Confronting challenges of managing degraded lake ecosystems in the Anthropocene, exemplified from the Yangtze River Basin in China

Abstract

Freshwater lake ecosystems have supported livelihoods for millennia. Yet, most lakes worldwide today are suffering severe degradation in response to anthropogenic pressures. Scientists and managers are facing pressing challenges to define safe operating space for successful lake management into the future. Achieving these goals requires a clear understanding of nonlinear trajectories of ecosystems response to changes in multiple interacting drivers at multi-decadal scales. Based on a synthesis and analysis of published paleo-environmental records, this paper shows that many lakes in the Middle and Lower Yangtze River Basin (MLYB) have undergone abrupt ecological shifts during the 1950s–1980s. The causes for these significant changes were multiple accumulating drivers acting together through time including climate change, land reclamation, agricultural intensification, and industrialization. This paper also explored the potential of defining the Anthropocene baseline for proper restoration and management in the MYLB based on paleo-environmental records. That is, we reveal that a return to the pre-1900s historical condition is no longer possible for many degraded lakes due to both socioeconomic and ecological constraints. For some severely degraded lakes with high sedimentation rates, restoration to the 1970s and 1980s levels within the historical range of variability might be more realistic, though constrained by socioeconomic limitations on reducing anthropogenic drivers. Management actions can still guide other lakes with low or moderate levels of sedimentation toward their pre-1900s conditions. This analysis is the first of its kind in illustrating within the MYLB that long-term paleo-environmental records can have useful applications in identifying the restoration targets in degraded aquatic ecosystems. It also suggests a pathway for potentially improving the resilience of such social-ecological systems toward sustainability.

Introduction

The extent and scale of human-biosphere interactions in recent centuries are unprecedented, altering the dynamics of ecosystems throughout the world (Johnson et al., 2017). Freshwater lake ecosystems are among the most altered and damaged on the planet while supporting the food security and livelihoods for millions of people worldwide (Beklioğlu et al., 2016). Increasingly, scientists and managers encounter previously unseen configuration of species in many lake ecosystems (Dubois et al., 2017). Land degradation, declining water quality, overharvesting, climate change, and other human-induced drivers have resulted in major alterations to the species composition and trophic structure of freshwater ecosystems. Extensive physical, chemical and biotic transformations in lake ecosystems have been observed in many locations around the world (Dubois et al., 2017). Observational, experimental and modeling studies suggest that many shallow lakes have experienced transitions from clear, macrophyte-dominated state to turbid, phytoplankton-dominated states, with the substantial reorganization of species and assemblages of aquatic communities (Scheffer and Jeppesen, 2007). These ecological shifts are important for the management and restoration of lake ecosystems because they often result in profound changes in ecosystem services, biodiversity, and esthetic values. They are also hard to predict and avoid, and are often costly, difficult, or even impossible to reverse (Scheffer et al., 2001).

Scientists and managers have increasingly accepted the impossibility of returning highly degraded lake ecosystem to their past configuration. The recent Anthropocene baseline concept proposed by Kopt and others (Kopf et al., 2015) acknowledges the conservation value of the remnants of historical ecosystems, but also emphasize that certain ecosystems have arrived at a new point of reference due to the accelerating human impacts in the Anthropocene. Managers and policymakers are facing pressing challenges to identify these reference conditions, maintain the ecosystem functions that are crucial for sustaining lake ecosystems, and to secure the ecosystem services that highly altered lake ecosystems can provide to people in the future (Hilt et al., 2017). Central to this endeavor is an improved understanding of long-term dynamics of lake ecosystems driven by both slow (i.e. climate change, catchment erosion) and fast (i.e. rapid socioeconomic development) drivers over various timescales, in order to define regional safe operating spaces (keep lake ecosystems within acceptable levels of boundaries) for future sustainability (Dearing et al., 2014; Scheffer et al., 2015).

The Middle and Lower Yangtze River Basin (MLYB) constitutes one of the largest groups of freshwater shallow lakes in the world (Fig. 1) (Wang, 1998). The catchment covers approximately 785,000 km², with more than 5900 (in number) or 15,000 km² freshwater lakes, ponds and reservoirs, accounting for nearly 25% of the freshwater lake area in East Asia (Wang et al., 2014). Freshwater lakes in MLYB provide essential ecosystem services (e.g. food, water, aesthetic and spiritual values) to sustain agricultural production and socioeconomic development in the region with nearly half a billion population. As a harbinger of China’s environmental change, shallow lakes in the MLYB have experienced serious degradation (Fig. 2) (Ding et al., 2015; Fang et al., 2006). Human has modified lake ecosystems in this region for millennia, but recent industrialization and urbanization have resulted in an exponential rise in the anthropogenic modification and use of lake resources (Ellis and Wang, 1997; Yang and Lu, 2014). A recent survey showed that more than 90% of the shallow lakes in these regions are experiencing eutrophy and hyper-eutrophy, and more than 1000 lakes (>1 km²) with a total area of 13,000 km² disappeared in the middle and lower Yangtze River Basin during the last five decades (Wang, 2015). Lake deterioration has led to the loss of biodiversity, damage to ecological services, as well as increased environmental disasters, such as the catastrophic Yangtze floods in 1998 and Taihu water crisis in 2007 (Qu et al., 2014; Wang et al., 2015).

China has made tremendous efforts to restore and manage the highly degraded lake ecosystems over the recent decade (Le et al., 2010). Billions of dollars are dedicated to lake ecosystem restoration via environmental protection actions (e.g. establishment of wastewater treatment plants, shutdown of heavily polluting factories, closure of aquaculture fisheries) (Wang, 2015; Zhou et al., 2017). Nevertheless, the extent to which China can protect and repair these degraded lake ecosystems, at what cost, and over what timeframe represent daunting challenges. Many current restoration and management practices inadvertently simplify and rescale these complex issues into a subsystem that is entirely ecological, and focusing on the symptom rather than the deep fundamental causes (Zhang et al., 2016). Lake management strategies ignoring these complex interactions frequently failed to achieve their anticipated outcomes and many lake improvements are sometimes short-lived (Crépin et al., 2012). For example, the cost of cleanup efforts in China's third largest body of freshwater, Taihu Lake, since 2007 has exceeded US $16 billion, yet massive blooms of algae and cyanobacteria still reoccur almost every year, threatening the water supply to 30 million people. Shallow lakes in this region will likely become extremely vulnerable to future changes in climate and other anthropogenic pressures (Qin et al., 2013).

Although progress has advanced considerably about changes in lake ecosystem in the Yangtze River Basin, most of studies mainly focus on recent “symptoms” of the problems rather than their deep historical causes (Wang, 2015). The extent and spatiotemporal variability of lake ecosystems change, in particular, is still poorly known. This paper analyzes existing paleoenvironmental records from the published literature for the MLYB to determine how shallow lakes have changed over the past century. In doing so, it also raises challenges for sustainable management of heavily degraded aquatic systems. Firstly, by reviewing and synthesizing the large body of published paleoecological records, we document the abrupt shift of lake ecosystems from the MLYB over the past century. Secondly, from this analysis, we identify how multiple pressures have changed and acted on lake ecosystems. Third, we explore the potential of defining the anthropogenic baseline in this region based on the long-term paleoenvironmental records, in order to assess proper and realistic restoration target. Finally, we provide suggestions and recommendations for sustainable lake management and restorations.