Pop bioaccumulation in aquatic trophic webs.

Persistent Organic Pollutants (POPs) are chemical substances that persist in the environment for long time since they are resistant to chemical, biological, and photolytic degradation, are long-range transported because they are semi-volatile, bioaccumulate in organisms due to their lipophilicity and have potential adverse effects on wildlife and human health. Starting from the first half of the last century, POPs have been developed and used in a wide range of products and they have been released in the environment through combustion. In the present PhD thesis, attention was posed to some “old generation POPs”: commercial mixtures of PCBs (PolyChlorinated Biphenyls), used as dielectric and hydraulic fluids, and some pesticides and their isomers/metabolites: DDTs (DichloroDiphenylTrichloroethane), HCHs (HexaChlorocycloHexanes) and HCB (HexaChloroBenzene). In the past century, million of tons of these chemicals were released into the environment. Nowadays, their use is banned in many areas of the world, including Italy, even if in certain countries, like in Africa for example, DDT is still widely used to fight against malaria vectors. Lake and marine sediments or the ice of remote areas are important POPs reservoirs which can be considered “chemical time-bombs” since they are not watertight environment and unexpected events can cause POP release with a boomerang effect. The present work was aimed at clarifying some aspects of POP bioaccumulation in the aquatic trophic webs. Although these compounds have been studied a lot in the last decades, there are still some open topics to understand the fate of these pollutants in real ecosystems. Many empirical and mechanistic models of different complexity have been developed since the ‘60s, in order to predict the fate of POPs and their potential for bioaccumulation in the aquatic food webs. However, some problems arise when filed data are used to verify these theoretical models. In this thesis, a bioenergetic steady-state model was applied to the pelagic trophic web of Lake Maggiore, one of the largest and deepest Italian lakes characterized by DDT contamination of industrial origin until 1996. In particular, the seasonal and spatial variability of water concentration and the role of zooplankton in the transfer of POPs were investigated. With this aim, water, zooplankton and fishes of Lake Maggiore were analysed for the determination of DDT content. Although DDT contamination along the water column became rather homogeneous starting from 2007, the hypothesis of a steady-state condition was rejected, probably due to the algal production. An apparent thermodynamic paradox was then found since zooplankton showed DDT concentrations and bioaccumulation factors on lipid basis (BAFL) higher than fishes. Beyond the uncertainty of the Log Kow values, the underestimation of bacteria and phytoplankton capability to bioconcentrate POPs and/or the underestimation of the trophic level of zooplankton, food consumption rates and other kinetic parameters were taken into account to explain the findings of this study. Moreover, the zooplankton species might be more endangered by POPs than predicted by the steady-state model. Concerning Italian deep subalpine lakes, other factors can obstacle the occurrence of a steady-state condition. The influence of secondary pollution sources was tested on two glacier-fed lakes (Como and Iseo), starting from the monitoring of a littoral filter-feeding mollusc (Dreissena polymorpha) and a pelagic fish species (Alosa fallax lacustris). In spite of the Italian DDT ban in 1978, in 2005 a large DDT contamination caused very high concentration in both monitored species. A plausible explanation for this phenomenon was that DDT, widely used from 1950s to 1970s for fruit treatment in valleys just below the glaciers, was trapped in the ice and then was released in the last 2 years as a consequence of the ice melting during the recent glacier retreat. In mussels, levels 150 times higher than those recorded before caused a degeneration of the reproductive apparatus, while concentrations in fish were of concern for human consumption. With the aim of confirming high-altitude glacier melting as the source of the secondary pollution by DDT, past and present contamination was reconstructed by the analyses of POPs stored in a sediment core collected in 2008 in an undisturbed deposition area of Lake Iseo. As expected, it was observed that DDT concentrations sharply increased starting from the mid ninety with a contamination peak in 2005, probably ascribed to the complete overturn of the lake. On the contrary of DDTs, PCBs showed different accumulation profile indicating that these pollutants reached the lake from different pollution sources. Since sediments can not be considered a watertight compartment their role as secondary pollution source can not be ignored. Within the national PRIN project “An Integrated Approach to the Conservation and Management of the European Eel in the Mediterranean Region”, POP contamination in European eel (Anguilla anguilla), a critically endangered species prone to POP bioaccumulation, was evaluated in order to understand the possible causes of its decline. POP bioaccumulation in eel poses concern even for human health because of its commercial importance and its wide distribution in Europe. The POP analysis of eels caught in three Italian coastal waters (Tevere River, Lesina Lagoon and Caprolace Lake) showed a high pollution variability within each subpopulation. Nevertheless, it was clear that Tevere eels were much more polluted by POPs with an industrial origin than the others collected in two coastal lagoons. As concern the risk for eel, levels of dioxine-like PCBs that could cause embryo fish mortality, were found in the river eels, also infested by the exotic swimbladder nematode Anguillicoloides crassus, which could jeopardize their swimming ability. Moreover, in eels collected in the Tevere River gonad alterations not directly related to POP contamination or to parasites were observed. The very low lipid contents measured in some Caprolace eels, probably due to the scarce food availability, could be the cause of an unsuccessful migration to the spawning site. The main conclusions of this study were that eel contamination by POPs seems to be of great concern for the survival both of this species and of its predators and, therefore, for the human consumption. The suitability of Anguilla anguilla as bioaccumulator species for POPs monitoring in European water bodies was also considered indicating the suitability of eels as bioindicators in brackish environments and rivers, with particular attention to the physiological condition of the fish. In the end the occurrence of DDT in equatorial African lakes, notable for the conservation of the endangered lesser flamingos (Phoeniconaias minor), was investigated. The monitoring of DDT pollution was carried out on sediments to evaluate the potential exposition. In spite of the residual usage of DDT in equatorial Africa to fight malaria, the remote areas of lakes Natron and Bogoria in the Rift Valley region were poorly affected by this kind of pollution.