Ursula Gaedke

• In large and deep Lake Constance, total phosphorus concentrations during winter mixing (TPmix) were reduced by a factor of three (> 80 to ca. 30 ;g/l) from about 1979 to 1993. This resulted in an amplification and lengthening of phosphorus (P) depleted conditions throughout the season and water column. The response of the phytoplankton community depended on the time of the year and the level of aggregation under consideration. Total phytoplankton biomass quantified in terms of algal biovolume or chlorophyll concentrations decreased in summer, i. e. during the period of most severe P depletion, to about half of the original values during the first decade. In subsequent years, summer chlorophyll concentrations remained at this lower level whereas total biovolume increased again despite further decreases of TPmix. Average algal biomass in spring and autumn fluctuated without a distinct relationship to TPmix although P was depleted below the detection level during parts of these time intervals in recent years. This moderate response byIn large and deep Lake Constance, total phosphorus concentrations during winter mixing (TPmix) were reduced by a factor of three (> 80 to ca. 30 ;g/l) from about 1979 to 1993. This resulted in an amplification and lengthening of phosphorus (P) depleted conditions throughout the season and water column. The response of the phytoplankton community depended on the time of the year and the level of aggregation under consideration. Total phytoplankton biomass quantified in terms of algal biovolume or chlorophyll concentrations decreased in summer, i. e. during the period of most severe P depletion, to about half of the original values during the first decade. In subsequent years, summer chlorophyll concentrations remained at this lower level whereas total biovolume increased again despite further decreases of TPmix. Average algal biomass in spring and autumn fluctuated without a distinct relationship to TPmix although P was depleted below the detection level during parts of these time intervals in recent years. This moderate response by community level parameters is attributed to changes in the temporal and internal organization of the algal community. Population dynamics and the relative importance of various taxonomical and functional groups such as mixotrophs and less-edible forms clearly changed in spring and summer. The renewed increase in algal biovolume in summer is mostly caused by species which are able to exploit additional P sources. For example, Dinobryon is an evidently mixotrophic organism which ingests P rich bacteria, its strongest competitors for soluble reactive phosphorus (SRP). Ceratium hirundinella might be migrating between the euphotic zone and deeper, P enriched water layers under suitable hydrodynamical conditions. At the level of genera and higher taxa, consistent trends in respect to TPmix were observed in spring and summer mostly indicating an adaptation to more oligotrophic conditions. In contrast, the functional group of well-edible algae showed little interannual variability and did not change in absolute numbers. This suggests that, in contrast to less-edible algae, well-edible forms are more strongly under top-down than bottom-up control, and that the nutritional basis of most herbivores changed less than it would be expected from the decrease in total algal biomass.…