Abstract
Critical transitions between ecosystem states can be triggered by relatively small external forces or internal perturbations and may show time-lagged or hysteretic recovery. Understanding the precise mechanisms of a transition is important for ecosystem management, but it is hampered by a lack of information about the preceding interactions and associated feedback between different components in an ecosystem. This paper employs a range of data, including paleolimnological, environmental monitoring and documentary sources from lake Erhai and its catchment, to investigate the ecosystem structure and dynamics across multiple trophic levels through the process of eutrophication. A long-term perspective shows the growth and decline of two distinct, but coupled, positive feedback loops: a macrophyte-loop and a phosphorus-recycling-loop. The macrophyte-loop became weaker, and the phosphorus-recycling-loop became stronger during the process of lake eutrophication, indicating that the critical transition was propelled by the interaction of two positive feedback loops with different strengths. For lake restoration, future weakening of the phosphorus-recycling loop or a reduction in external pressures is expected to trigger macrophyte growth and eventually produce clear water conditions, but the speed of recovery will probably depend on the rates of feedback loops and the strength of their coupling.
Highlights
The mean annual temperature has been rising since the 1990s with recent high temperatures equaling historical values found in the 1950s (Figure 3a, Mann–Kendall trend test: z = 2.50, n = 20, p-value = 0.01) and rainfall data show that droughts occurred in 1960, 1978, 1988 and 2006 (Figure 3b)
The implications were: (i) the lake can exist with alternative states over a total phosphorus (TP) range 0.018–0.030 mg/L; (ii) there was a strengthening positive feedback loop prior to 2001 that could transgress the nutrient threshold [18]; and (iii) a restoration of lake turbidity would likely involve the strengthening of another feedback loop that can shift the current state to its original form, or a similar state
The same study reasoned that the strong positive feedback loop in the period leading up to the critical transition in 2001 involved high levels of nutrient-driven planktonic productivity, oxygen depletion and anoxia in the hypolimnion, resulting in a secondary fertilization effect, whereby P is recycled from the lake sediments to the water column [9]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. With growing concerns about widespread ecological instability [1], global ecosystems are more at risk to rapid change from human-induced critical transitions than at any time in history [2]. The risk comes from a loss of ecological resilience such that a small force or perturbation can shift the system to a contrasting state. The presence of hysteresis means that some shifts in ecosystems may be effectively irreversible, requiring disproportionally large reversals of the key drivers that led to the transition [3]. Understanding and using the antecedent conditions to critical transitions as possible early warning signals is a major research priority [4,5,6]
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