Microbial communities in streambed sediments recovering from desiccation.

Climate change affects running waters not only by increasing temperatures but also by increasing discharge variability as more frequent and severe floods and more frequent and longer droughts occur, especially in upper reaches. Mediterranean streams are known to experience droughts, but Central European headwaters are also beginning to be affected. The development of bacterial communities (abundance, composition) and the recovery of microbial functions (bacterial production, extracellular enzyme activity) were explored after rewetting desiccated streambed sediments via a sediment core perfusion technique. The bacterial community composition changed only slightly in the sediments from the Central European stream Breitenbach (Germany), but distinctly in the Mediterranean Mulargia River (Sardinia, Italy) during 4 days of experimental rewetting. Breitenbach sediments probably enabled survival of bacterial communities more similar to indigenous streambed communities, because they were less dry. High activity of enzymes involved in polymer degradation at the beginning of rewetting in both sediments indicated the persistence of extracellular enzymes during drought. After 4 days, nearly all microbial activities reached a level similar to unaffected sediments for the Breitenbach, but not for Mulargia. Here, much more intense drying resulted in a more distinct change and reduction of the microbial community, responsible for slower recovery of structure and functions.