Sediment microbiology and grain-size distribution,as related to tidal movement,during the first mission of the West German Underwater Laboratory “Helgoland”

During the first 3 week mission of the Underwater Laboratory (UWL) Helgoland, July 28 to August 19, 1969, near Helgoland, North Sea, in situ studies were conducted on the bottom water and sediment microflora. The number and species composition of bacteria of the bottom water and sediments was influenced by (a) microtopographical conditions; (b) local changes in tidal currents; (c) artificially introduced changes of the bottom environment. During the UWL experiments comparisons were made of samples taken by conventional methods from R.V. Friedrich Heincke and hand samples taken by aquanauts. The process of hauling aboard to Van-Veen grabs and box samplers invariably changes the structure of the uppermost millimeters of certain sediments. These changes manifest themselves in reduction of bacteria numbers as well as in values obtained for water content and organic matter. Obviously, in ship collections, the uppermost layer of the sediment samples loses bacteria, water and organic matter to the supernatant water and to the lower parts of the sample due to water movement and to consolidation affected by sampling processes. Changes in sediment structure and composition do not cause immediately recordable variations in total numbers of bacteria; however, they are followed by changes in relative abundance of specific groups of bacteria (e.g. sulfate reducers). These facts suggest that sediment transport does not mechanically damage the microflora, but that resulting changes in sediment structure and grain-size distribution are followed by a more or less fast redistribution of the microflora.