The Baltic Sea is today one of earth's most endangered semi enclosed seas as a result of increased anthropogenic pressure manifested in e.g. eutrophication, overfishing, biodiversity loss, toxic pollutants and alien species. Eutrophication which causes hypoxia (very low oxygen concentration) in bottom waters is considered globally one of the most severe threats to marine ecosystems. More than 20 % of all identified hypoxic sites worldwide is recorded in the Baltic Sea coastal zone. In the open Baltic Sea, hypoxic events are recorded during three time periods (about 8000-4000 years ago known as the Holocene thermal maximum, 2000-800 years ago in Medieval times, and from AD 1800 up to present) recorded as laminated sequences in sediment indicating the absence of burying organisms. Data on long-term trends of hypoxia from the coastal zone are presently lacking. Thus, there is an urgent need for increased knowledge of the historical extent and driving forces for hypoxia in the coastal zone and a comparison with intermittently occurring hypoxia in the open sea. The impact of humans and climate on eutrophication and hypoxia in the Baltic Sea needs better understanding, especially today when the effect of increased nutrient discharge is superimposed upon the ongoing climate warming.

UPPBASER will disentangle the role of human- and climate-driven processes that have resulted in periods of high productivity and hypoxia in the Baltic Sea during the last 2000 years. Research focus is on the coastal zone, where the first responses to human activities on land can be expected to be registered, and the coupling to the open Baltic Sea, and will be accomplished by focusing on following over-arching objectives:

1. The long-term trends in coastal hypoxia in time and space
- How has the distribution of hypoxia in the coastal zone changed through time?

2. Influence of changes in land-use on coastal waters and interactions between the coast and open Baltic Sea
- How are historically documented changes in land-use recorded in sediments from the coastal zone?
- Is there a synchronicity between coastal areas and the open Baltic Sea in registered environmental changes?

3. The trigger for hypoxia in the coastal zone through time
- Is it possible to separate the relative importance of climate versus nutrients as unique or interactive predictors of environmental change?

4. Background nutrient conditions in the coastal zone as a target for sustainable Baltic Sea management
- How has the nutrients varied through time in the coastal areas?
- Is the choice of 1950 as reference for nutrient conditions for the Baltic Sea scientifically well-founded?
- How could knowledge of the past be used to establish sustainable goals for the management of the Baltic Sea?

Material

UPPBASER will be carried out as a multiproxy study on sediment cores sampled at carefully selected estuaries with suitable geological settings along the Swedish east coast, including Stockholm archipelago which record the run-off from severely industrial impacted areas of Stockholm. Comparison will be made with an open Baltic setting through the availability of the unique high-resolution sediment record from the deepest part of the Baltic Sea, the Landsort Deep 459 m, retrieved within The Integrated Ocean Drilling Program (IODP) Expedition 347.

Methods

Published historical data on changes in human impact onshore (e.g. changes in land-use and population density, technical improvements) will be compared to changes registered in different parameters in well-dated sediment archives of the coastal zone in order to understand the environmental consequences.

Nutrient proxies: Diatoms are an organism group present in the Baltic Sea with highly diverse communities responding quickly to changes in the environment. Diatom-based transfer functions (Molten, Detect and Define web page) will be used to infer and quantify total nitrogen from fossil diatom assemblages in sediment cores. The reconstructed TN will be supported by changes in diatom life-form and biodiversity. Geochemical analyses of total organic carbon content, stable nitrogen and carbon isotopes will be used to trace palaeo-primary production and nitrogen (natural/anthropogenic) and carbon (terrestrial/marine) sources.

Climate proxies: Sediment cores will be analyzed for silica isotope composition from diatom valves which will provide an estimate of historical air and water temperatures. The interpretation of silica isotope analyses will be supported by historical record of ice cover interpreted from ice-living diatoms. Variance partitioning using multivariate analysis will be used on the diatom stratigraphies to find the proportion of variance in the data that can be explained by climate variability through time.