Tracking the history of Baltic Sea hypoxia with bivalve shells

• Applicant: Professor Dr. Bernd R. Schöne
• Subject Area: Palaeontology ; Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions ; Oceanography
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 406557478

Bottom hypoxia (less than 2 mg/L dissolved oxygen, DO) is a growing environmental problem in coastal and offshore waters. Severe and prolonged low-oxygen conditions can be lethal for bottom dwellers, destroy benthic communities, cause ecosystem disturbances and eventually form dead zones. The spatial distribution, intensity and duration of hypoxia in the Baltic Sea, home of the largest oxygen-deficient zone, has increased strikingly during the last century, both in shallow nearshore and deeper offshore settings. Due to lacking data, the occurrence of coastal hypoxia in time and space is much less well characterized than in deeper waters. Yet, the main unresolved question concerns the causes for the drastic expansion of hypoxia in the Baltic Sea: How important are physical forcings (stratification) and nutrient-driven eutrophication? A strong halocline preventing downward-mixing of oxygenated surface waters is controlled by climate and maintained by riverine freshwater influx and precipitation as well as saline inflows from the North Sea. Conversely, anthropogenic nutrient input has greatly stimulated primary production and oxygen consumption by decomposing organic matter. To obtain a mechanistic understanding of these complex processes requires long, high-resolution records. Such data are currently not available. In fact, long-term monitoring data sets of DO levels are lacking, and knowledge of the history of Baltic Sea hypoxia (prior to the instrumental era) relies almost exclusively on proxies archived in laminated sediments. While the latter can provide unique insights into long-term DO trends of, they come with poor temporal resolution, are not available from shallow settings and do not permit to quantify DO levels.The main objective of the planned project is to establish a virtually untapped archive for hypoxia and eutrophic state, namely shells of long-lived bivalves. We will develop a multiproxy approach for the reconstruction of DO (Mn/Ca, Fe/Ca, I/Ca, delta34S) and apply this technique to infer the Late Holocene history of hypoxia in selected regions of the Baltic Sea with unprecedented temporal resolution. For this purpose, shells of live-collected and subfossil specimens of two long-lived bivalve species will be used, namely Arctica islandica and Astarte borealis. Master chronologies will be constructed that provide uninterrupted seasonally to inter-annually resolved information on DO levels during the last 200 years or so. In addition, radiocarbon (AMS) dated subfossil shells will be used to open windows into environmental/climate baseline conditions. In order to disentangle the relative importance of physical forcings and anthropogenic eutrophication on the drastic expansion of the hypoxic zone in recent decades, we will also develop and apply proxies for the eutrophic state (delta15N, P/Ca). Results of the planned study can be transferred to other oceanic settings and will thus significantly advance marine ecology research.

• DFG Programme: Research Grants