Environmental Pollution 141 (2006) 402e408 Identifying primary stressors impacting macroinvertebrates in the Salinas River (California, USA): Relative effects B.S. Anderson , B.M. Phillips , J.W. Hunt , V. Connor , N. Richard , a Department of Environmental Toxicology, University of California, Davis, CA 95616, USA b Division of Water Quality, State Water Resources Control Board, 1001 I. Street, Sacramento, CA 95814, USA Received 8 April 2005; received in revised form 15 August 2005; accepted 26 August 2005 Pesticides are the primary stressor impacting macroinvertebrates in sections of the lower Salinas River.
Laboratory doseeresponse experiments with organophosphate and pyrethroid pesticides, and doseeresponse experiments with increasing particle loads were used to determine which of these stressors were likely responsible for the toxicity and macroinvertebrate impacts previouslyobserved in the Salinas River. Experiments were conducted with the amphipod Hyalella azteca, the baetid mayfly Procloeon sp., and the midgeChironomus dilutus (Shobanov, formerly Chironomus tentans). The results indicate the primary stressor impacting H. azteca was pesticides,including chlorpyrifos and permethrin. The mayfly Procloeon sp. was sensitive to chlorpyrifos and permethrin within the range of concentrationsof these pesticides measured in the river. Chironomus dilutus were sensitive to chlorpyrifos within the ranges of concentrations measured in theriver. None of the species tested were affected by turbidity as high as 1000 NTUs. The current study shows that pesticides are more importantacute stressors of macroinvertebrates than suspended sediments in the Salinas River.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Pesticides; Macroinvertebrates; Toxicity; Suspended particles tests, chemical analyses and toxicity identification evaluations(TIEs), we recently showed that organophosphate pesticides The Salinas River is the largest of the three coastal rivers associated with agriculture drainwater were responsible for sed- flowing into the Monterey Bay National Marine Sanctuary in iment and water column toxicity in samples collected in the Sal- central California. Large areas in this watershed are cultivated inas River downstream of one of these tributaries ( year-round, primarily in row crops such as lettuce, strawberries, artichokes, and crucifer crops. Studies have shown that ambient throid pesticides might also be partially responsible for sedi- water samples from the river and specific tributaries are toxic to ment toxicity to the amphipod Hyalella azteca, but these standard test species in laboratory tests results were inconclusive due to a lack of pyrethroid analyses in river samples and limited doseeresponse information forselected pyrethroids using Hyalella (Our previous study also demonstrated * Corresponding author. c/o Marine Pollution Studies Laboratory, 34500 drainwater impacts on a number of macroinvertebrate commu- Highway 1, Monterey, CA 93940, USA. Tel.: C1 831 624 0947; fax: C1 nity metrics. There were significant negative correlations between the number of Ephemeroptera taxa, taxonomic 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.envpol.2005.08.056 B.S. Anderson et al. / Environmental Pollution 141 (2006) 402e408 richness and the percentage Chironomidae and concentrations three times, with each replicate containing five animals (7e14 days old), of diazinon and chlorpyrifos in the river. We also found signif- and C. dilutus treatments were replicated 10 times with one organism per rep-licate (3rd instar). Survival was monitored daily. Dissolved oxygen (mg/L), icant negative correlations between these metrics and sus- pH, temperature ( C) were measured at the beginning and end of all experi- pended particles (measured as turbidity) in the river. Thus, our ments using meters and probes. These instruments were calibrated in the results indicated that resident species might be impacted by laboratory as per manufacturer’s recommendations. Alkalinity (total as co-occurring stressors (i.e., multiple pesticides, and possibly, CaCO3) and hardness (calcium as CaCO3) were measured in well water suspended particles) associated with agriculture drainwater controls. The test temperature was 23 C. Containers were not aerated.
(Our ability to resolve the relative influ-ences of these stressors on macroinvertebrates was constrained by a lack of doseeresponse information for pesticides andsuspended particle effects on key resident species.
Mayfly larvae were collected from a reference station in the Salinas River in May 2000 for diazinon and chlorpyrifos tests, and again in March 2004 for The current study provides doseeresponse information for bifenthrin and permethrin tests. This reference station has been used in our the organophosphate pesticides diazinon and chlorpyrifos, previous studies, is 11 km upstream of our current study area, and has been and for suspended particles, using resident and surrogate mac- demonstrated to have uniformly low pesticide concentrations in sediment pyrethroid pesticides bifenthrin and permethrin. In terms of was verified by a qualified taxonomist. Larvae were transported to the lab inriver water then transferred to 20 L plastic trays where they were held until pounds per acre applied, diazinon, chlorpyrifos, and permethrin testing. Culture tray water was renewed daily with well water, but mayflies comprise the most commonly applied pesticides in the Salinas were not fed. Minor larval mortality was observed during the holding period.
Valley. Bifenthrin is also among the more commonly used py- Testing commenced within 4 days of the collection date. Larvae of unknown rethroid pesticides in the Salinas Valley. Experiments were con- age were used in these experiments. The approximate range of lengths of ducted using three species: the amphipod H. azteca, the baetid Proloeon sp. larvae tested was 0.5e1 cm (head to base of cerci). Water-onlydoseeresponse experiments with the organophosphate pesticides were 48 h mayfly Procloeon sp., and the midge Chironomus dilutus.
exposures, renewed at 24 h. Tests with pyrethroid pesticides were 48 h static These species were from genera or families that were shown exposures. Tests were conducted in 200 ml glass bowls containing 100 ml test to be impacted by agriculture drainwater in previous studies, solution. Each concentration was replicated 3e5 times, with each replicate but for which pesticide sensitivities or sensitivities to sus- containing five animals. Survival was monitored daily. Water quality and tem- pended particles are unknown. We compared LC50s from these perature were measured as described above. The animals were not fed and thecontainers were not aerated.
experiments to the ranges and mean concentrations of thepesticides and suspended particles previously measured in the Salinas River. Current results were combined with previousresults in a weight-of-evidence to determine which are the Experiments to assess the effects of suspended sediments associated with most important stressors impacting macroinvertebrates in this agriculture drainwater were conducted using soil from a certified organic farm.
Approximately 400 L of topsoil was obtained from Blue Heron Farms inWatsonville, California. This farm has been a certified organic operation since 1974 (personal communication Tim Voss, Blue Heron Farms); no pesticideshave been applied to soil on this farm for 30 years. The soil was sun driedand dry sieved to 500 mm. The soil was then suspended in well water and The pyrethroid pesticides bifenthrin and permethrin were tested with sequentially wet-sieved to a final sieve size of 64 mm. For these experiments, H. azteca, C. dilutus, and Procloeon sp., and the organophosphate pesticides the wet-sieved soil was dissolved in well water in four 114 L plastic bins to diazinon and chlorpyrifos were tested with Procloeon sp. Doseeresponse approximate four nominal suspended sediment concentrations that spanned data from the literature were used to determine the sensitivities of H. azteca the range reported in Nominal suspended sediment and C. dilutus to chlorpyrifos and diazinon. Stock solutions of permethrin concentrations, measured as turbidity, were 0, 250, 500 and 1000 NTUs.
and bifenthrin (100 mg/L in methanol) were obtained from Accustandard Sediments were kept in suspension by vigorously aerating each bin as their (New Haven, CT, 100% purity). These were used to prepare nominal concen- solutions were continuously pumped with a submersible pump (pump trations of 0, 500, 1000, 5000, 10,000, and 20,000 ng/L for tests with C. dilu- rate Z 60 L per min) to each of four 13 L head tanks that were also heavily tus. Secondary stocks (100 mg/L in methanol) were prepared from the primary aerated to keep the particles in suspension. Turbid water from the head tanks stocks to give final nominal concentrations of 0, 5.6, 10, 18, 32, and 56 ng/L flowed by gravity to a manifold fitted with five valves, each supplying water to for tests with Hyalella and 18, 32, 56, 100, 180, 320, and 560 ng/L for tests one of five replicate exposure chambers.
with Procloeon. Stock solutions of 250 mg/L chlorpyrifos and diazinon in Exposure chambers were 1.7 L polyethylene plastic food dishes fitted on methanol were prepared from pesticides obtained from ChemService both ends with 2.5 cm diameter 500 mm mesh screens. Each of the replicates (Westchester, PA, 99% purity). Secondary (2000 and 500 mg/L diazinon and contained 10 animals (amphipods, chironomids, or mayflies). The water chlorpyrifos, respectively) and tertiary (20 and 5 mg/L diazinon and chlorpyr- flowed through the exposure containers and into a catch basin that returned ifos, respectively) stocks were prepared from these to give final test concentra- it to the bin supplying that suspended sediment treatment. Approximately tions of 0.5, 1.0, 2.5, and 5.0 mg/L for diazinon, and 0.0625, 0.125, 0.25, and 300 ml of medium fine-grained sand was added to the exposure chambers 0.50 mg/L for chlorpyrifos. Responses of all animals to all pesticides were com- for the Procloeon and C. dilutus experiments to provide substrate for these spe- pared to both well water and methanol controls (1% methanol in well water).
cies. No substrate was used in the Hyalella experiments. The control water forthese experiments was well water and controls for the Procloeon and C. dilutus 2.1. H. azteca and C. dilutus 96-h survival tests exposure contained a sand substrate as described above. Turbidity was moni-tored at the beginning and end of each day. To account for settlement of sus- Water-only doseeresponse experiments with H. azteca (provided by pended sediment during the course of the experiments, additional suspended Aquatic Biosystems, Fort Collins, CO) and C. dilutus (provided by Chesa- sediment was added to each treatment after turbidity measurements to return peake Culture, Hayes, VA) were 96-h static exposures conducted in 20 ml the concentration to the target nominal concentration. Turbidity was monitored glass vials containing 15 ml solution. H. azteca treatments were replicated with a model 2100 turbidimeter (Hach, Loveland, CO). Total suspended solids B.S. Anderson et al. / Environmental Pollution 141 (2006) 402e408 were measured during one exposure in each nominal turbidity treatment for comparison using EPA procedures (). Suspended sediment ex-posures were conducted for 96 h, at which time the contents of the chambers were sieved through a 500 mm mesh screen and survival was recorded.
Measured concentrations of bifenthrin and permethrin were considerably lower than nominal concentrations in all cases.
For bifenthrin experiments with H. azteca, the bifenthrin re- Selected concentrations of 1 L samples of bifenthrin and permethrin were covery ranged from 19% to 56%, and average measured con- measured for comparison to nominal concentrations. Samples for pyrethroid centrations of bifenthrin were 37% of nominal concentrations.
analyses were prepared by mixing ten 100-ml samples (1 L total), each pre- For bifenthrin experiments with Procoleon sp., recovery pared from respective stock solutions of bifenthrin and permethrin. Each sam-ple was mixed at the time of test initiation in 100 ml Erlenmeyer flasks ranged from 55% to 77%, and average measured concentra- following the same procedures used for filling the exposure chambers. Pyre- tions of bifenthrin were 65% of nominal concentrations. For throid pesticides were measured using Gas ChromatographyeMass Spectros- bifenthrin experiments with C. dilutus, recovery ranged from copy (GCeMS; U.S. EPA method 1660) ECD-MS detector following methods 36% to 65%, and average measured concentrations of bifen- developed by the California Department of Fish and Game Water Pollution thrin were 54% of nominal concentrations. For permethrin ex- Control Laboratory (reporting limit bifenthrin and permethrin Z 0.01 mg/Land 0.02 mg/L, respectively). Chlorpyrifos and diazinon were measured using periments with H. azteca, recovery ranged from 0% to 61%, two methods, Enzyme Linked Immunosorbent Assays (ELISAs and average measured concentrations of permethrin were scribed below), and GCeMS (U.S. EPA methods 8140 and 8141A). Standard 54% of nominal concentrations For permethrin ex- quality assurance procedures including measurement of standard reference periments with Procoleon sp., recovery ranged from 32% to materials and quantification of surrogate recoveries and matrix spikes were 61%, and average measured concentrations of permethrin used in all analyses. All chemical analyses met prescribed quality assuranceguidelines.
were 43% of nominal concentrations ). For permethrinexperiments with C. dilutus, recovery ranged from 61% to75%, and average measured concentrations of permethrin were 68% of nominal concentrations Percent recov-eries of laboratory control spikes of bifenthrin and permethrin All chlorpyrifos and diazinon concentrations were measured using Enzyme in water during chemical analyses exceeded quality assurance Linked Immunosorbent Assays (ELISAs) following procedures recommendedby . ELISA readings were compared to a five-pointstandard curve, using standards provided by the manufacturer. After analysisof a group of samples, accuracy of the ELISA method was determined by mea- suring an external chlorpyrifos or diazinon standard. All standard measure- Nominal and measured concentrations (ng/L) of diazinon, chlorpyrifos, ments were within G20% of nominal. Precision of the ELISA method was permethrin and bifenthrin in water-only doseeresponse experiments with determined with duplicate measures of one sample by calculating the coeffi- cient of variation. CVs were always less than 20. The lowest detectable dose was 30 ng/L for diazinon and 50 ng/L for chlorpyrifos.
Median effect concentrations (LC50s) were calculated from mean mea- sured organophosphate concentrations and nominal pyrethroid concentrations using (ToxCalcÔ Statistical Software, Tidepool Software, McKinleyville, CA), using the trimmed SpearmaneKarber method ( The doseeresponse information resulting from the pesticide and suspended particle exposures was compared to data from our Salinas River study to de- termine whether effect thresholds were within the range of pesticide or sus- pended sediment concentrations measured in the river. For this comparison, we used the mean and range of diazinon and chlorpyrifos concentrations mea- sured at the station with the highest pesticide concentrations in our previousstudy. These concentrations were measured at Station #2, which was located at the confluence of the agricultural drainage creek and the Salinas River We also compared the turbidities from Station #2 in that study to those in the current doseeresponse experiments. Pyrethroid pesticides were not measured in our previous study, so to investigate the likeli- hood of exposure to these pesticides, we relied on pyrethroid concentrations reported as part of a California Department of Pesticide Regulation studyThese authors reported concentrations of pyreth- roids in water and sediments sampled weekly over 16 weeks from June through September, 2003 from a station in the agriculture drainage creek 0.5 km upstream of our Station #2 from . The mean and range of these measures were used to estimate the mean and range of con- centrations of permethrin in the Salinas River at Station #2 (these authors didnot detect any bifenthrin in their study).
B.S. Anderson et al. / Environmental Pollution 141 (2006) 402e408 thresholds. The range of percent recoveries of bifenthrin was bifenthrin and permethrin toxicity to Procloeon sp. were 89e116% (mean percent recovery Z 102%). The range of per- 84.3 and 89.6 ng/L, respectively ). The permethrin cent recoveries of permethrin was 69e115% (mean percent LC50s were within the mean and range of permethrin concen- trations in water in this system reported by Enzyme Linked Immunosorbent Assays (ELISAs) of diaz- . The mean and range of permethrin concentration inon and chlorpyrifos in the Procloeon sp. experiments reported by these authors were 104.8, and 71.2e162 ng/L, showed that nominal and measured concentrations were com- respectively (These authors did not detect any parable (Measured concentrations of diazinon in the experiments with Procloeon sp. ranged from 118% to 127% of H. azteca was more sensitive than Procloeon sp. to both the nominal concentrations. Measured concentrations of chlor- bifenthrin and permethrin. The mean LC50s for bifenthrin pyrifos in the experiments with Procloeon sp. ranged from and permethrin toxicity to H. azteca were 9.3 and 21.1 ng/L, 79% to 118% of the nominal concentrations ().
respectively (). The permethrin LC50 was also within The LC50s for all diazinon and chlorpyrifos tests with the mean and range of permethrin concentrations in water in C. dilutus are based on measured concentrations. The LC50s for all pyrethroid pesticides used in these experiments are pre- The midge C. dilutus was relatively insensitive to bifenthrin sented as nominal concentrations. For the experiments with and permethrin. The mean LC50s for bifenthrin and permeth- diazinon and chlorpyrifos we have shown that the nominal rin toxicity to C. dilutus were 26,150 and 10,450 ng/L, respec- concentrations were reasonable approximations of the mea- tively (). The permethrin LC50 was well above the sured concentrations. The actual LC50s and LOECs for bifen- mean (104.8 ng/L) and range (71.2e162 ng/L) of permethrin thrin and permethrin for Hyalella, Procloeon, and C. dilutus concentrations in this system reported by are probably considerably lower than those reported here.
Based on the fact that recoveries of spiked pyrethroids mea- Published doseeresponse data for diazinon and chlorpyrifos sured in the laboratory analyses were acceptable, it is likely toxicity to C. dilutus were used to assess risk of these pesticides that lower recoveries in samples collected at the initiation of to chironomids in the Salinas River. The LC50 for chlorpyrifos the tests were the result of loss of pyrethroids to the sides of toxicity to Chironomus tentans (C. dilutus) reported by was 70 ng/L, lower than the range of chlorpyrifosconcentrations we measured in our study (48e515 ng/L; The 96-h LC50 for diazinon toxicity to C. tentans (C.
The mayfly Procloeon was sensitive to chlorpyrifos within much higher than the range of diazinon concentrations we the range of chlorpyrifos concentrations measured in the measured in our previous study (190e790 ng/L; ).
Salinas River in our previous study. The mean LC50 for chlor-pyrifos toxicity to Procloeon was 81 ng/L. The mean concen- tration of chlorpyrifos measured at the most contaminatedstation in the Salinas River in our previous study was There were no apparent effects of suspended particles on mayflies, amphipods, or midges in our experiments. Mean sur- concentrations previously measured in the river was 48e vival of Procloeon at the highest turbidity tested, 1000 NTUs 515 ng/L ). Procloeon was less sensitive to diazinon.
(nominal), was 86.7%, while mean survival in the controls was The mean LC50 for diazinon was 1930 ng/L, which is greater 73.3% (). Mean survival of Hyalella at 1000 NTUs was than the concentrations of diazinon we previously measured in 98%, while mean control survival was 90.3%. Mean survival the river. The mean concentration of diazinon measured in the of C. dilutus at 1000 NTUs was 94.3%, while mean control Salinas River in our previous study was 460 ng/L, and the survival was 94.7%. Survival of all species tested at all lower range of concentrations measured was 190e790 ng/L.
turbidities was equal to or greater than their survival in the Procloeon sp. were relatively sensitive to the pyrethroid controls. Turbidities measured throughout the experiments pesticides bifenthrin and permethrin. The mean LC50s for Sensitivity of the amphipod Hyalella azteca to selected pesticides relative to Sensitivity of the baetid mayfly Procloeon sp. to selected pesticides relative to concentrations measured in the Salinas River in ng/L concentrations measured in the Salinas River in ng/L a Concentrations measured at the most impacted station, see text for a Concentrations measured at the most impacted station, see text for B.S. Anderson et al. / Environmental Pollution 141 (2006) 402e408 suspended sediment concentrations (as turbidity). When com- Sensitivity of the midge Chironomus dilutus to selected pesticides relative to bined with previous measures of pesticide concentrations, concentrations measured in the Salinas River in ng/L TIEs, and doseeresponse information from the literature, re- sults from the current study help resolve the relative contribu- tions of these stressors in this system.
Our previous data showed significant toxicity to the amphi- pod H. azteca at stations downstream of an agriculture drain where high concentrations of organophosphate pesticides were measured, and where amphipod field densities were im- Concentrations measured at the most impacted station, see text for pacted. Toxicity identification evaluations showed that sedi- ment toxicity was due to mixtures of the organophosphate pesticide chlorpyrifos, and some other non-metabolically acti-vated pesticide ().
were somewhat lower than the nominal values, due to settle- TIE evidence in these studies included increased mortality of ment of particles throughout the night ). For example, amphipods with the addition of the metabolic inhibitor piper- measured turbidities in the 1000 NTU treatment ranged be- onyl butoxide, which may suggest toxicity due to pyrethroid tween 755.9 and 909.8. The mean measured turbidities in tests pesticides (). Conclusive evidence of pyre- with Procloeon sp., H. azteca and C. dilutus in the 1000 NTU throid toxicity was constrained by a lack of doseeresponse in- treatment were 840, 836 and 824 NTUs, respectively. Despite formation with H. azteca for selected pyrethroids, and absence the variability of suspended sediment in these experiments, the of pyrethroid measurements in field sediments in our study range of turbidities was comparable to the range we previously measured in the Salinas River. The mean turbidity measured in ent study, we found that H. azteca is sensitive to the pyrethroid the river in the previous study was 521.4 NTUs, and the range pesticide permethrin within the range of permethrin concentra- tions measured in this system. The mean LC50 for permethrin suspended solid (TSS) concentrations corresponded to the toxicity to H. azteca in water is 21.1 ng/L California measured turbidities in these experiments. The mean TSS Department of Pesticide Regulation recently reported mean measured in the 250 NTU treatment was 297.7 mg/L. The water concentrations of permethrin in this system in 16 weekly mean TSS in the 500 NTU treatment was 483 mg/L, and the mean TSS in the 1000 NTU treatment was 848.7 mg/L ).
concentrations were measured in the agriculture drainagecreek approximately 0.5 km upstream of Salinas River Station#2, the station where our previous TIE work was conducted.
While we found strong negative correlations between turbidityand macroinvertebrate densities (), our Our approach to identifying the primary stressors impacting current data show that H. azteca tolerate concentrations of sus- macroinvertebrate communities in the Salinas River followed pended particles as high as those measured in the river (1000 an iterative process involving an initial toxicity assessment NTUs; When data from these studies are considered that described water column toxicity in selected agriculture as a weight-of-evidence, they confirm that one of the primary stressors impacting H. azteca in our study area on the Salinas more detailed studies that demonstrated effects in the river River was the organophosphate pesticide chlorpyrifos, and in- dicate that the pyrethroid pesticide permethrin is also a likely important stressor to amphipods in this system.
that Ceriodaphnia survival in toxicity tests and macroinverte- Our results suggest that chlorpyrifos and permethrin also brate densities were negatively correlated with the organo- impact the baetid mayfly Procloeon sp. in this system.
phosphate pesticides diazinon and chlorpyrifos, and with Procloeon sp. were sensitive to chlorpyrifos within the mean Table 5Effects of suspended particles on survival of Procloeon sp., H. azteca, and C. dilutus in 96-h experiments (results from three experiments for each species) Range concentrations measured in the rive a Concentrations measured at the most impacted station, see text for explanation.
B.S. Anderson et al. / Environmental Pollution 141 (2006) 402e408 and range of concentrations measured at Station #2 drainwater, the consistency of the input during the growing Procloeon sp. was also sensitive to permethrin within the season constitutes chronic exposure. The acute LC50s used mean and range of permethrin concentrations reported by to determine risk of diazinon, chlorpyrifos and permethrin to resident macroinvertebrates in the current study likely under- tolerated turbidities as high as those measured in our previous estimate chronic risk of these pesticides to Hyalella, Pro- study, suggesting this is a less important acute stressor than cloeon, and chironomids. In addition, pesticides in this system may influence macroinvertebrate community structure C. dilutus was used as a surrogate to assess the relative sen- through behavioral or indirect mechanisms. These include sitivity of chironomids to pesticides and suspended sediments sublethal influences on drift and predator avoidance behavior in this system. Previous results showed that chironomid densi- ties declined downstream of the agriculture drain input and exposures that only quantify mortality due to single chemicals that their densities were negatively correlated with diazinon may significantly underestimate sublethal risk due to mixtures and chlorpyrifos concentrations and suspended sediment con- of organophosphate and pyrethroid pesticides. The relative ef- centrations. Doseeresponse data from the literature indicate fects of suspended sediments on pesticide mixtures is beyond chironomids are very sensitive to chlorpyrifos and less sensi- the scope of this study, but previous work has shown that in the tive to diazinon in laboratory studies. re- case of more hydrophobic pesticides such as chlorpyrifos and ported a 10-day LC50 for chlorpyrifos toxicity to C. tentans pyrethroids, addition of sediment decreases toxicity of chlor- (dilutus) of 70 ng/L. The mean chlorpyrifos concentration measured in the Salinas River in our previous study was ) by reducing their bioavailability in water column toxic- ity tests. In addition, pesticide impacts on macroinvertebrate for diazinon toxicity to C. dilutus reported in populations may be tempered by additional biotic factors was 30,000 ng/L. The mean diazinon concentra- such as development of chemical resistance after prolonged tion previously measured in the river was 460 ng/L exposure to pesticides, influences of habitat refugia allowing Assuming C. dilutus is representative of other chironomids in for recolonization, and reduced bioavailability of pesticides the Salinas River, these results suggest that chlorpyrifos is a potentially important stressor to midge larvae in this system.
Despite the limitations of the current study, the weight-of- As with the other resident species we investigated, C. dilutus is evidence of our lab and field work in the Salinas River not acutely sensitive to suspended particles. Thus, although we demonstrates that pesticides are important stressors of macro- found strong negative correlations between macroinvertebrate invertebrates in parts of this system. Based on the current densities and turbidity in our previous study, this likely re- work, we conclude that suspended sediments associated with sulted from the co-occurrence of suspended particles and pes- agriculture drainwater probably play a minor role in directly ticides, in particular chlorpyrifos. Our current results show that affecting key components of aquatic invertebrates communi- C. dilutus is relatively insensitive to permethrin and bifenthrin.
ties in our study area, and that organophosphate and pyrethroid The LC50s for permethrin was well above the mean and range pesticides have a greater potential for impacting these of concentrations measured in this system, and no bifenthrin was detected (We could find no published data on There is growing awareness of environmental problems bifenthrin toxicity to this species, but data for permethrin sug- associated with agriculture runoff in Central California, and gest that C. dilutus (LC50 Z 10,450 ng/L) is considerably less growers have organized to form watershed monitoring groups sensitive to this pesticide than other species in this genus.
to better understand the extent and associated impacts of pes- The LC50s for permethrin toxicity to the midges C. plumulo- ticide pollution in this region. In addition, through cooperative sus and C. salinarius were 560 and 73 ng/L, respectively efforts of a number of stakeholders, on-farm practices to min- imize pesticide runoff are being actively investigated. Ours Our current data using three representative macroinverte- and similar studies have been helpful in identifying the most brates suggest that suspended particles are less important important pesticides to target for reduction in drainwater stressors in our study area in the Salinas River, and that macro- runoff. Future monitoring will be used to determine which invertebrate declines were caused by pesticides. These experi- practices are most efficient at reducing these chemicals.
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