WaterSubject

After 1990, external loads of Central European rivers with inorganic nutrients (nitrogen and phosphorus) and organic material were reduced because of changed environmental laws. However, in the eutrophic lowland River Warnow, North-Eastern Germany, nitrate concentrations remained high with 35–185 ?mol l^?1 without a significant decrease since 1992. In contrast, phosphate concentrations, varying between 0.3 and 5.2 ?mol l^?1 during the growth season 2002, decreased significantly over the years. However, its concentrations still exceeded 1 ?mol l^?1 regularly in the growth seasons. This load led to a substantial accumulation of organic matter additional to high terrestrial inputs. Despite the high organic load, the remineralising bacteria were mainly inactive in River Warnow. Therefore, the composition of the dissolved organic material, especially its bioavailability, were investigated during the growth season 2002 and discussed with other potential controlling factors. River Warnow carried a high load of dissolved organic carbon (14 mg l^?1), especially of humic substances (5.5 mg C l^?1). Bacterial abundance (12×10⁶ ml^?1) as well as production (1.7 ?g C l^?1 h^?1) depended on temperature. During late spring and summer at constantly higher temperatures, bacterial production correlated positively to readily utilisable substrates and to humic compounds. Thus, the bacterial community in River Warnow may be well adapted or contain enough species using the available amino acids and carbohydrates as well as humic matter compounds. However, calculated from protozoan biomass, grazing may control bacterial biomass and perhaps community composition profoundly, what lead to the low percentages of active bacteria.Many aquatic systems in Central Europe were exposed to continuous eutrophication for more than 30 years from the 1950s. Thus, even large systems, like Lake Constance, changed from an oligotrophic (1920–1924) to a mesotrophic (1952–1962) and finally to a meso/eutrophic status (1979–1995). Although especially the phosphorus load was reduced considerably since the early 1980s, the concentrations of particulate organic material and phytoplankton biomass did not decrease likewise.

Plant nutrients cause a massive phytoplankton biomass, which represents an important autochthonous particulate organic matter (POM) source. This load has to be remineralised in the pelagial, after sedimentation or in case of rivers after its export into the downstream regions. Furthermore, algae exude dissolved organic matter (DOM) into the water. In some aquatic food webs, about 50% of the photosynthetically fixed carbon is channelled via DOM to the bacterioplankton. Rivers are additionally loaded by high amounts of allochthonous material, such as leaf litter, leachate from soil or from peat bogs. This terrestrially derived organic matter consists predominantly of highly refractory material, like humic substances (HS), which are biologically difficult to degrade.

Bacteria are the most important remineralisers of organic matter to plant nutrients (inorganic nitrogen and phosphorus) and carbon dioxide. Factors controlling their abundance and productivity are of major importance for the prognosis of organic matter removal from aquatic systems, i.e. the so-called self-purification potential. Temperature is one of the most important factors controlling bacterial abundance and production. UV radiationcan also inhibit bacterial activity. Bacteria and their production correlated often to phytoplankton biomass, because the latter is an important source of readily utilisable dissolved organic material (uDOM). However, if inorganic nutrients, like nitrate, ammonium or phosphate, become limiting or the (allochthonous) organic matter is already N- and P-depleted, bacterial growth becomes limited inspite of sufficient organic carbon substrates. In contrast to these bottom-up control factors, bacteria are also top down controlled by grazing of protozoa or virus lysis. Especially rapidly growing bacteria are reduced by grazers and replicate viruses, which may lyse them.

Of course, the remineralisation rate of DOM must be highly dependent on its composition. Dissolved low molecular weight organic substances can be taken up by most bacterial species and are defined, therefore, as uDOM. Dissolved free amino acids (DFAA) were often the main substrates for bacteria. Dissolved free carbohydrates (DFCHO) were also important substrates, but an additional nitrogen source is needed for bacterial growth. However, the portion of uDOM is often very low, rapidly turned over and, therefore, limiting bacteria. Other substrate groups of the DOM pool, as dissolved combined amino acids (DCAA), can also be used after enzymatical hydrolysis. HS are much more resistant to bacterial degradation especially because of their lower nitrogen and phosphorus content. However, there may be a high portion of amino acids “bound loosely” or being adsorbed to humic matter that can be utilised by bacteria. Several bacterial species can also degrade humic matter itself. However, detailed information on DOM composition, especially uDOM, in (eutrophic) rivers is scarce, so that the fate of DOM components and their degradation rates are hard to estimate.

The slow-running lowland River Warnow has a rather high water residence time giving it properties of lakes and, thus, also a higher resistance to remesotrophication. River Warnow is situated in North-Eastern Germany and drains a 3224 km² catchment area of mainly agricultural land. At the end of the 1980s, it was heavily eutrophicated. At the beginning of the 1990s, the use of phosphorus in detergents was reduced in Germany, sewage plants were improved and agricultural land use became more regulated.

A first aim of the present study was to evaluate a decade of nutrient concentrations (1992–2002) for statistically significant downwards trends following these reduced loads. This evaluation will not only show, if the restoration and protection measures took effect, but define the degree of the lake character of the river. Additionally, the natural and the anthropogenically introduced DOM loads are very high representing a burden for the adjacent Baltic Sea as well as for the use of the river as a drinking water source. Additionally, the bacteria responsible for the DOM remineralisation were mainly inactive. Because bacterial inactivity might be caused by a poor bioavailability of DOM, the composition of DOM, especially the uDOM compounds, its effect on bacterial abundance and production were investigated in the present study. This is the first comprehensive DOM description in River Warnow after the intensive eutrophication period, whereas older data refer predominantly to HS. The main aim of this study was to discuss if the high portion of dissolved refractory material is directly responsible for a low bacterial activity or if there are enough uDOM compounds fuelling bacterial metabolism. Other limiting factors, such as N and P availability as well as temperature, are considered. The limiting factor for bacterial substrate utilisation strongly influences the removal of organic carbon from aquatic systems and, thus, controls the self-purification potential of this eutrophicated river.