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Dreissenid mussels (including the zebra mussel Dreissena polymorpha and the quagga mussel D. rostriformis) are among the world's most notorious invasive species, with large and widespread ecological and economic effects. However, their long-term population dynamics are poorly known, even though these dynamics are critical to determining impacts and effective management. We gathered and analyzed 67 long-term (>10 yr) data sets on dreissenid populations from lakes and rivers across Europe and North America. We addressed five questions: (1) How do Dreissena populations change through time? (2) Specifi- cally, do Dreissena populations decline substantially after an initial outbreak phase? (3) Do different measures of population performance (biomass or density of settled animals, veliger density, recruitment of young) follow the same patterns through time? (4) How do the numbers or biomass of zebra mussels or of both species combined change after the quagga mussel arrives? (5) How does body size change over time? We also considered whether current data on long-term dynamics of Dreissena populations are adequate for science and management. Individual Dreissena populations showed a wide range of temporal dynamics, but we could detect only two general patterns that applied across many populations: (1) Populations of both species increased rapidly in the first 1-2 yr after appearance, and (2) quagga mussels appeared later than zebra mussels and usually quickly caused large dedines in zebra mussel populations. We found little evidence that combined Dreissena populations declined over the long term. Different measures of population performance were not congruent; the temporal dynamics of one life stage or population attribute cannot generally be accurately inferred from the dynamics of another. We found no consistent patterns in the long-term dynamics of body size. The long-term dynamics of Dreissena populations probably are driven by the ecological characteristics (e.g., pred

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 0.20mgl−1, whereas its return occurred at less than 0.10mgTPl−1. Several regime shifts resulting in the occurrence of three alternative stable states were observed over a period of 30years. 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 pro

Deze memo beschrijft recente inzichten op het terrein van ecologische kantelpunten die zijn opgedaan in onderzoek en bij Rijkswaterstaat. Gegeven de korte projectduur (twee maanden) is er niet gestreefd naar volledigheid, maar zijn er inzichten bijeengebracht die behulpzaam zijn om de discussie aan te zwengelen over wat de kantelpuntentheorie kan betekenen voor het beheer, evaluatie en monitoring van de grote wateren. In het sterk gemodificeerde Nederlandse watersysteem is het moeilijk om vast te stellen of alternatieve evenwichten onder gelijke condities mogelijk zijn en dat kantelpunten hiertussen plaatsvinden. Alternatieve evenwichten en terugkoppelingsmechanismen zijn het best onderbouwd en omschreven in de literatuur voor de Waddenzee en Veluwemeer. In de estuaria zijn er veel indicaties maar ook veel onzekerheden. In het rivierengebied is het de vraag of alternatieve evenwichten kunnen bestaan. Het beter begrijpen van terugkoppelingsmechanismen in de Nederlandse zoete en zoute wateren, en de interacties hiertussen, wordt echter breed onderschreven. Terugkoppelingsmechanismenzijn met het kantelpuntenconcept goed te onderzoeken voor alle watersystemen en een beter begrip hierover is nuttig voor beleidsevaluaties, het inschatten van het effect van ecologische ingrepen en monitoringsplannen. Het is belangrijk om ook rekening te houden met ‘onbekende’ en niet kwantificeerbare factoren die de veerkracht van een ecosysteem kunnen beïnvloeden. Er is nog niet veel bekend over de invloed van microverontreinigingen, trofische cascades (vissen, vogels), microbiële bodemactiviteit en de rol van hydromorfologische dynamiek in rivieren en estuaria op de ecologische veerkracht van een watersysteem. Het is aan te bevelen om eerst met een aantal casestudies teverkennen wat de mogelijkheden en datavereisten zijn om met langjarige tijdreeksen (ecologie, waterkwaliteit) indicatoren te bepalen die iets zeggen over verandering in ecologische veerkracht. Voorts doen wij de aanbevelin

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 0.20mgl−1, whereas its return occurred at less than 0.10mgTPl−1. Several regime shifts resulting in the occurrence of three alternative stable states were observed over a period of 30years. 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 pro