Atmospheric contamination and ecological changes inferred from the sediment record of Lacul Negru in the Retezat National Park, Romania

Lacul Negru is a remote lake located in the Carpathian Mountains of Romania, within the Retezat National Park. In 2000, four sediment cores were taken from the deep area of the lake for a multi-pollutant, multi-proxy study to determine temporal trends in pollutant inputs and ecological change. One core was radiometrically dated and the other cores correlated to it by lithostratigraphic means. Analyses included persistent organic pollutants (POPs), trace elements, spheroidal carbonaceous particles (SCPs), carbon (C), nitrogen (N) and sulphur (S), pigments, and the preserved remains of cladocerans and chironomids. Indications of contamination from as early as the 16(th) century occur in the records of zinc (Zn) and cadmium (Cd) and from the 18(th) century for lead (Pb) and mercury (Hg). Temporal agreement between the records of Hg, Pb, arsenic (As) and SCPs suggests recent contamination from coal combustion sources, while the record of pyrolitic polycyclic aromatic hydrocarbons (PAHs) also implicates other industrial sources. The level of contamination in the sediments of Lacul Negru is typically higher than in mountain lakes in Scandinavia, but lower than that found in the Tatra Mountains, the Central Alps and in the UK. Chironomid, cladoceran, elemental (C, N and S) and pigment data all indicate changes in response to an increase in the trophic status of the lake since the beginning of the 20(th) century which may be linked to the introduction of grazing animals between 1920 and 1960. Elemental analysis also points to an increase in autochthonous production while pigment dataindicate an increase in invertebrate grazing pressure within the lake. This seems to correspond with trends in the fish population which, despite repeated attempts at re-stocking in the 1960s, has declined, possibly as a result of limited availability of spawning areas within the lake.