Baltic Earth Working Group on Biogeochemistry of the Baltic Sea – Linking observations and modelling

• How do physical, chemical, and biological processes interact to regulate the biogeochemistry of the
  Baltic Sea across spatial and temporal scales?
• How can observational and modelling approaches be integrated to better quantify and predict
  biogeochemical cycles under changing climate and nutrient-load scenarios?
• Which environmental drivers and feedbacks control key processes such as oxygen consumption,
  remineralisation, and organic matter burial?

The Baltic Sea is a complex and highly heterogeneous ecosystem shaped by its unique hydrographic setting, including large riverine freshwater input, rare intrusions of saline water from the North Sea, and persistent vertical stratification. These features, combined with historically high external nutrient loads – peaking between 1975 and 2000 – have led to widespread eutrophication. The combination of both water column stratification and eutrophication resulted in the expansion of the hypoxic and anoxic areas in the central Baltic, starting already in the 1950s–1960s. This has caused redox alterations in the nutrient cycles promoting denitrification and increasing phosphorus recycling or reducing its burial efficiency. On top of that, there is also an influence of ongoing climate change, exemplified in temperature increase, which in the Baltic Sea region is higher than the global mean, and in changes in the precipitation patterns. Although the major biogeochemical processes driving the elemental cycles of carbon, nitrogen, phosphorus, and oxygen are well established at a conceptual level, the detailed interactions among environmental forcing, biological communities, and physical dynamics remain insufficiently understood. Furthermore, the occurrence of strong biogeochemical gradients, both vertical, from air-sea interface through the stratified water column to sediments, and horizontal, along the land-ocean continuum, makes deciphering the role of individual biogeochemical processes even more challenging and requires the application of multiple drivers approach in research. Quantification of the regulatory effects that salinity, temperature, light, availability of oxygen, concentrations of nutrients and trace elements (including pollutants), biological community composition, and food web dynamics have on rates of important biogeochemical processes is fundamental for properly calibrating coupled hydrodynamic–biogeochemical models, which are the only tools to track the large-scale changes in the ecosystem and predict its future development under various climate and nutrient input scenarios. This can be done only by integrating and harmonising numerical studies with observations, as synergistic activities from both fields may significantly enhance the capacity to close existing knowledge gaps. Several important knowledge gaps remain. A clearer understanding is needed of how C:N:P stoichiometry evolves during spring and summer blooms and how this variability influences broader ecosystem functioning. Oxygen dynamics in deep-water layers also remain insufficiently quantified, including the magnitude of sinks and sources and the drivers controlling the expansion and contraction of hypoxic and anoxic zones. The processes underlying the decoupling between alkalinity and salinity – and its implications for ocean acidification – are still poorly constrained. The role of dissolved organic matter, particularly terrigenous inputs, in nutrient cycling and its transformation within the Baltic Sea is also not well understood. Furthermore, the spatiotemporal variability of organic-matter remineralisation and burial in sediments requires deeper investigation, including the extent to which these processes are shaped by changes in microbial communities and benthic fauna. Finally, interactions between contaminants and biogeochemical processes, including the combined effects of contaminant mixtures, remain largely unexplored.

Addressing existing knowledge gaps requires efforts beyond the capabilities of individual research groups, scientific disciplines, or research projects. Therefore, collaborative activities are needed to coordinate the enlargement and harmonization of data sets and improve the parameterization of biogeochemical processes. Achieving these goals can be facilitated by regular communication of needs and opportunities in both directions between modelers and experimentalists to build and continuously update a research strategy. This does not only concern the traditionally understood field of marine biogeochemistry but requires pursuing a more holistic approach, combining representatives of various scientific disciplines. This ambitious vision is the basis of the Baltic Earth working group, which will become an interdisciplinary platform enabling an efficient combination of numerical research and observations aimed at better understanding the biogeochemical functioning of the Baltic Sea across different spatial and temporal scales.