Seasonal dynamics of pelagic and benthic (littoral and profundal) bacterial abundances and activities in a deep prealpine lake (L. Constance)

Abstract

In order to obtain a first insight into the characteristics of pelagic, littoral and profundal bacterial communities and their potential role for the degradation of organic matter in the deep prealpine Lake Constance, we compared the seasonal dynamics of bacterial abundances and activities during the annual cycle in the epilimnetic water of a central station as well as in surface layers of sediments at a central profundal and a littoral site. For this purpose, bacterial abundances (DAPI counts), rates of leucine incorporation, respiration of 14C-labeled substrates (glucose, phenol) and ectoenzymatic activities were measured biweekly-monthly at the different sampling sites. Bacterial densities in lake sediments generally exceeded those of epilimnetic waters by at least two orders of magnitude with higher values in profundal sediments. A seasonal pattern was well expressed for bacterioplankton, less pronounced for benthic littoral bacteria and hardly visible for profundal bacteria. Compared to pelagic bacteria, benthic bacteria exhibited lower specific rates of leucine incorporation, especially during the warmer periods. For profundal benthic communities specific rates of glucose respiration and of enzyme activities were permanently low with little seasonal fluctuation. In contrast, littoral benthic communities showed the highest values for these specific metabolic activities among the three sites tested. Additionally, the capacity of metabolism of phenol (used here as a representative of more recalcitrant aromatic substances) was by far highest respired for littoral communities, and almost lacking for pelagic communities. Evidence from the literature suggests that the qualitatively and quantitatively enhanced level of degradation capacities observed for littoral bacteriobenthos may be predominantly a result of the combined effect of increased supply of additional organic matter (e.g. allochthonous matter and macrophytes), an enriched bacterial gene reservoir in sediments, and frequent resuspension. Littoral benthic communities showed the highest specific metabolic activities exceeding those of bacterioplankton and profundal bacteriobenthos by an order of magnitude. As a whole, our results indicate that the overall contribution of the littoral area to degradation of organic matter must be comparable to that of the total pelagic water body although it comprises less than 10% of the lake surface