Regime Shifts in Shallow Lakes

Every now and then the usual scenario of fluctuations around trends in ecosystems is interrupted by sharp shifts to a different regime (Scheffer and others 2001; Carpenter 2003). Similarly, reconstructions of past climatic dynamics reveal various spectacularly rapid changes punctuating episodes of relative stability (Rial and others 2004), and also societies are notorious for going through occasional drastic shifts (Brock 2006). The term ‘regime shift’ originally proposed to describe sharp changes in oceanic ecosystems (Steele 1996; Hare and Mantua 2000), is increasingly used in a general sense to refer to such sudden drastic transitions from one persistent dynamical regime to another. Regime shifts may be due to different mechanisms, such as a drastic impact on the system, or a stepwise change in some important external condition. However, of particular interest are regime shifts that arise because the system reacts sensitively to changing conditions around some critical threshold. One reason for this can be the existence of alternative attractors in a system over some range of conditions. Although this possibility has been postulated and intensively discussed during various episodes in the history of ecology (Holling 1973; May 1977), interest increased sharply after this phenomenon was convincingly demonstrated for the ecosystems of shallow lakes. In those ecosystems submerged macrophytes can become abundant if the water is clear enough, but the macrophytes also affect many key processes promoting water clarity (Jeppesen and others 1997). This implies a positive feedback that can cause the turbid and a clear state to be alternative attractors (Scheffer and others 1993). Much of the work on shallow lakes over the years has focused on understanding how the alternative states work, and how manipulation of fish stocks can be used to tip lakes from a turbid to a clear state (Moss and others 1996; Jeppesen 1998; Scheffer 1998). There have been anecdotal descriptions of lakes shifting in response to an exceptionally heavy storm (Hamilton and Mitchell 1988), a change in water level (Wallsten and Forsgren 1989), or unknown causes (Blindow and others 1997). However, little work had been done so far on unraveling the mechanisms behind natural regime shifts in shallow lakes. In this issue we present a cluster of papers that address this topic from different perspectives. Although some studies illustrate that speculations can remain inconclusive even after years of investigation, others arrive at fundamentally new insights and hypotheses that, just like the finding of alternative stable states, may have parallels and implications far beyond the realm of limnology. First, a 30-year in-depth study of Lake Veluwe provides a detailed view of a classical scenario of eutrophication and recovery of a shallow lake (Ibelings and others 2007). Hysteresis between alternative states is apparent in this case study, and it is argued that the resilience of the clear state depends critically on a few key species. The remaining studies all highlight deviations from this classical scenario. Two of the papers highlight the remarkable phenomenon of lakes oscillating between the contrasting states in a cyclic way. A Dutch and an English lake seem to have moved from a turbid published online 17 April 2007. *Corresponding author; e-mail: Marten.Scheffer@wur.nl Ecosystems (2007) 10: 1–3 DOI: 10.1007/s10021-006-9002-y