnia dubia is caused by the organophosphate (OP) pesticides chlorpyrifos and diazinon. We investigated chemical contamination and toxicity in waters and sediments in the river downstream of an agricultural drain water input. Ecological impacts of drain water were investigated by using bioassessments of macroinvertebrate community structure. Toxicity identification evaluations were used to characterize chemicals responsible for toxicity. Salinas River water downstream of the agricultural drain was acutely toxic to the cladoceran Ceriodaphnia dubia, and toxicity to C. dubia was highly correlated with combined toxic units (TUs) of chlorpyrifos and diazinon. Laboratory tests were used to demonstrate that sediments in this system were acutely toxic to the amphipod Hyalella azteca, a resident invertebrate. Toxicity identification evaluations (TIEs) conducted on sediment pore water suggested that toxicity to amphipods was due in part to OP pesticides; concentrations of chlorpyrifos in pore water sometimes exceeded the 10-d mean lethal concentration (LC50) for H. azteca. Potentiation of toxicity with addition of the metabolic inhibitor piperonyl butoxide suggested that sediment toxicity also was due to other non-metabolically activated compounds. Macroinvertebrate community structure was highly impacted downstream of the agricultural drain input, and a number of macroinvertebrate community metrics were negatively correlated with combined TUs of chlorpyrifos and diazinon, as well as turbidity associated with the drain water. Some macroinvertebrate metrics were also correlated with bank vegetation cover. This study suggests that pesticide pollution is the likely cause of ecological damage in the Salinas River, and this factor may interact with other stressors associated with agricultural drain water to impact the macroinvertebrate community in the system.

Ambient toxicity due to chlorpyrifos and diazinon in a central California coastal watershed.

The Salinas River watershed along the central coast of California, U.S.A., supports rapidly growing urban areas and intensive agricultural operations. The river drains to an estuarine National Wildlife Refuge and a National Marine Sanctuary. The occurrence, spatial patterns, sources and causes of aquatic toxicity in the watershed were investigated by sampling four sites in the main river and four sites in representative tributaries during 15 surveys between September 1998 and January 2000. In 96 hr toxicity tests, significant Ceriodaphnia dubia mortality was observed in 11% of the main river samples, 87% of the samples from a channel draining an urban/agricultural watershed, 13% of the samples from channels conveying agricultural tile drain runoff, and in 100% of the samples from a channel conveying agricultural surface furrow runoff. In six of nine toxicity identification evaluations (TIEs), the organophosphate pesticides diazinon and/or chlorpyrifos were implicated as causes of observed toxicity, and these compounds were the most probable causes of toxicity in two of the other three TIEs. Every sample collected in the watershed that exhibited greater than 50% C. dubia mortality (n = 31) had sufficient diazinon and/or chlorpyrifos concentrations to account for the observed effects. Results are interpreted with respect to potential effects on other ecologically important species.

Acutei and chronic toxicity of nickel to marine organisms: implications for water quality criteria.

Acute and chronic toxicity tests were conducted to determine the effects of nickel on three U.S. west coast marine species: a fish (the topsmelt, Atherinops affinis), a mollusk (the red abalone, Haliotis rufescens), and a crustacean (the mysid, Mysidopsis intii). The 96-h median lethal concentration (LC50) for topsmelt was 26,560 microg/L, and the chronic value for the most sensitive endpoint in a 40-d exposure was 4,270 microg/L. The median effective concentration (EC50) for 48-h abalone larval development was 145.5 microg/L, and the chronic value for juvenile growth in a 22-d exposure through larval metamorphosis was 26.43 microgAL. The mysid 96-h LC50 was 148.6 microg/L, and the chronic value for the most sensitive endpoint in a 28-d, whole life-cycle exposure was 22.09 microg/L. The abalone and mysid acute values were lower than other values available in the literature. Acute-to-chronic ratios for nickel toxicity to the three species were 6.220, 5.505, and 6.727, respectively, which were similar to the only other available saltwater value of 5.478 (for Americamysis [Mysidopsis] bahia) and significantly lower than the existing values of 35.58 and 29.86 for freshwater organisms. Incorporation of data from the present study into calculations for water quality criteria would lower the criterion maximum concentration and raise the criterion continuous concentration for nickel.

A model based upon pseudo regular spacing of cells combined with the randomisation of the nuclei can explain the significant changes in high-frequency ultrasound signals during apoptosis.

Recent ultrasound (US) experiments on packed myeloid leukaemia cells have shown that, at frequencies from 32 to 40 MHz, significant increases of signal amplitude were observed during apoptosis. This paper is an attempt to explain these signal increases based upon a simulation of the backscattered signals from the cells nuclei. The simulation is an expansion of work in which a condensed sample of cells, with fairly regular sizes, could be considered as an imperfect crystal. Thus, destructive interference could occur and this would be observed as a large reduced value of backscattered signals compared with the values obtained from a similar, but random, scattering source. This current paper explores the possibility that simple changes in the nuclei, such as their observed condensation or the small loss of nuclei scatterers from cells, could cause a significant increase in the observed backscattered signals. This model indicates that the greater backscattered signals can be explained by further randomisation of the average positions of the scattering sources in each cell. When these "microechoes" are added together, so that the destructive interference is reduced, a large increase in the signal is predicted. The simplified model strongly suggests that much of observed large increases of the backscattered signals could be simply explained by the randomisation of the position of the condensed nuclei during apoptosis, and the destruction of the nuclei could produce further signal amplitude changes due to disruption of the cloud of backscattered waves.