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Lake resilience: concept, observation and management

Worldwide, lake ecosystems face threats arising from multiple pressures, related to nutrient runoff (Abell et al. 2011), biological invasions (MacIsaac et al. 2004), hydrological modifications (Zohary and Ostrovsky 2011), exploitation of edible aquatic life (Youn et al. 2014) and climate change (Adrian et al. 2009). New Zealand boasts a great diversity of lakes, which are of cultural importance and environmental, social and economic significance. Their ecosystem services include the provision of water for various purposes, including power generation, transportation, fisheries, tourism, and recreational enjoyment (Schallenberg et al. 2013). Many lakes in New Zealand have been under increasing anthropogenic pressures such as the intensification of land use, urbanisation and the introduction of invasive species. Declining water quality, biodiversity loss, and cyanobacteria blooms are now observed frequently (Ministry for the Environment and StatsNZ 2017; David et al. 2018). In this special issue, attention is directed towards the implications of those pressures for lake ecosystem dynamics, under the unifying theme of ‘Lake Resilience’. This issue presents a series of forum and research papers promoting a better understanding of resilience mechanisms through the integration of applied research in a multidisciplinary framework. Resilience is defined as the ability of a system to maintain its state by absorbing both internal and external change and disturbances to its state variables, driving variables, and parameters (Holling 1973; Cumming et al. 2005). Resilience can be described mathematically as a property of nonlinear systems in which perturbations may move the system away from a point of equilibrium, but after the perturbation, balance is restored and the system returns to this steady state. Lake ecosystems, like most natural systems, have the capacity to maintain their natural ecological state when exposed to some level of pressure. For example, a moderately small gradual development in a lake’s catchment can increase nutrient inputs without causing concomitant deterioration of water quality. A lake with the ability to maintain its state in the face of both internal change and external shocks and disturbances is said to be resilient (Carpenter and Cottingham 1997). All possible ecosystem states may be thought of as a multidimensional surface where troughs and peaks represent stable and unstable steady states. This can be visualised by a marble moving around a three-dimensional surface (see cover image to this special issue for an abstract representation of this concept) or, simpler, as a marble on a two-dimensional track (Figure 1). A marble, representing current ecosystem state, would be drawn to troughs in which case resilience would be analogous to the depth of a depression that keeps an