Ecotoxicological evaluation of p,p’-DDE exposure in Antarctic krill (Euphausia superba): towards environmental risk assessment of persistent organic pollutants (POPs) in the Southern Ocean ecosystem

• Main Author: Anita Poulsen
• Format: Thesis
• Language: English
• Published: The University of Queensland, School of Medicine 2012
• Subjects: persistent organic pollutant (POP); Antarctic krill; p,p’-dichlorodiphenyl dichloroethylene (p,p’-DDE); polar ecotoxicology; critical body residue (CBR) approach; early life stage (ELS) testing; developmental toxicity; behavioural toxicity; baseline toxicity; contaminant uptake kinetics; 039901 Environmental Chemistry (incl. Atmospheric Chemistry); 050204 Environmental Impact Assessment; Antarctic; Southern Ocean; The Antarctic; Antarc*; Antarctic Krill; Euphausia superba
• Online Access: https://espace.library.uq.edu.au/view/UQ:274591/s41297483_phd_finalthesis.pdf; https://espace.library.uq.edu.au/view/UQ:274591/s42197483_phd_finalabstract.pdf; https://espace.library.uq.edu.au/view/UQ:274591

Persistent organic pollutants (POPs) are ubiquitous contaminants that reach remote areas far from their emission sources via long range environmental transport. POPs have high bioaccumulation potential in the food web and are toxic to wildlife and humans. The Antarctic ecosystem is a repository for POPs with ongoing reports of POP residues in Antarctic biota since the 1960s. Antarctic krill are a shrimp-like crustacean species that serves as key prey for most predators in the Southern Ocean ecosystem. Despite a suggested biomass decline of up to 80 % in recent decades, no previous research has assessed the toxicological sensitivity of the species to POP exposure. p,p’-dichlorodiphenyl dichloroethylene (p,p’-DDE) is the most stable metabolite of the insecticide DDT, which is used for malaria control. p,p’-DDE has been shown to cause toxic effects including neurotoxicity and endocrine disruption and was recently identified as a priority POP accumulating in Antarctic krill in the wild. This PhD project investigated the uptake kinetics and sublethal toxicity of p,p’-DDE in different life stages of Antarctic krill through a range of exposure scenarios. The aqueous uptake rate determined for p,p’-DDE in Antarctic krill larvae (84 ± 7.6 mL g-1 preserved weight h-1) is comparable to findings for other cold water organisms. For adult krill, the estimated aqueous uptake rate for p,p’-DDE is approximately half of that in larvae and several times lower than the volume based dietary uptake rate (200 ± 0.32 mL g-1 wet weight h-1), which is characterised by the volume of seawater stripped for p,p’-DDE. These findings indicate that diet is important for bioaccumulation of POPs in Antarctic krill, a hypothesis further supported by p,p’-DDE absorption efficiencies of up to 91.3 %. Egestion of p,p’-DDE with faecal matter reached 12 %, indicating that very little elimination took place. The modelled half life of dietary p,p’-DDE in Antarctic krill was 16 years exceeding two life times of the species. These results demonstrate that the majority of the POP load ingested by Antarctic krill is retained for biomagnification in the higher trophic levels of the food chain and that egestion of faecal pellets are important for the vertical cycling of POPs in the Southern Ocean ecosystem. Behavioural endpoints served as indicators of sublethal toxicity for all life stages tested. The observed responses were attributed to baseline toxicity (‘narcosis’), which was induced in Antarctic krill at body residues comparable to those established for small temperate fish. Internal effect concentrations (IECs) for sublethal narcosis in Antarctic krill ranged from 3.9 to 15 mmol/kg lipid weight (l.w.). The intensity of the response varied with body residue and exposure time. Significantly enhanced tail flicking and partial immobility were detected after two hours of exposure. A seven day delay followed from induction of partial immobility to manifestation of complete immobility with a further lag phase to mortality. p,p’-DDE exposure was found to cause accelerated development in the first three larval stages of Antarctic krill. Extended monitoring to the fourth larval stage revealed inhibitory responses including increased mortality and inhibition of metamorphosis up to 16 days after a 5 day exposure period had ended. Non-feeding larval stages of Antarctic krill are naturally compromised by limited lipid reserves. The observed delayed responses may represent indirect effects of exposure such as aggravated exhaustion of energy reserves. As these larval stages only have a brief time window to replenish their energy stores as they reach their first feeding stage, such effects could have serious long term implications (e.g., inhibition of development and/or initiation of feeding). Developmental toxicity was observed after 24 hour exposure at the lowest body residue tested (~ 0.4 mmol/kg l.w.) demonstrating an at least tenfold increase in sensitivity from sublethal narcosis to developmental effects. The effective body residues determined for sublethal toxicity in Antarctic krill were several orders of magnitude higher than environmental levels of single p,p’-DDE (up to 19 nmol/kg l.w.). In the wild, Antarctic krill are exposed to complex mixtures of contaminants that share some of the same modes of toxic action. The ecological risks of pollutant exposure must be viewed in light of the entire chemical load as well as other potential stressors. Further endpoints, compounds, non-chemical stressors and extended exposure scenarios need evaluation to fully understand the toxicological sensitivity of Antarctic krill. The presented threshold data for POP exposure to Antarctic krill indicate that this polar species is of comparable internal sensitivity as temperate species. These observations are in support of the critical body residue approach, which proposes that xenobiotics cause toxicity at similar tissue residues across species and geographical ranges, despite variable environmental conditions. This PhD research represents the first ecotoxicological data for POP exposure to Antarctic krill. The presented information is fundamental for understanding the role of Antarctic krill in POP biogeochemical cycling and food chain transfer and for environmental risk assessment in the Southern Ocean ecosystem.