Persistent organic pollutants (POPs) are carbon-based chemicals of anthropogenic origin . Due to their physical-chemical properties (Table 1), these compounds have been used extensively worldwide in agricultural, industrial and domestic applications (for animal, plant and human pest control). These unique properties make these chemicals harmful for organisms and the environment, leading the United Nations Environment Programme to implement the Stockholm Convention on POPs in 2004 to protect human health and the environment . These compounds are characterised by four criteria (Table 1), including persistence, the capacity of long-range environmental transport (LRET), the capacity to bioaccumulate and to elicit toxic effects in organisms, including humans [1,2]. The Stockholm Convention requires rigorous evaluation of chemicals’ properties before including them in its list of POPs (Table 1).
Table 1: Properties of POPs, their definition, and criteria for including a chemical in the list of POPs of the Stockholm Convention (modified from ).
Environmental variables like temperature, wind speed, solar radiation, precipitation and others directly affect POP biogeochemical cycling. In fact, the majority of POPs achieve LRET due to their semi-volatility, which allows worldwide dispersion through transboundary long-range atmospheric transport. These compounds volatilize from soils and water bodies and hence are prone to be transported far from source and use areas. Volatility is temperature-dependent and thus POPs undergo various volatilisation-deposition cycles before being deposited in cold areas  (Figure 1). Antarctica and Southern Ocean are considered a cold trap for them . POPs (both legacy compounds and recently used consumer chemicals) have been found in Antarctica and in the Southern Ocean despite their remoteness and geographical isolation. However, to date published information on POP environmental levels, fate and effects on organisms in the Antarctic Region are still scarce.
POPs have been studied in abiotic and biotic compartments of Antarctic ecosystems and their concentrations vary depending on the area, compartment and time of sampling . The most studied and detected POPs in the Antarctic environments have been polychlorobiphenyls (PCBs), hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCB), dichlorodiphenyltrichloroethane (DDT), and other chlorinated pesticides . POP concentrations in the Antarctic atmosphere were at least one order of magnitude lower than in other regions [6-7], which was also true for snow, sea ice, and seawater as well . Seasonal variations were observed with higher levels in summer than in winter .
The food web has been the most studied focus for POP contamination in Antarctica [9-13]. Antarctic ecosystems are characterized by short food webs and most organisms depend on only a few species, like krill (Figure 2).
POPs are assimilated by organisms at the base of the food webs and are gradually transferred to the next trophic levels (biomagnification [5,11-15]), where they can reach significant levels. Unfortunately, little information has been published so far regarding the concentrations in the primary producers (the base of the food web) . Several studies have been carried out to understand bioaccumulation and/or effects of POPs on Antarctic organisms [13-15]: organisms from extreme cold environments have higher lipid contents than species from temperate or tropical regions (they use lipids for thermal isolation and as energy reserve), thus they can bioaccumulate lipophilic contaminants like POPs; moreover, these organisms frequently show poor detoxifying ability, which makes them more vulnerable to POP accumulation . The Antarctic krill (Euphausia superba) and silverfish (Pleuragramma antarcticum) are key species in the Antarctic marine food-webs and play an important role in the POP transfer [14-16]. Penguins (Pygoscelids) have been used as bioindicators of POPs in Antarctica [5, e.g. 15-16] since these animals can accumulate relatively high concentrations of pollutants due to their trophic position. It is well established that HCB, DDTs, and PCBs are the predominant POPs in Antarctica and concentrations range from a few nanograms per gram in krill up to two orders of magnitude at the upper trophic levels (marine birds and mammals). Moreover, emerging pollutants like flame retardants (e.g. polybrominated diphenyl ethers, PBDEs) have also been detected in biota [10-11,15-17].
Published literature has indicated that concentrations in abiotic compartments are decreasing by a factor of 2 or 3, in agreement with modelling results . As a consequence global change will affect the POP partition and biogeochemistry, especially volatilization, diffusive exchange between water and air, gas-particle partition, and deposition . Moreover, there is evidence that climate change will increase productivity in surface waters  in conjunction with the melting of glaciers . Field measurements have shown that changes in temperature will increase the volatilization of PCBs from soils, results that are in agreement with models showing that volatilization of HCB will be increased by climate change and concentrations in surface waters will decrease [19-20]. However, this increase in the atmospheric concentrations could enhance the diffusive exchange between air and water. Sampling in Antarctica has revealed that primary productivity is the key process that links abiotic and biotic compartment in the fate of POPs . At present it remains unclear if the increase in primary productivity in the sea will increase the transfer through food webs reaching top predators or, on the other hand, there will be an increase in the environmental concentrations in sediments and benthic feeders.
The present paucity of data and lack of monitoring schemes hinder an accurate assessment of contamination levels and temporal trends, yet the more closely observed data from the Arctic have clearly demonstrated how instrumental continuous and coordinated monitoring can be in evaluation of the risks posed by these compounds (e.g. [6,21]). An international monitoring programme could help to provide answers to important questions such as:
a) How POP remobilization due to global change (ice melting , rising temperature) will affect the POP biogeochemical cycling and contaminant transfers through the food web? [4,19] (Figure 3)
b) How the increasing human activities in the region [4,22-23] will affect the POP levels in environmental compartments? At present the POP dispersion (including flame retardants and polycyclic aromatic hydrocarbon) from stations, research, fishing and tourist vessels are unavoidable and are certainly secondary sources [4,23-24].
The Antarctic continent and the Southern Ocean are considered of crucial importance for the global climate, freshwater mass balance, and ecosystem stability including human health. The global change may affect the worldwide distribution of POPs and contribute to their transport to the Antarctic regions, where they can be trapped in ice and snow. With the melting of glaciers, snow, pack ice, and the collapse of the ice shelves , those POPs trapped in the past (and perhaps not still in use, like many legacy POPs) and those recently transported southward (including also new contaminants) can be released into the environment. The influence of climate change on POP remobilization and deposition in Antarctica was already reported  as well as the following increasing concentrations in organisms . Therefore, the study of the influence of climate change on biogeochemical POP cycling, bioaccumulation and effects in Antarctic ecosystems should be of utmost importance. A possible near-future scenario in Antarctica would include an increase of temperature  and contaminant release, and organisms should adapt rapidly to both environmental change and increased POP levels: would they be able to do that over human timescale?