Biogeochemistry is the science of interactions between life and its environment. The term refers to the capacity of biochemical processes to change the abiotic environment in which they take place. The distributions and fluxes of most of the Earth´s elements are affected by biogeochemical processes. Water as the Earth´s ubiquitous solvent carries such elements like carbon, nitrogen, phosphorus, sulphur, oxygen, as well as trace elements, and governs their fluxes and reservoirs on land, in the sea and the atmosphere. All these elements are fundamental for life on land and in the sea. Biogeochemical cycles are thus closely interwoven with the hydrological cycle. These fluxes and reservoirs are expected to vary under climate change conditions with possible impacts on both marine and terrestrial ecosystems, as well as the socio-economy in the Baltic Sea catchment basin.

The expertise of BALTEX in describing the water and energy cycle and the development of coupled regional climate models embracing the atmosphere, the Baltic Sea with sea ice, and the land surface with rivers and lakes, has called for an application in biogeochemical matter flux estimations under changing conditions. The main goals of this research objective are understanding and quantifying biogeochemical processes and fluxes, and their integration into coupled regional climate models.

Improving the understanding of biogeochemical processes in the sea and on land

Biogeochemical processes in aquatic and terrestrial environments are similar in principle, but there are certain differences. For a good estimation of fluxes between land and sea, a good understanding of the different processes on land and in the sea is necessary. Emphasis is put on the biogeochemistry of carbon, nitrogen, phosphorus and oxygen, as those are key elements in the marine and terrestrial biogeochemical cycles. The carbon cycle in the Baltic Sea and in the terrestrial environment receive special attention.

Quantification of biogeochemical fluxes between sea, land and atmosphere

It is important to estmate how climate change will affect the fluxes of biogechemically relevant elements between land (including rivers and lakes), the atmosphere and the sea. An important question is, for example, if eutrophication will increase or decrease in the future. A warmer climate might result in increased eutrophication of the Baltic Sea, owing to a higher flux of nutrients into the Baltic Sea due to a higher runoff. On the other hand, the opposite may happen if we assume that runoff in the southern areas where intensive agriculture prevails will decrease, whereas additional nutrient loads from the northern rivers (where a higher runoff can be expected) may be insignificant. There is no clear answer to this question at the moment. This example demonstrates the necessity to quantify biogeochemical fluxes between land and sea, and between the atmosphere and land and sea.

Integration of biogeochemical components into coupled regional climate models

Coupling biogeochemical models with regional climate models is a major challenge for the coming years. Existing biogeochemical models should be gradually integrated and coupled with hydrological and atmospheric models, to comprehensively simulate the dispersion processes of biogeochemically relevant elements now and under changing climate conditions. With respect to atmospheric transport and deposition, the overall research needs should focus on the adaptation of the most advanced atmospheric chemistry and transport models suited for the simulation of transport, transformations and deposition of air constituents and aerosols on adequate spatial and temporal scales.

Some research questions:

• What is the fate of biogeochemcally relevant elements in the Baltic Sea basin as the hydrological cycle is altered due to climate change?
• How are the biogeochemical processes on land and in the sea affected by climate change?
• How is the atmospheric deposition of elements on sea and land affected by climate change?
• Will climate change lead to an increase or a decrease in eutrophication of the Baltic Sea? What are the regional differences? What are the differences between the elements (N, P, Si)?
• How will the carbon cycle on sea and land be affected by climate change?

The extended "BALTEX Box"

The extended BALTEX Box (compare with water and energy related BALTEX Box here). A simplified illustration of physical and biogeochemical fluxes altered by human impacts ("Anthroposphere"). C = Carbon fluxes (blue arrows); Nu = Nutrient fluxes (primarily N, P), green arrows; Po = Pollutants (heavy metals, persistent organic pollutants), red arrows.

Biological processes in the surface water determine the flux of CO₂ across the sea-atmosphere boundary

Annual cycle of CO₂ flux in the surface waters of the northern Gotland Sea, as measured automatically by the cargo ship FINN-PARTNER in 2005. Shown are the CO₂ values in the atmosphere (green) and in the surface water (blue). The concentrations in surface waters are strongly dependent on biological processes, i.e. respiration (CO₂ release by bacteria and zooplankton) and photosysthesis (CO₂ uptake by phytoplankton).

Throughout the winter, there is little phytoplankton growth due to low light availability, and respiration processes dominate, resulting in high CO₂ concentrations in the water. As soon as the phytoplankton spring bloom starts in March, the CO₂ value in the water drops dramatically. In June there is a stagnation phase with low phytoplankton growth as the spring bloom had taken up all available dissolved nitrogen compounds, resulting in a slight increase in CO₂ . In July, an extensive bloom of blue-green algae (cyanobacteria) cause a second drop in CO₂ values. As the season ends, phytoplankton growth slowly ceases and respiration processes get dominant until the winter values are reached again. Note that the atmospheric values also respond to the biological processes in the surface water, but to a much smaller degree.

(by courtesy of Bernd Schneider, IOW)

Terrestrial ecosystems may not be able to take up as much CO₂ in the future as they do today; they may switch from a "sink" to a "source"

Simulated carbon exchange by ecosystems of the Baltic Sea basin under the current climate (left panel, average of 1961-1990), and a representative regional climate model scenario for 2071-2100. Negative values (blue) represent a net uptake of carbon by ecosystems ("sequestration"); positive values (red) mean a net release to the atmosphere ("emission").

The figure shows that nowadays, there is a slight surplus of carbon uptake by the ecosystems (blue colour); i.e. more carbon is removed from the atmosphere than is released. By the end of the century, however, the situation might be reversed in the southern areas: more carbon will be released than is taken up. This finding may be important as it implies that with increasing temperatures, the ecosystems are less able to remove carbon from the atmosphere, which may thus aggravate the greenhouse effect.

(from BACC, 2008)