1Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), P.O. Box 50, 6700 AB Wageningen, The Netherlands
2Department of Aquatic Ecology and Water Quality Management, Wageningen University, The Netherlands
At the end of spring 2007 the drinking water station near Wuxi (southeast China) reports smelly drinking water and needs to stop its production (Guo, 2007; Qin et al., 2010). As a result, millions of people are without drinking water (Qin et al., 2010). The cause of the smell is found in a large algal bloom in the north of Lake Taihu (2338km2) persisting for several weeks (Paerl et al., 2011). However, these problems do not appear to be temporary as the blooms became more frequent and longer lasting in the following years, and even prevails today (Figure 1) (Chen et al., 2003; Duan et al., 2009).
Algal blooms like in Taihu are an increasing worldwide problem as a result of eutrophication. Increasing nutrient loads cause catastrophic shifts in lakes from macrophyte to phytoplankton dominance (Scheffer et al., 2001). Restoration from the eutrophic to oligotrophic state is often not straightforward due to hysteresis (Figure 2) (Scheffer et al., 1993). As result of hysteresis the trajectory of eutrophication is different from the trajectory of oligotrophication, a phenomena better known as alternative stable states. Alternative stable states have repeatedly been shown in small lakes like Lake Veluwe in the Netherlands (30 km2) (Meijer and Hosper, 1997) and the gravel pit lakes in England (< 1 km2)(Wright and Phillips, 1992). Whether alternative stable states also exist in large shallow lakes is less clear, however. In our review paper on alternative stable states in large shallow lakes this intriguing question is discussed based on lake size, spatial heterogeneity and internal lake connectivity (Janssen et al., 2014).
With increasing lake size, the potential for macrophyte presence decreases, here referred to as the size effect (Figure 3, ①). Indeed larger lakes have a longer fetch, allowing the wind to generate higher waves that destruct macrophytes and cause a high turbidity (Janse et al., 2008; Jeppesen et al., 2007). Additionally, larger lakes tend to be deeper which further negatively influences the light climate for macrophytes (Bohacs et al., 2003; Søndergaard et al., 2005).
Despite the size effect, macrophytes do occur and even flourish in large lakes, though they are often restricted to specific regions of the lake (Janssen et al., 2014). As a result of spatial heterogeneity (Figure 3, ②) some parts of the lake are more suited for macrophyte growth than others. The lake can for example vary in depth or fetch. The areas that are shallow and have a low fetch are then more suitable for macrophytes than deep parts with long fetch (Janse et al., 2008).
However, macrophyte presence itself does not prove alternative stable states’ existence in large shallow lakes. Positive feedbacks between macrophytes and algae need to be strong enough to exhibit hysteresis (Scheffer et al., 1993). The internal connectivity within a lake is an important factor that affects the positive feedbacks (Figure 3, ③). If, for instance, the macrophyte suitable areas are mostly isolated, they could be responding like separate small lakes. On the other hand, if the connectivity is high, they could influence other parts of the lake by introducing clear water or, vice versa, other parts of the lake could transfer more turbid water to the macrophyte suitable areas (Hilt et al., 2011).
With the ecosystem model PCLake, multiple bifurcation analyses have been performed using the standard parameter set (Janse et al., 2010; Janse et al., 2008). Therefore, the model has been run for different nutrient loadings ranging from oligotrophic to eutrophic and back to identify whether the lake shows hysteresis (Figure 2, orange dotted line) or not (Figure 2, blue dashed line). The bifurcation analysis has been repeated for multiple lake types differing in fetch and depth.
The multiple bifurcation analyses are a first estimation on which lakes might have alternative stable states and which do not. The model outcome has been compared with data from literature. This comparison between model and data shows agreement for lakes that are particularly suitable for macrophyte growth mainly because of their shallowness (Figure 4A, ①). For instance, Lake Istokpoga (113 km2, USA, Nr. 1) is overgrown with macrophytes and multiple attempts to remove them had only temporal effects. Other lakes are suggested by the model to have alternative stable states (Figure 4A, ②) as for example Lake Apopka (125 km2, USA, Nr. 4) where literature suggested hysteresis as well. Finally there are lakes that have, according to the model, too large a fetch or are too deep to be dominated by macrophytes (Figure 4A, ③). Indeed many of these lakes are partly free of macrophytes, however most of these lakes are not completely bare. However, the effects of spatial heterogeneity are not taken into account yet. As an example, when spatial heterogeneity is taken into account for Lake Taihu, a different picture may be seen (frequency distribution, Figure 4B). This illustrates that parts of the lake are suitable for macrophyte growth, whether or not with presence of alternative stable states and other parts will exhibit the phytoplankton dominated state. While this analysis ignores the internal connectivity, its effect can be logically deduced. With a large internal connectivity, lakes will act more homogenous and therefore the area of alternative stable states will be most likely more confined.
Discussion And Conclusion
To evaluate the presence of alternative stable states of large lakes it is important to look beyond the size effect of lakes and also take the spatial heterogeneity and internal connectivity into account. Under specific conditions, large shallow lakes may exhibit (spatial restricted) alternative stable states.
Full study published in the Journal of Great Lakes Research, December 2014, Vol. 40, Issue 4, p. 813–826.
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Featured Image: Lake Taihu. (Credit: Wikimedia Commons User Cocowind123 via Creative Commons 3.0)