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Drei Jahrzehnte kontinuierliche Untersuchungen an der Talsperre Saidenbach: Fakten, Zusammenhänge, Trends. Abschlussbericht zum Projekt „Langzeitstabilität der biologischen Struktur von Talsperren-Ökosystemen“ der Arbeitsgruppe „Limnologie von Talsperren“ der Sächsischen Akademie der Wissenschaften zu Leipzig
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... Retention time varies between about 6 and 12 months. The catchment is mainly agricultural and has a population density of 66 km )2 (for further details, see Horn et al., 2006). Eutrophication probably began before 1970, when investigations began showing continuously increasing phosphorus and nitrogen concentrations (Hofmann, 1971). ...
... In July 1990, detergents containing P were replaced by those without P. Additionally, the agricultural management of the catchment changed considerably, and there was a sudden, dramatic and sustained decrease in P imports from the second half of 1990. The average annual SRP load dropped by 69% in the 1990s compared with the 1980s (from 0.71 g to 0.22 g m )2 per year P), and the mean TP load decreased by 56% (from 2.5 g to 1.1 g m )2 per year P) (Paul, Horn & Horn, 2000;Horn et al., 2006). After the phosphorus loading concept of Vollenweider (1976), the reservoir has been classified as mesotrophic since 1991. ...
... The correlations of annual loads of SRP and TP with Qin are also significant but have changed since 1990. Nitrogen was not considered because of the very high molar NO 3 -N ⁄ PO 4 -P total ratio, ranging from about 200 : 1 in the 1980s to >300 : 1 thereafter (Horn et al., 2006). Frequent substantial fluctuations in water level cause resuspension of sediments, and remobilisation of nutrients is therefore likely. ...
Summary1. Long‐term data on the meteorology, hydrology, physicochemistry and plankton of a reservoir and its tributaries in SE Germany run from 1976 until now. This dimictic reservoir changed from mesotrophic to eutrophic in the 1970s, remained eutrophic in the 1980s and returned to the mesotrophic state after a sharp reduction in P loading in 1990.2. Phytoplankton biomass reaches an annual maximum in spring and consists almost entirely of diatoms. While Asterionella formosa was dominant until 1990, Aulacoseira subarctica became more frequent at the end of the 1990s and was particularly abundant in years with short winters.3. Statistical analyses suggested that these changes were triggered primarily by the mild winters that were frequent after 1988. Climate‐related hydrophysical variables and the initial biomass of the diatoms at the beginning of the year, considered as an ‘inoculum’, were identified as most important. These variables explained 39% of the total variance of the relative abundance, whereas the change in trophic conditions was responsible for about 20%.4. The absolute and relative abundance of A. subarctica was positively related to short ice cover, early ice‐out and a long‐lasting spring circulation. Owing to its physiological traits, and particularly its ability to survive under low‐light conditions, A. subarctica benefitted from short, mild winters. Under such conditions, it could sustain or establish a high initial biomass, whereas the concentrations of the other diatoms decreased over winter. However, this advantage may be lost if further warming causes an early onset of summer stratification. Because of its low population growth rate and requirement for high turbulence, A. subarctica needs long, cold springs to exploit the improved starting conditions and to become abundant.5. In contrast to A. subarctica, A. formosa required a substantial soluble reactive phosphorus supply to compete successfully. The eutrophic conditions until 1990 were the prerequisite for its mass growth under low‐light and low‐temperature conditions during the spring. After reduction in P concentration from 1990, A. formosa declined and other diatom species became more abundant.6. These other diatoms may be viewed as ‘stopgaps’ when conditions were not favourable for A. subarctica or A. formosa. Diatoma elongatum exploited brief circulation periods in years with low P loading. Synedra acus and Fragilaria crotonensis, because of their poor competitive ability at low light intensity, reached high density in the upper water column in the transitional period between spring circulation and summer stratification.7. Our study suggests that climate‐related variables have crucial impacts on the spring phytoplankton dynamics of deep stratified waterbodies. They can mask the consequences of changes in the trophic conditions and, corresponding to the functional traits of the different phytoplankton species, also decisively control their relative abundances. In this reservoir, the warmer winters and prolonged spring circulations did not only lead to high phytoplankton biomass (despite considerably reduced nutrient loads) but also cause a marked shift in the diatom assemblage during the spring bloom.
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