A main characteristic of the littoral zone of lakes is the increased concentration of organic matter due to autochthonous production (phytoplankton, phytobenthos, macrophytes) and allochthonous inputs of terrestrial origin. Bacteria as the primary users of this organic matter play an important role in its degradation and following conversion into bacterial biomass. The main part of the organic matter present in aquatic ecosystems is found in a polymeric form which cannot be directly absorbed by the cells without previously undergoing enzymatic hydrolisis. For this purpose, bacteria produce specific ectoenzymes and their activity is an indicator of the quality of the substrates undergoing hydrolization. The main purpose of this study was to evaluate the microbial degradation of the different substrates in the littoral zone of Lake Constance (water and sediment) and to compare them with the degradation in pelagic waters. In order to interprete the field results, several experiments were carried out in the laboratory. It was followed the production of different ectoenzymes used in the degradation of the main natural substrates found in the lake. This was done by adding each carbon source to lake samples. The sources were living algae, crustacean zooplankton, and detritus of macrophytes, algae, tree leaves and chitin. Living algae enriched the medium with small carbohydrates released by the cells, while crustacean zooplankton, due to algal grazing, contributed larger carbohydrates of algal origin, and chitin from their exoeskeleton. Microbial communities (bacteria and protozoa) growing on vegetal detritus exhibited a successional composition and a succession of ectoenzymatic activities. First, there was a hydrolisis of small molecules and nonstructural carbohydrates (starch) which was followed by the hydrolisis of structural polysaccharides (cellulose and hemicellulose). This process ocurred more rapidly in the case of the autochtonous substrates (algae and macrophytes) than in allochtonous substrates (leaves).

Higher temperatures and the presence of protozooans in the samples caused an increase in the speed of the succession. Bacterioplankton was present in similar concentrations in both the epilimnion of the pelagic and the littoral waters of the lake. However, the bacteria of the latter exhibited increased activities and production. Bacterial activities in the littoral water also showed a higher range, due to the frequent resuspension of the sediment caused by strong winds. The mixing caused by storms helped to decrease the high heterogenity found in the littoral sediments. The microbial communities present in these sediments showed higher rates of polysaccharide hydrolysis during the period of senescence of macrophytes and higher plants. The annual mean of ectoenzymatic activities were between one and two orders of magnitude higher than those found in the water, however the specific activities (activity per cell) were much lower in the sediment. The microbial communities living in the littoral zone of Lake Constance showed higher activities of hydrolysis of polymeric substrates, presumably due to the higher input of organic matter in this zone.