Lake sediments are important carbon sinks because carbon that was once present as CO₂ in the air is bound within organic matter and then deposited on the bottom of the lakes. However, when bacteria or archea “feed” on this carbon, they also consume oxygen – in some cases so much oxygen that it is in short supply for fish. As a result, species of fish that rely on deep spawning areas have died out in over-fertilized lakes.

Type and quantity of organic matter influences the distribution of bacteria

Overall, there is no clear difference in bacterial biodiversity between the lakes. There is, however, a strong variation from lake to lake in terms of the dominant species, which vary according to the conditions in which the sediments were formed. Indeed, the differences stemming from periods of intensive over-fertilization are still in evidence today. It was only in the old, deeper sediment layers deposited prior to eutrophication that the researchers did not encounter this distinction. This observation suggests that all five lakes still had a similar bacterial community structure at that time. In contrast to bacteria, archae communities display variations only in more recent deposits of organic matter.
According to Mark Lever from the Department of Environmental Systems Science at ETH Zurich, who is the study’s corresponding author, this might be an indication of how organic matter is broken down in the sediment: it may be that bacteria perform the first, decisive steps, whereas archaea primarily feed on intermediates produced by the bacteria. These intermediates – formed, for example, by fermentation – are considered to be universal and are therefore less dependent on the composition of the organic matter.
The original publication can be found here.