Carbon burial and evasion in European lakes and reservoirs:
a contribution to the search for the missing sink
 

Martin Kastowski
PD Dr. Adam Vecsei

Prof. Dr. M. Hinderer (TU Darmstadt)


Introduction

It has become a generally accepted fact that the anthropogenic release of greenhouse gases like CO2 has a significant effect on our climate. Since early industrialization to now the atmospheric CO2 content increased from 280 ppm to over 360 ppm due to burning of fossil fuels and changes in land use. Man has become a global ecological force. Possible consequences are global warming, sea level rise an increasing frequency of climatic extreme events with strong implications for all domains of human life.
Large parts of the biogeochemical process are as yet not well understood. A key to this problem is the global carbon cycle. Estimates have been made for marine and terrestrial carbon budgets. These estimates are still very rough and contain major errors. Anyway, these calculations suggest an unknown carbon sink of ca. 1 Gt/a: the missing sink.
Lakes and artificial water bodies have been neglected in most calculations. Although lakes and reservoirs cover only about 2 % of the land surface, they seem to play a significant role within the global carbon cycle. Lakes and reservoirs have high carbon accumulation rates compared to the ocean and they are naturally oversaturated with CO2 relative to the atmosphere. Thus, they seem to have an ambivalent function as carbon sink and source.

 
map

Fig. 1: Lakes and reservoirs in Europe. Presentation is limited by the resolution of the image and the used dataset.

 

Furthermore the rising numbers of rivers affected by damming necessitate more investigations. About half of all rivers are dammed and supposed 800'000 artificial waterbodies exist worldwide. Most of them are insufficiently documented. Their effects on the water and carbon cycles are still a point of discussion.


Appearance of lakes and reservoirs

In Europe most lakes are of glacial origin and therefore related to Pleistocene ice cover. High density of lakes can be found in Scandinavia, northwest Russia and eastern Germany. Tectonic, volcanic and other origins are minor. Lakes are generally geologically young. Lakes reaching ages of several million years are very rare in Europe.

 
 
Fig.2
Fig.2b
Fig. 2: Different distribution of lakes in European regions: The western alps with large lakes within the foreland (left) and numerous lakes covering the south of Finland. Cut-Out from Landsat satellite images.

 

The distribution of reservoirs depends on demand, political and social circumstances, and geomorphology. The common applications of reservoirs are: irrigation, public water supply, flood control, hydropower, low flow enhancement and fish farming.

 
 
 fig3

Fig. 3: The Lago di Vogorno (Switzerland) features the highest retaining wall
in Europe (220 m).

Reservoirs are numerous in states with semi-arid regions, e.g. Spain. Also in mountainous regions dams are often used for hydropower (Austria, Switzerland). Beside large dams there are countless small artificial waterbodies like fire water ponds which are difficult to quantify.

Carbon in lakes and reservoirs

Carbon in lakes and reservoirs appears in different forms: as Dissolved Inorganic Carbon (DIC), Dissolved Organic Carbon (DOC) and Particulate Organic Carbon (POC).
 
 
 fig. 4

Fig. 4: Simplified model of the main relevant carbon fluxes within a lake.

DOC is generally the dominant species in lakes and is delivered from the catchment area. Major source are carbon rich soils, e.g. peatlands.
The stable phase of DIC in most water is bicarbonate (HCO3-). It is either produced by rock weathering or by decomposition of organic material within the lake or the catchment. About half of the DIC from weathering of carbonate rocks is of atmospheric origin. The rest is derived from dissolution of carbonate minerals. Silicate weathering produces DIC of nearly entirely atmospheric origin.
POC is transported into the lakes through rivers and air. It is also generated autochthonously by primary production within the lake. POC that is not lost by decomposition or outflow is buried in lake sediments and thereby withdrawn from the carbon cycle for a presumable long time. Reservoirs have much higher sedimentation rates compared to lakes and are for that reason of special interest.
Carbon can be stored in lake sediments as carbonate or organic matter. This depends basically on alkalinity, primary production, water hardness and sedimentation rates which are in turn related to other parameters like catchment characteristics or climate.
Carbon is emitted from the lake through the outflow or by degassing. The mainly released gases are CO2 and methane from decomposition of organic matter. The lake surface is generally not in equilibrium but several times oversaturated in CO2 with the atmosphere.
Water transport, degassing and burial make lakes and reservoirs important carbon exchange sites.

Objectives

The aim of the study is to make a reliable estimate for the carbon budget of European lakes and reservoirs.
Thus, a database of relevant facts of European lakes, reservoirs and related catchment areas will be created. This includes all available information on water geochemistry, catchment characteristics, hydrological conditions and sediment composition. Data will be implemented into a GIS and tested for correlations among parameters. These results will be used for extrapolations of carbon burial in and emission from all European lakes, including error assessment.
The result of the study should allow important insights into the role of lakes and reservoirs within the carbon cycle. We intend to continue the study on a global scale at a later date.