SOS, it's POPs!

Remember we have talked about DDT in the previous post? We are going to shift our focus to the “big family” of DDT which is the Persistent Organic Pollutants (POPs).

POPs are resistant to chemical degradation, giving them long half-lives in the environment. This long-lived organic contaminants biomagnify in food chains and may ultimately cause toxic effects. Biomagnification refers to chemical concentrations is higher in higher trophic level organisms and exceed those concentrations in the organism’s prey.

This is a review on a study investigated the extent to which various organic chemicals biomagnify in lichen, caribou, wolf food chains of Canada’s Arctic tundra region. (KELLY & GOBAS, 2001)

Figure 1 Illustration on the studied food chain

Chemical input to the tundra ecosystem is from atmospheric sources, resulting in gaseous partitioning between air and lichens, particulate (dry) deposition, and wet deposition (e.g., snow). Chemical uptake by lichens can occur from air-lichen equilibrium partitioning when directly exposed to the air, or from water-lichen exchange during exposure to melting snow. Following chemical uptake in lichens, the transport of contaminants in the food chain is controlled by biomagnification processes in caribou and wolves.

The reason to choose these species is because the food web structure in the studied region is approximately linear (lichens à caribou à wolves). Lichens and willow are the dominant food source to caribou and caribou is the dominant food for wolves. Hence, it is more representative to investigate the ability of contaminants to biomagnify in terrestrial food chains.

Concentrations of several hydrophobic organic contaminants in lichens, caribou, and wolves were collected and. Concentrations were then expressed in terms of fugacities and compared by applying the statistical methods. Fugacity is most often regarded as the "escaping tendency" of a chemical from a particular phase. (Appendix P Earthworm Fugacity Estimation )In this study, fugacity is used to express the concentration of chemical.

Figure 2 Sampling Locations

During the spring of 1997 (i.e., May-June), Cl.rangiferina and Ct. nivalis samples were collected east and west of Bathurst Inlet along the Huikitak and Hood Rivers, respectively. During the summer of 1998 (i.e., July), additional samples of Cl. rangiferina and Ct. nivalis east and west of Bathurst Inlet were obtained near the communities of Omingmaktok (east) and Brown Sound (west), while at Cambridge Bay only Ct. nivalis samples were collected.

Figure 3 Fugacities of lichens and willows

Data are presented on log scales. Error bars represent the standard deviation. ND indicates nondetectable concentrations during chemical analysis. An asterisk (*) indicates statistical significance difference (p < 0.05) between mean chemical fugacities in lichens collected in spring (Huikitak and Hood River samples) as compared to lichens collected nearby in summer (Omingmaktok, Brown Sound, and mid-Bathurst samples). Two asterisks (**) indicate a statistically significant difference (p < 0.05) between vegetation species at a given sampling location.

Bioaccumulation in Lichens

The result showed that fugacities of organochlorines such as α-HCH, γ-HCH, and HCB were greater than fugacities of PCBs (congener 153) in vegetation samples at all sampling locations. At some locations, samples of Ct. nivalis, Cl. rangiferina, and S. glauca exhibited statistically different fugacities of α-HCH,  γ-HCH, HCB, and PCBs (e.g., Inuvik, Brown Sound, and Cambridge Bay). There were no statistically significant differences in fugacities of α-HCH, γ-HCH, HCB, and PCBs in lichens and willow leaves collected during the summer. This indicates that the spatial distribution of these contaminants is fairly homogeneous, possibly reflecting atmospheric concentrations. The available data indicate that differences in chemical concentrations among the various food items in the caribou diet are small during summer months.

PCB congeners demonstrated comparable fugacity increases in spring-collected lichens relative to lichens collected during summer. This might be due to accumulation of these substances in spring meltwater and subsequent uptake in lichens. Collection of lichen samples in the spring of 1997 occurred during the spring snowmelt period. Snowpacks contain contaminants scavenged during the previous winter’s snowfall events. Snow sublimation tends to “concentrate” these contaminants in the snowpack; especially for low volatile PCBs evaporate very slowly while the more volatile HCHs and HCB evaporate more rapidly, reducing their degree of snowpack accumulation. During snowmelt events, when snow turns into meltwater, a significant drop in the fugacity capacity may further elevate the fugacity in meltwater over that in the snowpack. 

Figure 4 Fugacities (nPa) of individual compounds from various classes of POPs in food chains

Data are presented on log scales, where bars represent the chemical fugacities in lichens Cl. Rangiferina and Ct. nivalis in summer, caribou during fall (males and females), and wolves during fall (males and females). The left-hand scale is for HCHs and chlorobenzenes (CBz). The right-hand scale is for DDTs, PCBs, and chlordanes. Error bars represent the standard deviation. ND indicates nondetectable concentrations during chemical analysis. An asterisk (*) refers to statistically significant biomagnifications for the lichen-caribou trophic transfer (i.e., f_CARIBOU > f_LICHEN), while two asterisks (**) represent statistically significant biomagnifications for the caribou-wolf transfer (i.e., f_WOLF > f_CARIBOU).

Bioaccumulation in Barren-Ground Caribou:

Predominant compounds detected in caribou tissue samples were α-HCH, β-HCH, 1,2,4,5-TCB, HCB, oxychlordane, p,p’-DDE, and PCB congeners 118, 138, 153, and 180. The fugacities of chlorobenzenes (1,2,4,5-TCB and HCB) and HCHs (α and β isomers) were greater than those of PCB congeners and organochlorine pesticides. Fat concentration data were used to represent the fugacities in caribou for the biomagnification analysis because higher chemical concentrations in fat tissue was tested, which imply smaller analytical error than liver and muscle tissues.

The fugacities of HCHs, DDTs, chlordanes, chlorobenzenes, and PCBs in the tissues of male caribou from Bathurst Inlet were significantly greater in the summer than in the fall. The observed drop in chemical fugacities between the summer and the fall can be explained by increased lipid production and growth. In the Arctic, caribou gain substantial deposits of fat and protein over the short summer period to utilize energy during the winter, resulting in greater body weights in the fall as compared to early summer. This tends to “dilute” the chemical concentration in the fat.

Bioaccumulation in Wolves

α-HCH, β-HCH, 1,2,4,5-TCB, HCB, oxychlordane, and PCB congeners 153, 138, and 180 were the predominant compounds observed in wolf tissues.

Biomagnification of POPs in Lichen-Caribou-Wolf Food Chains:

Hexachlorocyclohexanes (HCHs)

• β-HCH appears to biomagnify in the food chain as indicated by an increase in fugacity with each trophic level. The fugacities of β-HCH in wolves were significantly greater than those in caribou at all three locations.
• No substantial biomagnifcation or trophic dilution of α-HCH was observed. α-HCH fugacities were not statistically different between lichens, caribou, and wolves.
• The fugacity of γ-HCH is shown to decrease with increasing trophic level, indicating trophic dilution, probably due to metabolic transformation.
• Both caribou and wolves can efficiently eliminate γ-HCHand to a smaller extent β-HCH but not α-HCH.

Chlorobenzenes

• No biomagnification of HCB was observed in Cambridge Bay male wolves.
• 1,2,4,5-TCB biomagnified in wolves from Bathurst Inlet and Inuvik but not in wolves from Cambridge Bay
• Food chain bioaccumulation of 1,2,4,5-TCB was only observed at Bathurst Inlet due to nondetectable levels of this compound in either lichens or caribou samples from Cambridge Bay and Inuvik.

PCBs

• fugacities of PCB 153 and 180 increase significantly with increasing trophic level
• fugacities of PCB 52 were not statistically different between trophic levels, suggesting that relative to PCB 153 and 180, PCB 52 is eliminated and/or metabolized efficiently in both caribou and wolves

Chlordanes

• Technical grade chlordane, a pesticide mixture consists mainly of chlordane (cis and trans), heptachlor, and nonachlor (cis and trans) compounds.
• Oxychlordane is the predominant metabolite of chlordane and nonachlor compounds. Hence, extensive biomagnification of oxychlordane in relation to chlordane and nonachlor components indicates metabolic transformation of technical chlordane.
• Oxychlordane biomagnified in the caribou wolf trophic transfer at all three locations while transnonachlor only biomagnified in Bathurst wolves.

DDTs

• the fugacity p,p’-DDT (DDT related compound) in caribou was significantly lower than that in lichens, indicating the ability of caribou to metabolize p,p’-DDT
• the fugacity of p,p’-DDE (DDT related compound) is shown to increase from lichen to caribou at Bathurst Inlet
• data suggest that caribou may have the ability to metabolize or eliminate p,p’-DDT but not the persistent metabolite p,p’-DDE
• Neither p,p’-DDT nor p,p’-DDE biomagnify in wolves, indicating these animals can metabolize both p,p’-DDT and p,p’-DDE

The two important parameters affecting POPs biomagnifications are gastrointestinal absorption efficiencies and lipid-to-air elimination rates in the animals, which are strongly dependent on the chemical’s K_OW and K_OA.

• Octanol-Water Partition Coefficient (K_OW) - A coefficient representing the ratio of the solubility of a compound in octanol (a non-polar solvent) to its solubility in water (a polar solvent). The higher to K_OW, the more non-polar the compound. (Octanol-Water Partition Coefficient (KOW), 2014)
• Octanol-air partition coefficient (K_OA) - The "octanol-air" partition coefficient (K_OA) characterizes POP partitioning between air and organic films. (Erdman, Gusev, & Pavlova, 1999)

Hydrophobic organic chemicals with high K_OW can relatively easily pass through biological membranes via passive diffusion. When absorbed, organic chemicals are quickly distributed within the organisms and partition predominantly in the lipids. (CHAPTER 4 TOXICITY ASSESSMENT)

The significance of a high K_OA for terrestrial biomagnification is that lipid-to-air elimination (through exhalation) is low, causing, in absence of metabolic transformation, a very low depuration rate to counteract uptake from the gastrointestinal tract, hence resulting in high tissue concentrations.

Different animal classes (e.g., herbivores and carnivores) are expected to exhibit differences in these two parameters due to their distinct taxonomic and physiological characteristics.

BMFs for individual animals can differ substantially with differences in gender, age, season, and dietary preference. Biomagnification factors (BMFs) is calculated as fB/fD, where fB is the fugacity in the higher level organism (e.g. wolves) and fD is the fugacity in the diet of the organism (e.g. caribou).

Note: All the information and figures in this post is extracted the journal "Bioaccumulation of Persistent Organic Pollutants in Lichen-Caribou-Wolf Food Chains of Canada’s Central and Western Arctic", unless they are cited with other resources.

Works Cited

Appendix P Earthworm Fugacity Estimation . (n.d.). Retrieved October 17, 2014, from US Environmental Proection Agency: http://www.epa.gov/espp/litstatus/effects/redleg-frog/trifluralin/appendix-p.pdf

CHAPTER 4 TOXICITY ASSESSMENT. (n.d.). Retrieved October 17, 2014, from US Environmental Protection Agency: http://www.epa.gov/oswer/riskassessment/ragse/pdf/chapter4.pdf

Erdman, L., Gusev, A., & Pavlova, N. (1999, April). ATMOSPHERIC INPUT OF PERSISTENT ORGANIC COMPOUNDS TO THE MEDITERRANEAN SEA. Retrieved October 17, 2014, from Meteorological Synthesizing Centre - East: http://www.msceast.org/reports/mediterranean.pdf

KELLY, B. C., & GOBAS, F. A. (2001). Bioaccumulation of Persistent Organic Pollutants in Lichen-Caribou-Wolf Food Chains of Canada’s Central and Western Arctic. ENVIRONMENTAL SCIENCE & TECHNOLOGY , 325-334.

Octanol-Water Partition Coefficient (KOW). (2014, June 2). Retrieved October 17, 2014, from U.S. Geological Survey: http://toxics.usgs.gov/definitions/kow.html