Abstract. This review summarizes winter conditions from six polymictic European shallow lakes. The lakes range from oligotrophic to hyper-eutrophic. Four of the lakes freeze regularly while ice cover is absent or rare in the two others. Ice duration and timing of ice-out are significantly influenced by climate signals in three of the lakes. Winter water temperature remains higher in nonice-covered lakes. No long-term trend in temperature is detectable except for one lake where winter water temperature began to increase in 1986. Secchi depth in winter is equal or greater than summer values in all six lakes indicating relatively better light conditions in winter. Total phosphorus concentration in winter ranges from 10 to 130 µg L -1 , which is equal or lower than summer values and is unrelated to chlorophyll a in five of the sites. Phytoplankton species composition during winter differs largely at the six sites. The winter assemblages largely depend on the trophic level and the conditions during the previous season. Winter chlorophyll a and phytoplankton biomass are usually lower than summer values because of reduced photosynthetic rates. Bacterial production often exceeds primary production. Epipelic algal assemblages tend to proliferate during winter in both ice-covered and non-ice-covered lakes. Primary production is low during winter because of insufficient light. Zooplankton abundances and biomass critically depend on conditions during the previous season and the winter situation and are quite variable from year to year, but their values correlate with the trophic status of the lakes. As a result, winter conditions are important to understand seasonal and annual changes in shallow lakes.

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Size related changes of phytoplankton biovolume and species composition have been analysed for forward and reverse regime shifts in a shallow, urban, seepage lake. As a consequence of changes in the hydrology, the pelagic switched from a clear water, macrophyte dominated state to a turbid stable phase with an abundant cyanobacterial population. Experimental nutrient reduction forced the system into a recovery phase. Each state change was associated with significant changes in total biovolume, species composition and size structure affecting surface to volume ratios (S/V). Chlorophyll-a content and S/V drastically increased during the early recovering phase due to small cell sized species developing. As expected, state transitions were associated with significant alterations in size structure and composition.

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The effects of experimentally reduced total phosphorus concentrations (TP) from 2.4 to 0.6 mol L Ϫ1 on plankton community in a shallow lake were evaluated from the proportion between dissolved reactive P (DRP), dissolved nonreactive P (DOP) and particulate organic P (POP), and the stoichiometry among particulate organic carbon (POC), nitrogen (PON), and POP. In both triple ratios, DRP : DOP : POP and POC : PON : POP, we used POP as a key component to indicate shifts between P fractions and between nutrients in particulate organic matter. The enhanced P accumulation by the planktonic assemblage by 14% at reduced P supply was achieved by two steps, in the first year mainly at the expense of DOP and in the second year by DRP. The evidence that this increase of %POP of TP reflects the adaptive P utilization of the organisms under low P supply was substantiated by significant relationships between P fractions and changes in the community. A higher %POP of TP was associated with higher specific alkaline phosphatase activity and with tighter coupling between producers and consumers respectivelythat is, driven from both the nutrient-producer and the producer-consumer interface. A lowered %DOP of TP was significantly related to smaller overall loss rates for the carbon pool of bacteria and algae, whereas %DRP of TP was correlated to the decrease of the biomass of most organism groups. Stoichiometric shifts toward a P-rich assemblage under low TP have been justified by compositional shifts from N-rich cyanobacteria to P-rich eukaryotic algae and bacteria. The shifts toward POP in both triple ratios strongly indicated that the plankton community acted as a sink for phosphorus under reduced total pool size.Ecosystems are regulated by abiotic and biotic factors. The controversial debate over which of the two is more important finally led to the concept of bottom-up and top-down control. These are not mutually exclusive but act alternatively or simultaneously. Abiotic factors such as nutrient supply establish potential biomass increases of producers, whereas food web structure determines actual biomass increase. The concept is complicated by feedback and coupling 1 Corresponding author (Katrin.Teubner@oeaw.ac.at). AcknowledgmentsWe thank Dan Danielopol, Gernot Falkner, Martin Hahn, Alois Herzig, Thomas Posch, and Bettina Sonntag for discussion on adaptation of organisms, ratios, and food chain structure. We are very grateful to Thomas Weisse, Richard Robarts, and two anonymous reviewers for improving an earlier draft of the manuscript.