Sediment Evidence of Early Eutrophication and Heavy Metal Pollution of Lake Mälaren, Central Sweden

Abstract

Lake Mälaren is the water supply and recreation area for more than 1 million people in central Sweden and subject to considerable environmental concern. To establish background data for assessments of contemporary levels of trophy and heavy metal pollution, sediment cores from the lake were analyzed. Diatom-inferred lake-water phosphorus concentrations suggest that pre-20th century nutrient levels in Södra Björkfjärden, a basin in the eastern part of Mälaren, were higher (c. 10-20 micrograms TP L-1) than previously assumed (c. 6 micrograms TP L-1). Stable lead isotope and lead concentration analyses from 3 basins (S. Björkfjärden, Gisselfjärden and Asköfjärden) show that the lake was polluted in the 19th century and earlier from extensive metal production and processing in the catchment, particularly in the Bergslagen region. The lake has experienced a substantial improvement of the lead pollution situation in the 20th century following closure of the mining and metal industry. The lead pollution from the old mining industry was large compared to late-20th century pollution from car emissions, burning of fossil fuels and modern industries.

Distribution and retention of effluent nitrogen in surface sediments of a coastal bay

“…Further, the absence of a defined gradient in sediments deposited prior to ca. 1970, despite elevated ␦ 15 N in L. Mälaren at that time (Renberg et al 2001), indicates that present-day gradients in N isotopes arise from effluent inputs since the initiation of the STP. Thus, we assume that the isotopic gradient in Himmerfjärden primarily reflected a decreasing contribution of effluent N seaward.…”

“…The influence of effluent N was most pronounced in the innermost basins, with sedimentary ␦ 15 N values generally above 7‰ within 12 km of the outfall ( Fig. 2A) (Renberg et al 2001). Thus, we cannot rule out the possibility that some transfer of N from L. Mälaren has contributed to the enriched N in sediments near the inlet, although the overall trend indicates that this is only a localized effect.…”

“…1970 (pre-STP) revealed that ␦ 15 N values varied little among sites located at Ϫ5 km (ϳ5.2‰), Ϫ2 km (ϳ6.0‰), and 7.5 km (5.7‰) (Savage unpubl. data), despite signals of ϳ8‰ in inflowing water from Lake Mälaren (Renberg et al 2001). …”

Saving the Baltic Sea, the Inland Waters of Its Drainage Basin, or Both? Spatial Perspectives on Reducing P-Loads in Eastern Sweden

“…In 2012 wheat, barley, and oat were the dominating crops; however, 33 % of arable land is used for haymaking and pasture, and approximate 10 % lie fallow. Diatom analyses have shown that the lake Mälaren was culturally eutrophicated already in the medieval time (Renberg et al 2001). Phosphorus has been monitored in Lake Mälaren since the 1960s, and the monitoring shows that the introduction of chemical precipitation to the sewage water treatment process led to large improvements in the 1960s and 1970s, reducing the P input to the lake by 60 %.…”

Determination of the historical variation of the trophic state in lakes using sediment stratigraphies

“…The content of the isotope Cs-137 was established in core 3. The history of SCP presence in sediments is established by Goldberg (1975, 1979), Wik (1992), Odgaard (1993), Rose et al (1990Rose et al ( , 1995Rose et al ( , 1999Rose et al ( , 2001), Bindler et al (2002) and Renberg et al (2001), on which the dating of modern sediments in Karlskärsviken is based.…”