The influence of temperature and ecosystem dynamics on the partitioning of a Persistent Organic Pollutant (POP) in Antarctic marine food webs

Abstract

Persistent organic pollutants (POPs) are toxic man-made chemicals that do not readily degrade in the natural environment, and may be subject to long-range environmental transport, potentially reaching remote regions far from the emission sources. Chlorinated POPs are hydrophobic and lipophilic chemicals with octanol-water partition coefficients (KOW) typically of the order of 10 4 to 10 7 . This combination of properties means that POPs have a propensity to bioconcentrate and bioaccumulate in living organisms, representing a significant risk to human and environmental health (Wania and Mackay, 1999). The movement of chemicals such as POPs in the environment can be described using the concept of fugacity (Mackay et al., 2009). Fugacity (literally 'fleeing tendency', f ) is an equilibrium criterion related to chemical potential that determines how a chemical will partition between multiple phases (Mackay, 2001). The rate of chemical movement between phases is proportional to the difference in fugacity. A chemical in diffusive equilibrium between two phases is unlikely to have the same concentration in the phases, but it will have the same fugacity. Whilst it is known that high latitude environments can act as repositories for semi-volatile POPs, little is known about the movement and behavior of POPs in Antarctic ecosystems. Polar marine environments are characterised by strong, seasonal variations in light and temperature that affect primary productivity and thereby drive the population dynamics of polar biological systems (De Laender et al., 2010). Spring and summer phytoplankton blooms play a major role in biogeochemical cycling and potentially also in cycling of POPs. Temperature changes also affect the partitioning of POPs in the environment. To provide useful insights into the seasonal behavior and fate of these compounds in Antarctic ecosystems, any model simulation must accommodate the large seasonal plankton biomass changes as well as temperature variation. The approach adopted involved fugacity-based models of the physical and biological environments together with a dynamic plankton food web model. The physical fugacity model and the plankton food web model may be considered sub-models that are coupled by the biological fugacity model (Cropp et al., 2011). This work then describes a mass-conserving, fugacity-based model and its application to investigate the effect of seasonal variations in temperature and light on the movement of POPs in Antarctic physical and plankton systems. The use of a plankton ecosystem model to simulate the movement of POPs requires that a detritus compartment, composed of dead phytoplankton and zooplankton and their respiration products, be included. This is because it provides a pathway for POPs in dead organic matter to exchange with the water. The model is applied to the movement of hexachlorobenzene (HCB), still a prevalent POP in remote regions (Weber and Goerke, 2003). The model reveals that seasonal variations in temperature and plankton biomass have different impacts on the partitioning of POPs in polar environments. It predicts that the burden of HCB in the plankton varies with the seasonal cycle of plankton biomass in Antarctic waters, and that this variation is primarily due to plankton dynamics. Seasonal variations in temperature have little effect on the distribution of HCB in terms of mass, with the exception of detritus in which it generates a subtle seasonal cycle. Temperature variations do affect fluxes between the physical compartments of the model (especially diffusive exchange between water and air, and water and sediment), but have negligible effect on the mass distribution of HCB in these compartments.