Grantee Research Project Results

 

No toxic responses were seen for any of the DDD seawater (30S) exposure concentrations of 11.8, 22.5, 45, and 90 μg/L.  At 35-days, 90 μg/L exposures produced maximum lipid-normalized body burdens of 594 ± 54 and 775 ± 54 pg-DDD/μg-lipid for gravid females and males respectively.  Significant sublethal toxic responses (p < 0.05) were observed for lindane (LD) corresponding to delayed total development from nauplius to adult (60.0 ± 30.5 and 44.5 ± 7.31 pg-LD/μg lipid for gravid females and males), decreased mean viable offspring production (122 ± 58.6 pg-LD/μg lipid for gravid females), decreased fertilization success and increased total chronic mortality (510 and 607 pg-LD/μg lipid for gravid females and males).  A Trimmed Spearman Karber analysis was performed to predict an EC₅₀ for decreased reproductive output of 28.2 μg-LD/L with a 95% CIE of 22.3 - 35.7 μg-LD/L.  From these values, sublethal lipid-normalized CBRs of 354 (281-448) pg-LD/μg lipid and 253 (192-331) pg-LD/μg lipid were calculated for males and gravid females, respectively, relative to decreased reproductive success.  Sex-specific lindane log BCF values of 3.92 ± 0.13 and 4.06 ± 0.17 for gravid females and males respectively were measured at these CBRs.  Lastly, a Leslie-matrix based modeling approach was used to predict lipid-normalized population growth CBRs (for a 50% projected population reduction after three generations) of  60 and 58 pg-LD/μg lipid for males and gravid females respectively. 

 

For the persistent and predominant sediment degradation product of fipronil -- fipronil sulfide (FS) -- copepod life-cycle toxicity tests were conducted using A. tenuiremis at measured concentrations of 0.10, 0.20, 0.39, and 0.85 μg/L.  Tandem microplate bioaccumulation experiments were conducted to establish dry weight and lipid normalized body burdens at each exposure concentration.  Significant toxic effects ranged from delayed naupliar development (at 0.10, 0.39, and 0.85 μg/L), delayed copepodite development (at 0.85 μg/L), delayed total development (at 0.20, 0.39, and 0.85 μg/L), decreased reproductive success (at 0.39, and 0.85 μg/L), and decreased mean viable offspring (at 0.85 μg/L). A sublethal Critical Body Residue (CBR) relating to reproductive success/failure under FS exposure was computed.  Using Trimmed Spearman-Karber analysis, a Repro-EC₅₀ of 0.16 μg/L (0.12 to 0.21 μg/L 95% CIE) was calculated corresponding to adult CBRs (averaged across sexes) and lipid-normalized CBRs of 0.38 pg-FS/μg dry weight (95% CIE: 0.27 to 0.52 pg/μg dry weight) or 2.8 pg-FS/μg lipid (95% CIE: 2.2 to 3.6 pg-FS/μg lipid), respectively.

 

Leslie matrix modeling was used to integrate all observed life table parameters including mortality, developmental failures, sex ratio shifts, infertility and fecundity into a predictive model of population growth and age structure over time.  Copepod population size projections as a function of FS exposure were produced from empirical microplate data and regressed against corresponding tissue residues (as above) using best-fit regression (r² > 0.99) to predict the median effective tissue concentration (i.e., lipid and non-lipid normalized population EC₅₀) that would produce a population size 50% of the FS-free control.  Intrinsic population growth rates (λ) predicted from empirical transition and fecundity rates using the Leslie matrix declined with dose > 20% from 1.27 in the controls down to 0.98 at the highest FS exposure.  A λ  of unity means the population growth rate is just replacing itself over time.  A rate of 1.27 means the population is expected to double every 2.6 lifecycles, but at 1.27 pg-FS/μg-dry weight, more than 7 generations would be required to double population size.  Consequently, by the third generation, sharply depressed population sizes as a function of FS body burden were predicted by the model compared to controls.  The model predicted fewer than five females would be gravid in the population by the third generation of exposure to both 0.39 and 0.85 μg-FS ^. L^-1 (i.e., 9.6 – 10.2 pg-FS/μg-lipid).  A 50% population growth suppression would be predicted to occur at an aqueous exposure concentration between 0.10 and 0.20 μg-FS/L.   The median population growth response, (i.e., EC₅₀ ), for the Leslie matrix modeling results was defined a priori as the lipid-normalized body burden where matriarchal population size after 3 generations would be predicted by regression to be 50% of the control.  In this study that critical body burden corresponded to a lipid-normalized CBR of 1.6 pg-FS · μg^-1 lipid, which yielded a third generation EC₅₀ prediction of 968 copepods in the population.  This concentration is 1.75 times lower than the lipid-normalized CBR predicted to suppress reproductive success by 50% .  Other treatment factors such as stage-specific (e.g., naupliar) FS mortality and sex ratio shifts that were captured and integrated in the Leslie matrix model, but not in the reproductive success prediction, likely drove the CBR prediction to a lower effective tissue concentration.  Eigenanalysis of the copepod stage matrix for the control microplate populations showed highest “elasticities” linked to the higher proportions of adult virgin females able to mate and become gravid in controls, and to the correspondingly higher reproductive output (fecundity). Conversely, the two highest FS concentrations showed highest elasticities for the low proportions of adult virgin females able to mate and become gravid, and for their correspondingly lower reproductive outputs.  [“Elasticities” are simply the proportional sensitivities of λ to small changes in the Leslie matrix elements for each empirical stage matrix of the FS exposed populations.]

 

The BCF values and sublethal CBR’s reported here for lindane and FS in A. tenuiremis are highly novel, as lipid normalized sublethal CBRs have never been reported for either legacy or current use pesticides for any meiobenthic organism to date.  Non-normalized CBR estimates for a few meiobenthos and most macrobenthos have been typically mortality based and reported for other types of nonionic contaminants such as DDT (and metabolites), PAHs and PCBs. All POP body burdens in this project were predicted using a simple model assuming 13.7% lipid mass per gravid female copepod (determined empirically) and 15.2% for males, a lipid density of 0.8 g/mL, and sex-specific dry weight body masses of 1.4 μg per gravid female, and 0.6 μg per adult male.  Calculations assume ideal equilibrium interactions between water and lipid where lipid has similar contaminant loading properties as octanol.  Predicted burdens were then calculated from mean lipid volumes per copepod, micro-well water volumes (250μL), log K_OW values for each compound, and concentrations of each POP in aqueous solution.  “Concentration in lipid” values and predicted body burdens were reported here on an individual copepod basis since our original objective was developing ability to measure and link critical body burdens to the individualized ASTM E2317-04 microplate standard method.