Distribution of Persistent Organic Pollutants and Mercury in Freshwater Ecosystems Under Changing Climate Conditions - Climate Change Impacts on Freshwater Ecosystems - page 186

Organohalogen compounds and temperature
Many POPs have a vapour pressure that enables a certain level of volatilization.
Hence, they can enter the atmosphere in warm or temperate zones of the planet,
and then condense or accumulate as solids or liquids in cold areas. This
phenomenon, known as the global distillation effect , primarily depends on
temperature (Wania & Mackay 1995).
Variations in temperature due to climatic change can have considerable
influence on the dynamics of these pollutants, affecting their long-range transport,
bioaccumulation, bioavailability, biodegradation and, above all, environmental
persistence and incorporation into trophic chains. For example, the total quantity
of PCBs produced on Earth is about 1.3 million tons, 97% of which have been
produced in the northern hemisphere. The majority of these compounds have
remained trapped in the soil of the zones in which they were produced or used
(i.e. temperate latitudes) (Meijer et al . 2003). Nonetheless, a fraction entered the
atmosphere and subsequently accumulated in cold zones (e.g. Arctic zones).
However, it has recently been observed that this process of accumulation via
condensation affects not only distant zones, but also high mountain regions
(Grimalt et al . 2001; Vives et al . 2004a). In other words, the industrialized
countries have not only exported part of their pollution abroad, but they have
also transferred part of it to what were previously the best-preserved areas of the
industrialized world.
Figure 8.1 shows the distribution of various OCs in fish from European lakes.
It reveals that the most contaminated lakes are those which are furthest from the
sources of the pollution (i.e. cities and factories). This observation might seem like
a paradox: traditionally, dumping of waste to distant zones has been considered
to be associated with dilution, which diminishes the effects on the environment.
In this case, what is observed is a net transfer of pollutants to ecosystems in distant
zones owing to a process of evaporation (i.e. dilution) and subsequent condensation
(i.e. concentration). Hence, the effects of POPs on ecosystems are not diluted;
they simply move from one location (warm) to another (cold).
The same phenomenon is observed for compounds with very different
applications. DDE is present in the environment because it is the main metabolite
of DDT insecticide. PCBs, including the least volatile congeners of the mixtures
(e.g. No. 101, No. 118, No. 153, No. 138 and No. 180), exhibit the same trend
as DDE, and behave differently from the most volatile ones (e.g. No. 28 and
No. 52) (Fig. 8.1). The determining factor in their environmental behaviour is
volatility. Compounds with vapour pressures lower than 10 −2.5 Pa accumulate in
high-mountain zones. Compounds with vapour pressures above this value are
not retained in the range of the annual average temperatures represented by the
mountain lake series but rather in the high latitude areas (Grimalt et al . 2001).
Temperature is also a key factor in the environmental redistribution of the
recently manufactured or used organobromine compounds. This is true for
PBDEs, which were first used upon prohibition of PCBs in the late 1980s. The
distribution of these two types of compounds was studied in a transect of high-
mountain Pyrenean and Tatra lakes, encompassing altitudinal differences of
about 1000 and 550m, respectively. Figure 8.2 reveals that for the Pyrenean