Assessing annual, seasonal and spatial trends in copper sediment concentrations from a California agricultural waterbody.

The objective of this study was to assess annual, seasonal and spatial trends in copper sediment concentrations in an agricultural California stream (Cache Slough) based on sampling 12 sites over two seasons (spring and fall) in 2012, 2013 and 2014. The relationship between copper sediment concentrations and precipitation was also evaluated. The results from this study showed that copper sediment concentrations from Cache Slough ranging from 19 to 63 ug/g dw were similar to concentrations reported in European agricultural streams. Copper sediment concentrations were not reported to increase over time in this agricultural waterbody where copper was used as a fungicide. However, spatial differences in copper sediment concentrations were reported among sites with the highest concentrations in the middle waterbody section. Seasonal analysis showed no significant difference in copper sediment concentrations for both spring and fall for 2012 and 2013 but 2014 spring concentrations were statistically higher than fall concentrations when precipitation was lower than the two previous years. There were no statistically significant relationships between copper sediment concentrations and precipitation for the three-year period based on an analysis by year and season.

Long term bioassessment multiple stressors study in a residential California stream.

The relationship of various benthic metrics to physical habitat metrics, pyrethroids, metals and sediment parameters was evaluated for a 10 year data set in Pleasant Grove Creek (Roseville, California) using univariate linear models, stepwise multiple regressions, and canonical correlation analysis. In general, total physical habitat scores in this residential stream were considered to be marginal to suboptimal. The most dominant benthic taxa were generally considered to be tolerant of environmental stressors and the benthic communities were rated as impaired based on a benthic index. Potentially toxic sediment concentrations of cadmium, chromium, copper, nickel and zinc were reported at various sites based on a comparison with existing threshold effect levels. The sum of pyrethroid Toxics Units (TUs) indicated that 10 of 21 sites based on a sensitive Hyalella laboratory toxicity test had TUs greater than one thus suggesting toxicity at various sites. In summary, the effects of the physical habitat, as reflected by certain habitat metrics that were indicative of stream-flow, hydrology, habitat diversity, and substrate quality overshadowed any apparent effects of pyrethroids and metals on shaping resident benthic communities when all environmental variables were considered in multivariate analyses.

A Comparison of Different Statistical Methods for Addressing Censored Left Data in Temporal Trends Analysis of Pyrethroids in a California Stream.

This study compared four different statistical methods, involving six estimation procedures, for addressing censored left data in measuring temporal trends of eight different pyrethroids measured in sediment from a 10-year data set in a residential California stream (Pleasant Grove Creek). The statistical methods used were: the Kaplan-Meier (km) method; the robust regression on order statistics (ros using normal and log normal distributions rosln); the maximum likelihood estimation (mlen using normal and log normal distributions mleln); and a substitution method (sub) using ½ the detection limit. For five of the eight pyrethroids (bifenthrin, cyfluthrin, cypermethrin, lambda-cyhalothrin, and permethrin), the six statistical methods generally agree, with one exception, that the data set exhibit significant declining trends. In the case of bifenthrin, the slight disagreement among statistical methods only occurred for the mleln estimate that did not show a significant declining trend, whereas the other five methods did. For deltamethrin, esfenvalerate, and fenpropathrin, all six statistical methods were in agreement showing no significant trends. Possible reasons for declining sediment concentrations of pyrethroids in Pleasant Grove Creek are urban label changes effective in 2012-2015 that reduced residential use, variable annual rainfall, and more responsible homeowner use based on outreach/education programs.

Bootstrap simulations to estimate relationships between Type I error, power, effect size, and appropriate sample numbers for bioassessments of aquatic ecosystems.

The Extended Bootstrap (EB) assessment approach was developed for the examination of relationships of Type I error, power, sample size (n), and effect size (ES) for statistical tests of ecological data. The EB approach was applied to univariate and multivariate statistical analyses of a large data set collected from an ongoing, multiple stressor bioassessment study of watersheds in the Central Valley, San Francisco, and Central Coast areas of California. Benthic metrics were created that either increased or decreased monotonically with stress (toxicants or metrics indicative of habitat quality). Type I errors were stable for all statistical tests that were evaluated. The relationships between n and ES displayed patterns of "diminishing returns" for all statistical tests: i.e. an increasingly larger n was required to detect decreasingly smaller ES. Nonetheless, the n's collected across the watersheds and within a selected watershed were sufficient to detect even small correlations between representative benthic metrics and potential stressors with high power. The power and robustness of a novel method using EB and previously described statistical techniques designed to address multicollinearity were shown to approach those of simpler univariate regressions. Potential applications of the EB approach for experimental design, data assessment and interpretation, and hypothesis testing are discussed.

Multivariate spatial patterns analysis of environmental variables and benthic metrics in five California waterbodies.

Multivariate spatial patterns of 38 environmental variables (habitat metrics, metals, pyrethroids and sediment characteristics) and 14 benthic metrics were determined from over a decade of sampling in five California waterbodies. Canonical discriminant analyses produced very highly significant separations of the five watersheds based on environmental variables. The discriminant analyses based on benthic metrics also produced highly significant separations, although confidence ellipses were not quite as well separated as the environmental variables. Separation of creeks made ecological sense as Pleasant Grove Creek (PGC), which appeared to have greater habitat quality also had benthic metrics indicative of less stressed benthic communities, while the Santa Maria River watershed (SM) appeared to have lower habitat quality and more toxicants in the sediments also appeared to have benthic communities indicative of stress-tolerant taxa. The benthic community health of the other three creeks (Arcade Creek [ARC], Kirker Creek [KC] and Salinas Streams [SAL]) were considered intermediate in apparent stress between PGC and SM. Overall, benthic communities were somewhat more similar between creeks than were the environmental conditions. Benthic communities have greater overlap in characteristics than would be expected from environmental conditions, perhaps due to a greater resilience since they are generally dominated by tolerant benthic taxa.

Ranking the importance of benthic metrics and environmental stressors from over a decade of bioassessment multiple stressor studies in five California waterbodies.

Comprehensive bioassessment multiple stressor field studies were conducted in five waterbodies in the Central Valley, San Francisco and Central Coast areas of California between 2006 and 2017. Samples were collected over multiple years in Pleasant Grove Creek (PGC - 8 years), Kirker Creek (KC - 2 years), Arcade Creek (ARC - 3 years), Salinas streams (SAL - 3 years) and the lower Santa Maria River watershed (SM - 3 years). In each watershed, metrics indicative of physical habitat quality and the relative health of benthic communities were collected, along with measurements of various potential toxicants in sediment (metals and pyrethroids), total organic carbon (TOC) and sediment grain size characteristics. For all waterbodies, pyrethroids have been reported at concentrations suspected to be toxic based on single-species laboratory toxicity tests. The data from all of these studies were assembled into a single data base which was analyzed by a series of univariate and multivariate statistical analyses. These analyses were designed to examine overall relationships between benthic community health and environmental quality conditions across the watersheds, as well as to address the following two key questions: (1) Which benthic metrics (response indicators) were the most important to relationships with environmental quality conditions and how do these metrics rank from most to least important? and (2) Which environmental variables (stressors) were the most important in terms of relationships to benthic metrics indicative of community health and how do the stressors rank from most to least important? Consensus ranking of 14 benthic metrics showed that stress tolerant (% Tolerant taxa, Tolerance value and Collectors/gatherers) and stress sensitive (EPT taxa and taxa richness) were the most important. The consensus ranking of 38 environmental variables from most to least important showed that habitat metrics, metals and grain size characteristics were most important for shaping benthic communities while pyrethroids were not reported to be an important stressor across the geographic scope of the watersheds.

Spatiotemporal trend analysis of metal concentrations in sediments of a residential California stream with toxicity and regulatory implications.

The objective of this study was to determine if concentrations of arsenic, cadmium, chromium, copper, lead, nickel and zinc measured in the sediments of a residential stream in California (Pleasant Grove Creek) have changed temporally or spatially from 2006 to 2016. Threshold Effect Levels (TELs), conservative ecological effects benchmarks, and exceedances for the seven metals were also evaluated over the 11-year time period to provide insight into potential metal toxicity to resident benthic communities. In addition, the bioavailability of metals in sediments was also determined by calculating Simultaneous Extracted Metal/Acid Volatle Sulfide (SEM/AVS) ratios to allow an additional assessment of toxicity. Regulatory implications of this data set and the role of metal toxicity are also discussed. Stream-wide temporal trend analysis showed no statistically significant trends for any of the metals. However, spatial analysis for several sites located near storm drains did show a significant increase for most metals over the 11-year period. TEL exceedances during the 7 years of sampling, spanning 2006-2016, were reported for all metals with the number of exceedances ranging from 47 for copper and zinc to 1 for lead. A spatial analysis showed that the highest number of TEL exceedances and the highest number of SEM/AVS ratios greater than one with at least one metal exceeding a TEL occurred at upstream sites. The potentially toxic metal concentrations reported in Pleasant Grove Creek should be used in the 303 (d) listing process for impaired water bodies in California.

The influence of multiple chemical and non-chemical stressors on benthic communities in a mid-west agricultural stream.

The objective of this 3-year study was to characterize benthic communities and physical habitat in an agricultural stream in the mid-west area of the United States (Big Bureau Creek, Illinois). Concurrent basic water quality parameters and seven nutrients were measured in the water column. Sediment measurements from depositional areas were conducted for bifenthrin, Total Organic Carbon, grain size, polychlorinated biphenyls (PCBs) and eight metals. All parameters were measured at 12 sites annually during the late summer for a 3-year period (2014, 2015 and 2016). Univariate regressions, stepwise multiple regressions and canonical correlation statistical analyses were used to determine the relationship between various benthic metrics (i.e., taxa richness and abundance) and all the measured parameters for the 3-year database. Benthic communities comprising 108-110 taxa were collected annually, and were generally dominated by sensitive caddisflies and mayflies. These communities were rated as good to exceptional using the Ohio Invertebrate Community Index. Physical habitat for the various sites was rated as good using the Ohio Qualitative Habitat Evaluation Index, thus suggesting that habitat is not a significant stressor that would likely impact resident benthic communities. Based on a comparison of measured in-stream total nitrogen and total phosphorus concentrations and criterion value exceedances, it appears that the in-stream nutrient concentrations could be potentially stressful to resident benthic biota. Metal concentrations were below established NOAA Threshold Effects Levels at all sites. Measured PCB concentrations were below levels of detection at all sites. Toxic units' (TUs) calculations based on using sensitive laboratory strains of Hyalella were less than 0.1 for bifenthrin, thus suggesting that bifenthrin sediment toxicity was unlikely. Thirty significant relationships reported between benthic metrics and the various environmental variables based on the 3-year database were as follows: 12 relationships with habitat metrics, 8 relationships with metals, 7 relationships with nutrients, 2 relationships with bifenthrin and 1 relationship with sediment characteristics. Relative habitat quality, habitat preferences, and ecological competition appear to be factors that shape the characteristics of the benthic communities of Big Bureau Creek, rather than stress imposed by toxicants (bifenthrin and metals), nutrients, or sediment characteristics. Complex relationships between benthic metrics and environmental variables are discussed.

Spatiotemporal Trends Analysis of Pyrethroid Sediment Concentrations Spanning 10 Years in a Residential Creek in California.

The objective of this study was to assess temporal and spatial trends for eight pyrethroids monitored in sediment spanning 10 years from 2006 to 2015 in a residential stream in California (Pleasant Grove Creek). The timeframe for this study included sampling 3 years during a somewhat normal non-drought period (2006-2008) and 3 years during a severe drought period (2013-2015). Regression analysis of pyrethroid concentrations in Pleasant Grove Creek for 2006, 2007, 2008, 2012, 2013, 2014, and 2015 using ½ the detection limit for nondetected concentrations showed statistically significant declining trends for cyfluthrin, cypermethrin, deltamethrin, permethrin, and total pyrethoids. Additional trends analysis of the Pleasant Grove Creek pyrethroid data using only measured concentrations, without nondetected values, showed similar statistically significant declining trends for cyfluthrin, cypermethrin, deltamethrin, esfenvalerate, fenpropathrin, permethrin, and total pyrethroids. Spatial trends analysis for the specific creek sites showed that six of the eight pyrethroids had a greater number of sites with statistically significant declining concentrations. Possible reasons for reduced pyrethroid concentrations in the stream bed in Pleasant Grove Creek during this 10-year period are label changes in 2012 that reduced residential use and lack of precipitation during the later severe drought years of 2013-2015.

The relationship of benthic community metrics to pyrethroids, metals, and sediment characteristics in Cache Slough, California.

A bioassessment multiple stressor study was conducted at 12 sites in Cache Slough, California during the Spring and Fall of 2012, 2013 and 2014. Specific study goals were to: (1) collect and identify benthic macroinvertebrates and develop a suite of benthic metrics; (2) measure total organic carbon (TOC), grain size, bulk metals, simultaneously extracted metals (SEM) and acid volatile sulfides (AVS), and 8 pyrethroids in sediment; (3) measure basic water quality parameters; and, (4) use univariate and stepwise multiple regressions and canonical correlation analysis to determine the relationship between various benthic metrics (i.e., taxa richness, abundance) and TOC, grain size, metals (bulk metals and SEM/AVS) and pyrethroids using the 3-year database. Five benthic metrics showed statistically significant relationships with environmental variables. Taxonomic Richness, a metric that decreases with stress, tended to be greater in less organic-rich, coarser sediments and the apparent relationships with toxicants such as pyrethroids or metals were diminished when these conditions are taken into account. The % Amphipod metric, which has a variable response to environmental stressors, showed a direct relationship with arsenic, an inverse relationship with chromium and an inverse relationship with % silt. The % Corbicula metric, which has a variable response to environmental stressors, was directly correlated with cypermethrin and nickel. Therefore, it appears that Corbicula tends to be associated with more contaminated sediments in Cache Slough. The metric % Collectors/Filterers & Collectors/Gatherers, a metric that increases in stressed environments, was reported to increase in sediments with higher arsenic concentrations. This relationship makes ecological sense because this metric should increase with an increase in arsenic concentrations. The benthic metric Abundance, a metric that decreases with stress, was reported to be inversely correlated with % TOC and % silt. The abundance of benthic communities increases in coarser, less organic rich sediments. In general, relatively few statistically significant relationships were observed between the various combinations of benthic metrics and environmental variables. Benthic communities in Cache Slough appear to be more closely associated with sediment characteristics and with metals concentrations than with the pyrethroid concentrations.

Temporal trends analysis of 2004 to 2012 toxicity and pesticide data for California's Central Valley water quality coalitions.

This study was designed to assess temporal trends of 2004-2012 toxicity data from three water column tests, one sediment test and concentrations of three pesticides from four Central Valley Water Quality Coalitions in California. Major conclusions based on this extensive toxicity and pesticide data from 150 mainstem and tributary sites showed a significant decline in toxicity over 9 years from Ceriodaphnia dubia water column toxicity tests and a significant decline in toxicity from three water column and one sediment toxicity test combined. The organophosphate insecticides diazinon and chlorpyrifos also showed a significant decline in concentrations from 2004 to 2012 and also a significant decline in the percent of samples that exceeded water quality objectives for both of these insecticides. The analysis conducted did not show any cases of either increased toxicity or pesticide concentrations over the 9-year period. The final conclusion from this analysis is that water quality conditions in the Central Valley Region of California, as measured by the toxicity and pesticide data from this study, have improved from 2004 to 2012.

Assessment of periphyton, aquatic macrophytes, benthic communities, and physical habitat in midwestern United States streams coinciding with varying historical concentrations of atrazine.

The objectives of this pilot study were to: (1) characterize periphyton and benthic communities using standard collection methods in six Midwest watersheds with varying historical levels of atrazine (low range, medium range and upper range); (2) qualitatively assess presence of aquatic vascular plants at each site; (3) assess and compare physical habitat at each study site in order to evaluate how physical habitat structure may influence the biological communities and (4) analyze the periphyton and benthic macroinvertebrate community data (i.e., series of metrics) among sites to evaluate possible differences or similarities among sites with different historical atrazine exposures. Five of the eight physical habitat metrics (including total physical habitat score) were different among the six study sites. There appeared to be no substantial difference in the structure of periphtyon communities at the six Midwest sites based on 9 of 12 metrics. For the three metrics that showed differences among sites-percentage of sensitive diatoms, percent Achnanches minutissima and percent motile diatoms - there was no consistent pattern with previous degrees of atrazine exposure and the scoring of these metrics. There were also no statistical differences in aquatic macrophyte spatial coverage among the six study areas. Thus, based on the spatially and temporally limited periphyton and aquatic macrophyte data, varying historical atrazine exposure was not associated with impact on resident plant communities (the target receptor group for atrazine). All 10 benthic community metrics showed significant differences among the six Midwest sites. Although no consistent pattern existed with varying historic levels of atrazine, benthic communities at one site with lower historical levels of atrazine were of higher quality than the other five sites. However, this one site also had a higher quality habitat compared to the other sites which was most likely the reason for this benthic condition.

Relationship of bifenthrin sediment concentrations to grain size and total organic carbon in California waterbodies: implications for ecological risk.

A summary analysis of data sets from six California waterbodies was conducted to determine the relationship of bifenthrin sediment concentrations to: % sand/gravel; % silt; % clay; % silt/clay and % total organic carbon (TOC). The relationship of TOC to % sand/gravel, % silt, % clay, and % silt/clay was also analyzed. Statistically significant and meaningful direct relationships were reported between bifenthrin and % TOC, % silt, % clay and % silt/clay while a significant and meaningful inverse relationship was reported between bifenthrin and % sand/gravel. A significant and meaningful inverse relationship was reported between % TOC and % sand/gravel, while a significant and meaningful direct relationship was reported between % TOC and % silt, % clay and % silt/clay. Significant bifenthrin sediment concentrations would not be expected in non-depositional (sand/gravel) areas which have been reported to be dominant in various streams in California's Central Valley and are also the preferred habitat for many benthic macroinvertebrate taxa.

Relationship between bifenthrin sediment toxic units and benthic community metrics in urban California streams.

The objective of this study was to use ecologically relevant field measurements for determining the relationship between bifenthrin sediment toxic units (TUs) (environmental concentrations/Hyalella acute LC50 value) and 15 benthic metrics in four urban California streams sampled from 2006 to 2011. Data from the following four California streams were used in the analysis: Kirker Creek (2006, 2007), Pleasant Grove Creek (2006, 2007, and 2008), Arcade Creek (2009, 2010, and 2011), and Salinas streams (2009, 2010, and 2011). The results from univariate analysis of benthic metrics versus bifenthrin TU calculations for the four California streams with multiple-year datasets combined by stream showed that there were either nonsignificant relationships or lack of metric data for 93 % of cases. For 7 % of the data (4 cases) where significant relationships were reported between benthic metrics and bifenthrin TUs, these relationships were ecologically meaningful. Three of these significant direct relationships were an expression of tolerant benthic taxa (either % tolerant taxa or tolerance values, which are similar metrics), which would be expected to increase in a stressed environment. These direct significant tolerance relationships were reported for Kirker Creek, Pleasant Grove Creek, and Arcade Creek. The fourth significant relationship was an inverse relationship between taxa richness and bifenthrin TUs for the 3-year Pleasant Grove Creek dataset. In summary, only a small percent of the benthic metric × bifenthrin TU relationships were significant for the four California streams. Therefore, the general summary conclusion from this analysis is that there is no strong case for showing consistent meaningful relationships between various benthic metrics used to characterize the status of benthic communities and bifenthrin TUs for these four California streams.

The relationship of metals, bifenthrin, physical habitat metrics, grain size, total organic carbon, dissolved oxygen and conductivity to Hyalella sp. abundance in urban California streams.

The objectives of this study were to determine the relationship between Hyalella sp. abundance in four urban California streams and the following parameters: (1) 8 bulk metals (As, Cd, Cr, Cu, Pb, Hg, Ni, and Zn) and their associated sediment Threshold Effect Levels (TELs); (2) bifenthrin sediment concentrations; (3) 10 habitat metrics and total score; (4) grain size (% sand, silt and clay); (5) Total Organic Carbon (TOC); (6) dissolved oxygen; and (7) conductivity. California stream data used for this study were collected from Kirker Creek (2006 and 2007), Pleasant Grove Creek (2006, 2007 and 2008), Salinas streams (2009 and 2010) and Arcade Creek (2009 and 2010). Hyalella abundance in the four California streams generally declined when metals concentrations were elevated beyond the TELs. There was also a statistically significant negative relationship between Hyalella abundance and % silt for these 4 California streams as Hyalella were generally not present in silt areas. No statistically significant relationships were reported between Hyalella abundance and metals concentrations, bifenthrin concentrations, habitat metrics, % sand, % clay, TOC, dissolved oxygen and conductivity. The results from this study highlight the complexity of assessing which factors are responsible for determining the abundance of amphipods, such as Hyalella sp., in the natural environment.

Mapping of depositional and non-depositional areas in Salinas, California streams with concurrent pyrethroid and benthic macroinvertebrate assessments.

This study used sediment mapping to determine the spatial extent of depositional and non-depositional areas in the wetted stream bed of four urban streams in Salinas, California. After the stream mapping was completed, 8 pyrethroids were analytically measured from randomly selected sites in 12 depositional and 12 non-depositional areas in the four Salinas streams. Benthic macroinvertebrate samples were collected and identified from depositional and non-depositional areas where pyrethroids were measured. In addition, physical habitat was also evaluated at each site where benthic communities were collected. Based on a random sampling design, 24 % of the 96 sediment sampling sites in the Salinas streams were classified as predominately depositional areas. Mean total pyrethroid concentrations were approximately 2× to 61× times higher in depositional areas of the Salinas streams when compared to non-depositional areas. Physical habitat scores from the 12 depositional and 12 non-depositional areas in the Salinas stream sites were extremely low compared with other California streams thus demonstrating that impaired physical habitat is a critical stressor in these streams. Approximately 6,300 individual macroinvertebrates were picked and identified from 70 taxa from the 24 Salinas stream sites. The most dominant taxa collected were all considered tolerant of environmental stressors and dominant taxa from both depositional and non-deposition areas were similar. Ten different benthic metrics for the Salinas streams were similar for the depositional areas, where pyrethroid concentrations consistently exceeded laboratory based toxicity thresholds, and non-depositional areas where pyrethroid concentrations were much lower. These results suggest that factors other than pyrethroids are responsible for impacting resident benthic communities in these urban Salinas streams.

Grain size analysis of California waterbodies to determine the spatial extent and temporal trends of depositional areas where hydrophobic organic compounds could accumulate.

The objective of this study was to summarize the grain size data from 1993 to 2016 from California water bodies and determine the number and percent of sites where sediment samples are primarily depositional sediment (more than 50% silt/clay) or non-depositional sediment (less than 50% silt/clay). Fine grain depositional areas were of interest because these are areas where hydrophobic organic compounds (HOCs) can accumulate if sources exist. Trends analysis of percent silt/clay data were conducted for all nine California Water Board Regions if adequate data for this type of analysis were available. Sediment sampling protocols were reviewed for all the grain size measurements to determine if depositional areas were targeted or random sampling was conducted. Twelve of the 13 sediment sampling protocols targeted depositional areas. Based on 23 years of data from 685 sites (1859 observations) in all 9 California Regions, the summary analysis showed that only 58% of the sites were considered depositional areas even when targeted sampling for depositional areas was used. Since only slightly more than half the sites were reported as depositional areas based on sampling that targets depositional areas, these results would certainly suggest that depositional areas are not dominant. In fact, for the one protocol where random sampling was used, depositional areas were not reported to be dominant. From an ecological risk perspective, the implication of this finding is that HOCs would not be expected to accumulate in the dominant type of sediment found in most California waterbodies. Trends analysis of percent silt/clay data showed mixed results for the various California Regions.

Monitoring of Irgarol 1051 concentrations with concurrent phytoplankton evaluations in East Coast areas of the United States.

The objectives of this study were to measure: (1) Irgarol and GS26575 (major metabolite) during the peak 2004 boating season at selected marinas and reference areas in the Carolinian Zoogeographic Province of the Eastern United States; (2) Irgarol and GS26575 at selected stations during the summer months in the Back Creek/Severn River area in Maryland in 2003 and 2004; and (3) structural and functional characteristics of resident phytoplankton communities concurrently with Irgarol and GS26575 monitoring in Back Creek/Severn River area. Irgarol concentrations from 14 marinas in the Carolinian Province ranged from non-detectable (<1 ng/L) to 85 ng/L; concentrations were less than 16 ng/L at all reference sites. The probability of exceeding the plant 10th centile for Irgarol (251 ng/L) was less than 0.6% for all marinas and 0.01% for all reference areas. These data suggest low ecological risk from Irgarol exposure for both marina and reference areas in the Carolinian Province. Irgarol concentrations ranged from 5 ng/L at the Severn River reference site to 1,816 ng/L in Port Annapolis marina during the two year study. Ecological risk from Irgarol exposure was high for the Port Annapolis marina sites based on a probability of exceeding the plant 10th centile. However, risk was low for Severn River and Severn River reference sites. Functional and structural measures of resident phytoplankton communities in the Back Creek and Severn River did not suggest that these target species are impaired in the Port Annapolis marina area where probabilistic analysis predicted adverse effects from Irgarol exposure.

Comparative sensitivity of field and laboratory populations of Hyalella azteca to the pyrethroid insecticides bifenthrin and cypermethrin.

Hyalella azteca are epibenthic invertebrates that are widely used for toxicity studies. They are reported to be more sensitive to pyrethroid insecticides than most other test species, which has prompted considerable use of this species in toxicity testing of ambient surface waters where the presence of pyrethroids is suspected. However, resident H. azteca have been found in some ambient water bodies reported to contain surface water and/or sediment pyrethroid concentrations that are toxic to laboratory reared H. azteca. This observation suggests differences in the sensitivities of laboratory reared and field populations of H. azteca to pyrethroids. The goal of the present study was to determine the sensitivities of laboratory reared and field populations of H. azteca to the pyrethroids bifenthrin and cypermethrin. Specimens of H. azteca were collected from resident populations at field sites that are subject to varied land-use activities as well as from laboratory populations. These organisms were exposed to bifenthrin- or cypermethrin-spiked water in 96-h water-only toxicity tests. The resulting data demonstrated that: 1) field-collected populations in urban and agricultural settings can be >2 orders of magnitude less sensitive to the pyrethroids than laboratory reared organisms; 2) field-collected organisms varied in their sensitivity (possibly based on land-use activities), with organisms collected from undeveloped sites exhibiting sensitivities similar to laboratory reared organisms; and 3) the sensitivity of field-collected "tolerant" organisms increased in subsequent generations reared under laboratory conditions. Potential mechanisms for these differences are discussed.

Occurrence of Irgarol 1051 and its major metabolite in Maryland waters of Chesapeake Bay.

Irgarol and its major metabolite (GS26575) were measured in Maryland waters of Chesapeake Bay: (1) in and near 10 marinas, a mainstem Bay site and two Severn River locations during a general survey in July and December of 2002; (2) at various sites in the Port Annapolis Marina and the Severn River area during March of 2002 before the boating season began; and (3) during July (peak boating season) in the same Port Annapolis Marina and Severn River sites area during both an ebb and flood tide. Irgarol concentrations ranged from 1.82 ng/l at the mid-Bay site to 585 ng/l in Port Annapolis marina during the July and December general survey. An Irgarol 90th centile of 239 ng/l was reported for the 10 marina sites, two Severn River sites and one mainstem site sampled during the general survey conducted in July and December. Temporal analysis of all pooled data showed that 90th centiles were over seven times higher in July when compared to December. A comparison of Irgarol concentrations at 12 sites in the Port Annapolis marina and Severn River area during both an ebb and flood tide in July showed no consistent trend with tidal cycle by site although significant reductions in concentrations were reported with distance from the three Port Annapolis marina sites. Ecological risk from Irgarol exposure was judged to be low for most Chesapeake Bay sites sampled. Possible exceptions were Port Annapolis marina, Severn River sites in close proximity to this marina and Chesapeake Harbor marina where Irgarol concentrations exceeded a conservative effects threshold during the peak boating season in July. Ecological risk from GS26575 exposure was low for all sites.