Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 7 8 2010 10.5194/bg-7-2567-2010 http://www.biogeosciences.net/7/2567/2010/ http://www.biogeosciences.net/7/2567/2010/bg-7-2567-2010.html http://www.biogeosciences.net/7/2567/2010/bg-7-2567-2010.pdf 2567 2580 2010-08-31 Hypoxia and cyanobacteria blooms - are they really natural features of the late Holocene history of the Baltic Sea? L. Zillén lovisa.zillen@geol.lu.se D. J. Conley Department of Earth and Ecosystem Sciences, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden During the last century (1900s) industrialized forms of agriculture and human activities have caused eutrophication of Baltic Sea waters. As a consequence, the hypoxic zone in the Baltic Sea has increased, especially during the last 50 years, and has caused severe ecosystem disturbance. Climate forcing has been proposed to be responsible for the reported trends in hypoxia (< 2 mg/l O₂) both during the last c. 100 years (since c. 1900 AD) and the Medieval Period. By contrast, investigations of the degree of anthropogenic forcing on the ecosystem on long time-scales (millennial and greater) have not been thoroughly addressed. This paper examines evidence for anthropogenic disturbance of the marine environment beyond the last century through the analysis of the human population growth, technological development and land-use changes in the drainage area. Natural environmental changes, i.e. changes in the morphology and depths of the Baltic basin and the sills, were probably the main driver for large-scale hypoxia during the early Holocene (8000–4000 cal yr BP). We show that hypoxia during the last two millennia has followed the general expansion and contraction trends in Europe and that human perturbation has been an important driver for hypoxia during that time. 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