Der Stoffhaushalt in Seen: Bedeutung zeitlicher Variabilität und räumlicher Heterogenität von Prozessen sowie des Betrachtungsmaßstabs : Eine Analyse am Beispiel eines eutrophen, geschichteten Sees im Einzugsgebiet der Ostsee (Belauer See, Schleswig-Holstein)
Creators: name: Schernewski, Gerald
Publisher: name: Leibniz-Institut für Ostseeforschung Warnemünde
Year: 1999

description: Meereswissenschaftliche Berichte No 36 1999 - Marine Science Reports No 36 1999
description: Subject of this investigation are Lake Belau and to a minor degree Lake Schmalensee and Lake Bornhöved. These small lakes, with surfaces between 0.73 km² and 1.13 km², a maximum depth between 7.5 m and 26 m and water residence times between 0.3 years and 0.76 years belong to the Bornhöved Lakes Chain, which is of glacial origin and situated in Schleswig-Holstein (North Germany). The stratified eutrophic to hypertrophic lakes (OECD 1982) are linked by the Alte Schwentine and drained into the Baltic Sea.

Goal is an interdisciplinary and integrative analysis of the lake ecosystem from a geographical viewpoint. The analysis of spatial ecological pattern and heterogeneity as well as the temporal variability of processes and their ecologieal meaning, the role of different scales in limnological analysis and last not least aspeets of the representativity of the investigated systems are major objects of this work. Beside this emphasis is put on nutrient fluxes, cycles and budgets, their driving forces and their role in the system.

Compared to the OECD study (OECD 1982), the relationship between chlorophyll-, phosphorus- and nitrogen-concentrations on an average annual basis can be regarded as typical in all lakes. Phosphorus has to be regarded as the main limiting element for primary production in the long-term run. The close molar TN/TP quotient of 15 in Lake Belau indicates a potential high importance of nitrogen as a limiting element. Especially in late summer nitrogen shortages can be observed. The diatom bloom in spring is usually limited by silicon and in winter light limitation dominates. The clear water phase between May and June is top-down controlled by intensive daphnia grazing. In all other seasons trophic interactions play a minor role and nutrients and their availability are most important for the intensity of primary production in Lake Belau. The detailed discussion shows that the traditional simplified picture of one limiting resource per time is questionable. Taking into account the spatio-temporal variability of processes, the maintenance of a non-equilibrium state in a lake by disturbances and handling the resource limitation question as a varying problem in different temporal scale yields a more complex picture. It is very likely that there is more or less always an intermittent or simultaneous co-limitation by several factors.

For the total phosphorus input into Lake Belau, his own small catchment (4.5 km²) plays a minor role, compared to the input by the brook Alte Schwentine and the lake sediment. The annual phosphorus release from the sediment, the internal eutrophication, accounts for 30 % of the total input. Sources are sediment layer down to a sediment depth of 50 cm. These layer possess high P-concentrations due to strang P-load in earlier years. In summer and autumn this release under anoxic conditions is the main phosphorus source for the lake. The increased phosphorus supply during late summer is at least one reason, why nitrogen can become a limiting element at that time. The P-release by internal eutrophication is very close to the annual phosphorus fixation by sedimentation. Therefore the net P-fixation in the sediment is close to zero and the lake has lost its function as a P-sink in the landscape. It is reduced to a P-transformation unit. Simple calculations after VOLLENWEIDER (1976) show that the recent P-load of 16 kg TP ha-1 a-1 is 8-times higher than the critieal load which is sufficient to keep the lake in its eutrophic state. The P-input by internal eutrophication alone is sufficient to keep the lake in that eutrophic state. It causes a slow response ofthe in-lake concentrations on external load reductions. It is very likely, that no realistic nutrient load reduction measure will be able to transfer the lake into a mesotrophic state during the next two decades.

The brook Alte Schwentine is the main source of nitrogen and silicon for Lake Belau as well. Diffuse nitrogen input with the highly polluted ground water gains more and more importance. About 12 % of the nitrogen input is fixed in the sediment. About 36 % are release from the lake by denitrification. Approximately 50 % of the silicon input is fixed in the sediment. Altogether the lake can be seen as an effective sink in the landscape regarding these two elements that helps to reduce the nitrogen load into the N-limited Baltic Sea.

The investigated lakes are small but functional important units in the landscape. One very important question that effects the value of the yielded results is, whether their properties and  hydrochemical development is representative for other lakes in this region and to what degree the results can be generalised. For this purpose automatic classification methods, cluster-analyses, are used on a basis of 68 lakes scattered over the country Schleswig-Holstein. Lake Schmalensee and Lake Bornhöved are characterised by comparatively low P-, N- and POC-concentrations and can morphometrical be described as small, shallow lakes with a catchment area below the average. These properties are in concordance with these of 45 % of all lakes. Especially Lake Bornhöved can be regarded as a good representative of this large group. Due to its relatively high average depth Lake Belau is a less typical lake and representative for only 12 % of all lakes. The classification clearly points out the linkage between a large catchment area and high N-concentrations in the lakes as well as the connection between a high average depth and high P- and POC-concentrations. Between 1983 and 1993 the phosphorus (TDP)-concentrations declined by 40 % in Schleswig-Holsteins's lakes. With 12 % the reduction of the N-concentrations in the lakes were less pronounced. This trend is typical for German lakes in general and Lake Belau reflects these changes very well.

Long-term trends are superimposed and covered by a broad spectrum of temporal variable processes. Two aspects are discussed in detail: the impact of short term disturbances and their meaning for primary production and species diversity as well as the observed strong interannual variability. After a hypothesis by HARRIS (1994) interannual variability is higher in eutrophic than in oligotrophic systems. Two processes, the diatom bloom in spring and the onset of turnover in autumn are identified as key processes for the strong interannual variability of species composition and production in Lake Belau and are used to support this hypothesis.

Spatial organism patchiness, hydrochemical pattern and heterogenous physical processes in Lake Belau are presented and discussed in the light of the scale dependency of hydrophysical/biological interactions and concerning their meaning for species diversity. The 'multiple forces' hypothesis by PINEL-ALLOUL (1995) is tested and generalised.