Baltic Sea Coastal Eutrophication in a Thousand Year Perspective

Abstract

Sediment cores from three sites along the east-coast of Sweden, northwestern Baltic Proper, have been studied with respect to lithologies, geochemistry, and diatom assemblages to trace and date early human impact with emphasis on nutrient discharge. The three sites Bråviken, Himmerfjärden, and Ådfjärden, have been impacted to various degree during the last millennia by multiple stressors like excessive nutrient discharge and hazardous substances, leading to coastal hypoxia, eutrophication, and pollution. These stressors are mainly caused by drivers in the drainage area as increased human population, changed land use, and point sources as industries and a sewage treatment plant. Even though their detailed history differs, the results show similar general patterns for all three sites. We find no evidence in our data from the coastal zone supporting the hypothesis that the extensive areal distribution of hypoxia in the open Baltic Sea during the Medieval Climate Anomaly was caused by human impact. Timing of the onset of man-made eutrophication, as identified from δ 15 N and changes in diatom composition, differs between the three sites, reflecting the site specific geography and local environmental histories of these areas. The onset of eutrophication dates to ∼1800 CE in Bråviken and Himmerfjärden areas, and to ∼1900 CE in the less urban area of Ådfjärden. We conclude that the recorded environmental changes during the last centuries are unique in a thousand year perspective.

Climate Change in the Baltic Sea Region: A Summary

“…It had been suggested that agricultural nutrient input was large enough to influence oxygen conditions already during the Medieval Warm Period, perhaps also modulated by changes in river runoff due to the described climate fluctuations (Zillén and Conley, 2010). However, a detailed analysis of new sediments records find little evidence of anthropogenic eutrophication before the industrial period (Norbäck Ivarsson et al, 2019;Ning et al, 2018;van Helmond et al, 2018). In view of the large temperature increases projected for this region in the next decades, further study of the influence of climate on oxygen conditions is warranted.…”

Iron‐Phosphorus Feedbacks Drive Multidecadal Oscillations in Baltic Sea Hypoxia

“…Although enhanced nutrient loading to the Baltic during the MCA has been suggested (Zillén & Conley, 2010 ), little evidence for eutrophication has been found in coastal paleoenvironmental data from this period (Jokinen et al., 2018 ; Ning et al., 2016 ; Norbäck Ivarsson et al., 2019 ). Therefore, we imposed an external loading of 8,000 tonnes P/year for both the MCA HI and HTM HI simulations, as derived from estimates of the preindustrial loading to the Baltic Sea (Gustafsson et al., 2012 ).…”

Iron-phosphorus feedbacks drive multidecadal oscillations in Baltic Sea hypoxia

“…Although enhanced nutrient loading to the Baltic during the MCA has been suggested (Zillén & Conley, 2010), little evidence for eutrophication has been found in coastal paleoenvironmental data from this period (Jokinen et al, 2018;Ning et al, 2016;Norbäck Ivarsson et al, 2019). Therefore, we imposed an external loading of 8,000 tonnes P/year for both the MCA HI and HTM HI simulations, as derived from estimates of the preindustrial loading to the Baltic Sea (Gustafsson et al, 2012).…”