Greenhouse gases from lakes
Berlin water ecologists suspect that the importance of inland waters for the emission of greenhouse gases has been underestimated so far. The World Climate Conference in Kyoto in December will also focus on reducing greenhouse gas emissions. Traditionally, the focus is on the industrial sources. Water ecologists from the Berlin Institute for Water Ecology and Inland Fisheries (IGB) suspect that inland waters are of greater importance than previously assumed. After all, aquatic ecosystems are responsible for 20-40 percent of global methane emissions.
Methane, with its chemical formula CH4, includes carbon dioxide, CO2 and nitrous oxide N2 O among the most important greenhouse gases. Its atmospheric concentration is steadily increasing and is expected to have doubled in 2000 from 1900. It is then about 1.8ppm (parts per million=1 part in a million parts). Since the gas is very effective (approx. 25 times more than carbon dioxide), even the slightest increase in concentration is significant. This necessitates scientific investigations into the emission, function and regulation of the processes in the water and the function of the organisms involved. It is true that rice fields, cotton wool, swamps and flooded areas also release methane and are important because of their enormous extent. However, they are not as suitable as a model for understanding the function and regulation of the processes as long-term, stable habitats in lakes, which also allow insight into the microbial ecology.
Dr. Peter Casper (IGB Department of Limnology Stratified Lakes, Neuglobsow am Stechlinsee) used a six-month research stay in the Department of Microbial Ecology of the Institute of Freshwater Ecology (IFE) in Windermere, England, to study the emission of carbonaceous gases CH4 and CO2 to explore further. With a local team led by Bland J. Finlay, he surveyed the hypertrophic (highly nutrient-loaded) Lake Priest Pot, which was 3.5 acres in area and 12 feet deep. It turned out that this lake released both methane and carbon dioxide into the atmosphere during the entire study period (May-October 1997). The main path of CO2 emission is molecular diffusion across the water/atmosphere interface – unlike methane, which is mainly released via gas bubbles, which can often also be easily observed optically. CO2 reached 39 mmol per square meter per day, 100 times that of methane diffusion (0.39 mmol CH4). In other words, during the summer, more than 12,000 l of gas (or 6.2 kg of carbon as CH4 and CO2) were extracted daily from this 1-ha released into the atmosphere. Inland waters cover an area of 2.5 million square kilometers globally. If only half of this lake area were to release the amount of methane determined in the Priest Pot, this would correspond to approx.50 million tons of methane per year, or almost 8 percent of the annual global CH4 emission (640 million t).
Emissions from inland waters could be reduced by reducing eutrophication. The work in the nutrient-poor Stechlinsee in Brandenburg shows the conditions in an unpolluted body of water. Interestingly, there can be pulsatile release events. When sufficient gas has accumulated in the sediments, falling air pressure allows these bubbles to be released. So found Dr. Casper e.g. at the end of August a 10-fold increase in emissions with an air pressure drop of 10 mbar.
Methane is formed by microorganisms under oxygen-free conditions. In terms of evolutionary history, these microorganisms are very old forms, which is why they are also called archaea. They differ from the eubacteria in a large number of taxonomic properties. The Archaea include above all microbes that live in extreme biotopes and z. B. master extremely high s alt concentrations, pressures or temperatures. Using molecular biological methods, the team was able to detect methane producers in different sediment depths of the Priest Pot Archaea. The Institute for Freshwater Ecology and Inland Fisheries now wants to work within the framework of long-term cooperation with the IFE to identify these organisms and to compare their diversity with that in the lakes of the Stechlin area. Above all, the change in ecological conditions, as they affect the microbial community through the stratification rhythm in our waters, will play a role. Since the emission of greenhouse gases from aquatic ecosystems will also be a focus of the European Commission's 5th framework program for research funding from 1998, Dr. Casper to coordinate a project application in which European working groups work together both quantitatively on emissions from different biotopes and qualitatively on the microbial ecology of the processes and organisms.