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Langenargen, Germany

Wessels M.,Institute for Lake Research | Bussmann I.,Alfred Wegener Institute for Polar and Marine Research | Schloemer S.,Federal Institute for Geosciences and Natural Resources | Schluter M.,Alfred Wegener Institute for Polar and Marine Research | Boder V.,HafenCity University Hamburg
Limnology and Oceanography | Year: 2010

Pockmarks are well known in the marine environment but almost unreported from limnic systems. In Lake Constance we investigated pockmarks near the mouth of the main tributary, Alpine Rhine River, using sediment cores and hydroacoustics (sidescan sonar, subbottom profiler, horizontal scanning sonar, multibeam echo sounder). The maximum dimension of an individual pockmark is about 16 m in diameter and 3-4-m deep, often with a round shape. Most pockmarks are located on top of morphological high points such as sediment waves or shoulders of old channels in front of the Alpine Rhine River. They are collapse structures formed when a reservoir of biogenic methane rapidly empties into the water column. Ebullition of gas very often was observed, indicating that biogenic methane still migrates toward pockmarks. © 2010, by the American Society of Limnology and Oceanography, Inc. Source

Lindhorst K.,Leibniz Institute of Marine Science | Vogel H.,University of Cologne | Krastel S.,Leibniz Institute of Marine Science | Wagner B.,University of Cologne | And 5 more authors.
Biogeosciences | Year: 2010

Ancient Lake Ohrid is a steep-sided, oligotrophic, karst lake that was tectonically formed most likely within the Pliocene and often referred to as a hotspot of endemic biodiversity. This study aims on tracing significant lake level fluctuations at Lake Ohrid using high-resolution acoustic data in combination with lithological, geochemical, and chronological information from two sediment cores recovered from sub-aquatic terrace levels at ca. 32 and 60 m water depth. According to our data, significant lake level fluctuations with prominent lowstands of ca. 60 and 35 m below the present water level occurred during Marine Isotope Stage (MIS) 6 and MIS 5, respectively. The effect of these lowstands on biodiversity in most coastal parts of the lake is negligible, due to only small changes in lake surface area, coastline, and habitat. In contrast, biodiversity in shallower areas was more severely affected due to disconnection of today sub-lacustrine springs from the main water body. Multichannel seismic data from deeper parts of the lake clearly image several clinoform structures stacked on top of each other. These stacked clinoforms indicate significantly lower lake levels prior to MIS 6 and a stepwise rise of water level with intermittent stillstands since its existence as water-filled body, which might have caused enhanced expansion of endemic species within Lake Ohrid. © Author(s) 2010. Source

Bussmann I.,Alfred Wegener Institute for Polar and Marine Research | Schlomer S.,Federal Institute for Geosciences and Natural Resources | Schluter M.,Alfred Wegener Institute for Polar and Marine Research | Wessels M.,Institute for Lake Research
Limnology and Oceanography | Year: 2011

In the eastern part of Lake Constance, the second largest prealpine lake in Europe, about 500 pockmarks (morphological depressions on the lake floor) were recently discovered. The diameters of these pockmarks are as large as 16 m, and about 24% of them continuously release methane in the form of visible bubbles. The isotopic composition of the escaping gas indicated that the methane was of biogenic origin and was predominantly produced by the CO2-reduction pathway. In shallow-water pockmarks (9 m and 12 m), pore-water analysis revealed increased methane concentrations of > 1100 μmol L-1 in the sediment. Diffusive methane fluxes and oxidation rates within these shallow pockmarks were much higher than in sediments outside pockmarks. An estimated methane ebullition into the water column of about 40 L h-1 per pockmark showed that methane ebullition is by far the dominant pathway of methane release from these shallow pockmarks. Meiofauna abundance and organic carbon contents of the sediment were also higher in the shallow pockmarks, but there were no differences in macrofauna. In a deep pockmark (80 m), even though bubble release was observed, the sediment inside the pockmark had almost the same methane concentrations and isotopic compositions as a site outside of the pockmark. At both sites, the shallow and deep pockmark act mainly as sedimentation traps, the emanating gas has only minor influence on the methane inventory of the sediments and, therefore, gas bubbles enter the water column essentially unaffected. © 2011, by the American Society of Limnology and Oceanography, Inc. Source

Matter M.,Eawag - Swiss Federal Institute of Aquatic Science and Technology | Anselmetti F.S.,Eawag - Swiss Federal Institute of Aquatic Science and Technology | Jordanoska B.,Hydrobiological Institute Ohrid | Wagner B.,University of Cologne | And 3 more authors.
Biogeosciences | Year: 2010

To date, little is known about the role of spring waters with respect to authigenic carbonate precipitation in a shallow lacustrine setting. Lake Ohrid, located in Southeastern Europe, is a large lake fed to over 50% by karstic springs of which half enter subaquatically and influence significantly its ecology and species distribution. In order to evaluate how sedimentological processes are influenced by such shallow-water springs, the Kališta subaquatic spring area in the north west of Lake Ohrid was investigated by a sidescan sonar survey and with sediment traps and three transects of gravity short cores. Results indicate that sedimentation in the spring area is dominated by authigenic carbonate precipitation. High sedimentation rates and evidences for bio-induced precipitation processes were observed in the water column and in the sediments. Two distinct stratigraphic units characterize the shallow subsurface, both composed of carbonate silts with high carbonate contents of up to 96%, but differing in color, carbonate content and diatom content. A chronological correlation of the cores by radiocarbon dates and 137Cs activities places the transition between the two stratigraphic units after ∼1955 AD. At that time, coastal sedimentation changed drastically to significantly darker sediments with higher contents of organic matter and more abundant diatoms. This change coincides with the recent human impact of littoral eutrophication. © 2010 Author(s). Source

Vogel H.,University of Cologne | Wessels M.,Institute for Lake Research | Albrecht C.,Justus Liebig University | Stich H.-B.,Institute for Lake Research | Wagner B.,University of Cologne
Biogeosciences | Year: 2010

Lake Ohrid is likely of Pliocene age and thus commonly referred to as the oldest existing lake in Europe. In this study spatial variability of recent sediment composition is assessed using >50 basin wide distributed surface sediment samples. Analysis of biogeochemical bulk parameters, selected metals, pigment concentrations as well as grain size distributions revealed a significant spatial heterogeneity in surface sediment composition. It implies that sedimentation in Lake Ohrid is controlled by an interaction of multiple natural and anthropogenic factors and processes. Major factors controlling surface sediment composition are related to differences in geological catchment characteristics, anthropogenic land use, and a counterclockwise rotating surface water current. In some instances processes controlling sediment composition also seem to impact distribution patterns of biodiversity, which suggests a common interaction of processes responsible for both patterns. Source

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