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Logistic regression was used to analyse the relationship between six submerged macrophyte taxa (Chara spp., Potamogeton perfoliatus, Potamogeton pectinatus, Potamogeton pusillus, Myriophyllum spicatum, Alisma gramineum and sum of all species) and four environmental variables (turbidity, effective wind fetch, water depth and sediment silt percentage, including interactions and some quadratic terms). The models were based on intensive vegetation samples (total c. 72 000) and other monitoring data carried out in five Dutch shallow lakes in the IJsselmeer area from 1988 to 1998. Water depth and light extinction were the most important factors determining the occurrence of all studied species in Veluwemeer, while effective wind fetch had a moderate effect and sediment silt had only a minor effect on the occurrence. Water depth had a negative impact on all species, except A. gramineum, which showed an optimum response. Three species showed an optimum response at intermediate turbidity (M. spicatum, P. pusillus and P. pectinatus), whereas the other taxa were negatively related. Three species (Chara spp., P. perfoliatus and M. spicatum) were positively related to wind fetch or showed an optimum response at intermediate value, whereas P. pusillus, P. pectinatus and A. gramineum were negatively related. The models including interactions between the explanatory variables showed a high goodness of fit for Veluwemeer for Chara, P. pectinatus and M. spicatum (in more than 8 of 10 instances, a cell originally scored `1' are predicted to have a higher probability than the ones originally scored `0'). The models of the other species showed a moderate goodness of fit (between 6 and 8 of 10 instances correctly predicted). The models developed for Veluwemeer were valid for Chara in all the four other lakes (more than 8 of 10 instances correctly predicted), while the models of other species were valid in some instances. A succession of vegetation was recognised based on water depth and turbidity in Veluwemeer. P. perfoliatus and P. pectinatus dominated the shallow zones under turbid conditions, but a change to dominance by Chara occurred in clear water. Based on the observation of co-occurrence, competition has played an important role for the shift from P. pectinatus to Chara. Chara became dominant 2 years after initial colonisation of Chara in P. pectinatus beds. Competition between P. perfoliatus and Chara may have been of less importance, due to a more distinct habitat (deeper colonisation) of dense P. perfoliatus beds. The analysed species showed large differences in vegetation stability from year-to-year. Chara showed the highest year to year stability (c. 65% of the cells remained covered from one year to another), while A. gramineum showed the highest dynamics (c. 10% of the cells remained covered from one year to another). Species producing specialized vegetative propagules for over wintering showed a higher local stability than species without such propagules. [ABSTRACT FROM AUTHOR]

In this paper we analyze a long-term dataset on the recovery from eutrophication of Lake Veluwe (The Netherlands). Clear hysteresis was observed in a number of ecosystem variables: the route to recovery differed significantly from the route that led to loss of clear water. The macrophyte dominated state disappeared in the late 1960s at TP above 0.20 mg l−1, whereas its return occurred at less than 0.10 mg TP l−1. Several regime shifts resulting in the occurrence of three alternative stable states were observed over a period of 30 years. The turbid state showed resistance to change, despite a strong and prompt reduction in Chl -a following reduction of external P-loading. The most important component that determined hysteresis in the return to clear water was not internal P-loading, but a high level of nonalgal light attenuation (through sediment resuspension) maintained by the interaction between wind and benthivorous fish. Although Chara was able to re-colonize the most shallow parts of the lake, recovery stalled and for a number of years clear (above charophyte beds) and turbid (deeper parts of the lake) water co-existed, as a separate alternative state on route to full recovery. Lake-wide clear water was re-established after bream density had been reduced substantially. This allowed a return of zebra mussels to the lake, whose high filtration capacity helped in maintaining clear water. In this study, we were able to identify the main drivers of hysteresis and regime shifts, although formal demonstration of cause and effect was not possible on the basis of field data alone. We argue that resilience of the present clear water state of Lake Veluwe very much depends on sizable populations of a few keystone species, especially Chara (stoneworts) and Dreissena (zebra mussels), and that careful management of these species is equally important as control of nutrients. Lake management should strive to maintain and strengthen resilience of the ecosystem, and this should offer protection against a renewed collapse of the clear state. [ABSTRACT FROM AUTHOR]