Interactions between microbial degradation of sedimentary organic matter and lake hydrodynamics in shallow water bodies: insights from Lake Sarbsko (northern Poland)

In this paper we demonstrate spatial and seasonal changes in the pathways of microbial decomposition of organic matter within the surface sediments of Lake Sarbsko, a coastal water body located on the middle Polish-Baltic coast. We studied lake waters and bottom sediments at 11 sampling stations throughout the basin and in different seasons between November 2007 and September 2008. It was established that, in this very productive and shallow lake, microbial activity increases in warmer seasons and ceases during winter. In spring, bacterial activity is fuelled by increased influx of highly reactive planktonic organic matter, which is decomposed via methanogenesis, reduction of NO3 -, SO4 2-, and Fe and Mn oxides. On the other hand, during summer, oxidation processes (mainly oxidation of CH4) tend to predominate. The change from reduction to oxidation is attributed to wind-induced vertical mixing of Lake Sarbsko waters and resuspension of bottom deposits. Degradation of sedimentary organic matter in Lake Sarbsko results in appreciable changes in the pH and the concentrations of red-ox sensitive ions in pore waters, but it has little effect on the chemistry of bottom and surface waters. However, release of PO4 3- from the sediments might be a source of this nutrient in the lake. Internal loading of phosphates in Lake Sarbsko occurs under both oxic/mildly reducing and anoxic conditions.