The role of diazotrophic cyanobacterial blooms in the expansion of bottom water hypoxia in the Holocene and late Weichselian Baltic Sea

• Applicants: Professor Dr. Thorsten Bauersachs; Professor Dr. Lorenz Schwark
• Subject Area: Palaeontology
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 261181348

• Final Report Year: 2020

Final Report Abstract

The modern Baltic Sea experiences massive blooms of N 2-fixing heterocytous cyanobacteria. These blooms facilitate the spread of hypoxic bottom waters due to the massive export or organic matter and its subsequent decay, rendering one-third of the Baltic Sea an ecological “dead zone”. Anthropogenic loading of nutrients (in particular phosphorous) is commonly considered the main reason for the proliferation of N2-fixing cyanobacteria in the Baltic Sea over the last decades. Other factors that may facilitate cyanobacterial bloom formation, however, are only poorly constrained. In this study, we analyzed sediments recovered during IODP Expedition 347: “Baltic Sea Paleoenvironment” to study the deglacial history of cyanobacterial blooms in the Baltic Sea. Bulk-geochemical, isotope and lipid biomarker data indicate that the postglacial Baltic Sea was a low productivity environment with a predominantly oxygenated water column. With the establishment of brackish conditions about 8000 years ago, the Baltic Sea repeatedly experienced blooms of heterocytous cyanobacteria that were tightly coupled to a spread of bottom water anoxia. All time periods of enhanced bloom formation were associated with an increase in surface water temperatures above 15 °C, suggesting that climate warming exerts a major control on the formation of cyanobacterial blooms in the Baltic Sea. Our results thus indicate that the combined effect of nutrient over-enrichment and increasing surface water temperatures may result in a significant intensification of cyanobacterial bloom events in the future Baltic Sea.

Publications

• (2015) IODP expedition 347: Baltic Sea basin paleoenvironment and biosphere. Scientific Drilling 20, 1-12
  Andrén, T.; Barker Jørgensen, B.; Cotterill, C. & Green, S.
  
• (2017) A multi-proxy study of climate and environmental change from the Bornholm Basin (Baltic Sea). 28 th International Meeting on Organic Geochemistry. Prague, Czech Republic
  Lorbeer, N., Schwark, L., Bauersachs, T.
  
• (2017) Climate change led to the formation of massive cyanobacterial blooms in the Holocene Baltic Sea. 28th International Meeting on Organic Geochemistry. Prague, Czech Republic
  Bauersachs, T., Lorbeer, N., Schwark, L.
  
• (2017) Lipid biomarker signatures as tracers for harmful cyanobacterial blooms in the Baltic Sea. PLoS ONE 12, e0186360
  Bauersachs, Thorsten; Talbot, Helen M.; Sidgwick, Frances; Sivonen, Kaarina & Schwark, Lorenz
  
• (2017) Reconstructing Holocene temperature and salinity variations in the western Baltic Sea region: A multi-proxy comparison from the Little Belt (IODP Expedition 347, Site M0059). Biogeosciences 14, 5607-5632
  Kotthoff, Ulrich; Groeneveld, Jeroen; Ash, Jeanine L.; Fanget, Anne-Sophie; Krupinski, Nadine Quintana; Peyron, Odile; Stepanova, Anna; Warnock, Jonathan; Van Helmond, Niels A. G. M.; Passey, Benjamin H.; Clausen, Ole Rønø; Bennike, Ole; Andrén, Elinor; Granoszewski, Wojciech; Andrén, Thomas; Filipsson, Helena L.; Seidenkrantz, Marit-Solveig; Slomp, Caroline P. & Bauersachs, Thorsten
  
• (2018) Holocene history of the Ångermanälven Estuary. Boreas 47, 593-608
  Warnock, Jonathan P.; Bauersachs, Thorsten; Kotthoff, Ulrich; Brandt, Hauke‐Tom & Andrén, Elinor
  
• (2018) Climate warming and its effect on the formation of diazotrophic cyanobacterial blooms in the Baltic Sea. 13 th European Nitrogen Fixation Conference. Stockholm, Sweden
  Bauersachs, T., Foster, R., Lorbeer, N., Schwark, L.
  
• (2018) Controls on the onset and termination of past hypoxia in the Baltic Sea. Palaeogeography, Palaeoclimatology, Palaeoecology 490, 347-354
  Papadomanolaki, Nina M.; Dijkstra, Nikki; van Helmond, Niels A.G.M.; Hagens, Mathilde; Bauersachs, Thorsten; Kotthoff, Ulrich; Sangiorgi, Francesca & Slomp, Caroline P.
  
• (2019) Cyanobacteria and Anoxia in the Baltic Sea: High-resolution community profiling. 29th International Meeting on Organic Geochemistry. Gothenburg, Sweden
  Bauersachs, T.; Lorbeer, N.; Dieck, P. & Schwark, L.