Lake ecosystem tipping points and climate feedbacks

Abstract. Lakes and ponds experience anthropogenically forced changes that may be non-linear and sometimes initiate ecosystem feedbacks leading to tipping points beyond which impacts become hard to reverse. In many cases climate change is a key driver, sometimes in concert with other stressors. Lakes are also important players in the global climate by ventilating a large share of terrestrial carbon (C) back to the atmosphere as greenhouse gases and will likely provide substantial feedbacks to climate change. In this paper we address various major changes in lake ecosystems and discuss if tipping points can be identified, predicted, or prevented, as well as the drivers and feedbacks associated with climate change. We focus on potential large-scale effects with regional or widespread impacts, such as eutrophication-driven anoxia and internal phosphorus (P) loading, increased loading of organic matter from terrestrial to lake ecosystems (lake “browning”), lake formation or disappearance in response to cryosphere shifts or changes in precipitation to evaporation ratios, switching from nitrogen to phosphorus limitation, salinization, and the spread of invasive species where threshold-type shifts occur. We identify systems and drivers that could lead to self-sustaining feedbacks, abrupt changes, and some degree of resilience, as opposed to binary states not subject to self-propelling changes or resilience. Changes driven by warming, browning, and eutrophication can cause increased lake stratification, heterotrophy (browning), and phytoplankton or macrophyte mass (eutrophication), which separately or collectively drive benthic oxygen depletion and internal phosphorus loading and in turn increase greenhouse gas (GHG) emissions. Several of these processes can feature potential tipping point thresholds, which further warming will likely make easier to surpass. We argue that the full importance of the vulnerability of lakes to climate and other anthropogenic impacts, as well as their feedback to climate, is not yet fully acknowledged, so there is a need both for science and communication in this regard.