AME 15:115-125 (1998)  -  doi:10.3354/ame015115

ABSTRACT: The abundance, composition and bacterial colonization of macroscopic organic aggregates (lake snow) were studied from March until November 1993 in Lake Constance, Germany. In addition, the concentration of particulate organic carbon (POC) and the amount, composition, and bacterial colonization of particulate matter (PM) collected in a sediment trap deployed at 50 m were investigated. The seasonal and vertical dynamics of lake snow and of sinking PM were closely linked to those of phytoplankton blooms in spring, summer, and fall and to sedimentation losses of zooplankton during the clear-water phase. POC in lake snow comprised between 0.15 and 28% of total POC in the surrounding water. On the other hand, sinking losses of POC bound in lake snow as estimated from the abundance, POC content and potential sinking rates of lake snow comprised much higher percentages, indicating the significance of these aggregates for the POC sinking flux. Particulate combined amino acids comprised 8 to 51% of lake snow POC and were turned over in <20 h as determined from high rates of aminopeptidase activity. A rapid release of dissolved amino acids from sediment trap material into the surrounding water was shown experimentally indicating that the rapid POM decomposition in the aphotic zone leads to a supply of dissolved organic matter (DOM) to planktonic bacteria at this depth. However, in the same type of experiments we found that dissolved amino acids were taken up by the bacteria associated with the settled material when it was deprived of labile organic substrates. This finding complicates the 1-way concept of bacterially mediated interactions between POM and DOM in aquatic environments.

KEY WORDS: Lake snow · Lake Constance · Sinking flux · POM · Amino acids · Sediment trap · Aminopeptidase · POM decomposition