The contribution of anthropogenic activities on hypoxia in the Stockholm Archipelago during the past two millenia

Abstract

Hypoxia (O2 < 2 mg L-1) is spreading in coastal zones worldwide and especially in the Baltic Sea, which has been identified as the biggest aquatic dead zone in the world. This has been destructive for ecosystem services like fisheries, biodiversity and recreation, emphasizing the need for solutions to reduce the hypoxic zones in the Baltic Sea. The cause of hypoxia in the Baltic Sea is a complex interplay between climate, geomorphology/hydrology and human activity. To better understand the causes and consequences of hypoxia in the Baltic Sea it is necessary to disentangle the effects of these different actors which can be done by studying how past hypoxic periods have developed in relation to these actors. We use marine and terrestrial palynology (i.e. dinoflagellate cysts and pollen/spore assemblages), existing sedimentary geochemical records and known archaeological and historical sources to investigate the interactions between human population pressure, eutrophication and the development of hypoxia during the past two millennia for a densely populated coastal area in central Sweden: Erstaviken in the Stockholm Archipelago. Our pollen record indicates that agricultural land use is first evident at c. 400 anno Domini (AD) and clearly matches European trends of population increase during the Medieval Expansion (c. 1000-1300 AD) and decline during the plague epidemics (c.1350-1450 AD). The accumulation rates and relative abundances of heterotrophic/autotrophic dinocysts in our record indicate that economic activities during the Viking Age and population growth in the Medieval period did not have a marked effect on hypoxia. Based on our results we conclude that not human activity but geohydrological changes resulting from isostatic uplift of the basin and climate (i.e. temperature) have predominantly controlled hypoxia in the Stockholm Archipelago before preindustrial times.