engleski

Pockmarks vs sediments methane emission from a lentic environment

Freshwater lakes account for 6 to 16% of the total emissions of the potent greenhouse gas methane (only 1% from the oceans) and are therefore an important part of the global methane cycle. Freshwater CH4 emissions are enough to offset an estimated ~25% of the terrestrial carbon sink in CO2 equivalents. Within individual lakes, up to 77% of CH4 emissions can come from production in littoral sediments, where warm temperatures and accumulated organic matter (OM) promote methanogen activity and ebullition, where shallow waters and wave action facilitate rapid diffusion. The major part of methane is formed biologically by the activity of methanogenic archaea in anoxic environments, where alternative electron acceptors are lacking. Some methane is lost from the sediments due to ebullition or mixing events, but most of it is readily oxidized by aerobic methanotrophic bacteria when it reaches the oxic biosphere. Pockmarks, concave depressions of the sediment surface of various sizes, depths, and shapes are important methane sources. They generally occur in fine-grained sediments and result from fluid flow from the sediment into the water and are rarely described in lentic sediments. Since organic-rich sediments in lakes and reservoirs are thought to be hot spots of methanogenesis and hence are sources of methane released into the atmosphere, the fate of organic carbon and methane within the sediment is of specific interest. Recently we have detected numerous pockmarks in the Vrana Lake with high organic matter concentration. This lake was formed in a karst-type depression (doline) within karstified carbonate rocks (dolomites and limestones). With a surface area of 5.75 km2 and a catchment basin of 33 km2, it is the largest lake in Croatia. The aim of this study was to identify the bacterial diversity and chemical composition of two deep sediment samples, one in the sandy sediment and the the other one in a pockmark (both at ~3 m deep). The two sites are showing remarkable differences in bacterial community composition along all depths analysed. More than 50% of the bacterial phylotypes identified in the pockmark were belonging to the Chrloroflexi, followed by Acidobacteria and Atribacteria. In the sandy sediment sample, Chrloroflexi accounted for only 25- 40% of the bacterial community, followed by Deltaproteobacteria (20% decreasing with depth) Acidobacteria and Planctomycetes.

methane emission ; bacteria, sediments

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