Assessing the safe operating space of aquatic macrophyte biomass to control the terrestrialization of a grass-type shallow lake in China

Highlights

• Aquatic macrophytes expansion accelerates the lake terrestrialization.

• Plant biomass was identified in different terrestrialized zones at multi-scales.

• A valuable method was developed for the safe operating space of macrophyte biomass.

• The approach provides a scientific reference for shallow lake management.

Abstract

As an important type of aquatic ecosystem, lake ecosystems play an irreplaceable role in providing water resources, controlling floods, regulating the regional climate, and maintaining the regional ecological balance. However, multiple lake ecosystems have been threatened by the expansion of aquatic macrophytes and the resulting bioaccumulation, which accelerates the process of lake terrestrialization. Therefore, it is necessary to identify the safe operating space of macrophyte biomass in order to control the terrestrialization of shallow lakes. In this study, we investigated the biomasses of dominant species and community types at different growth stages in different terrestrialized zones for a typical shallow lake in North China. Then, we developed a suitable method for estimating the safe operating space for the aquatic macrophyte biomass in different terrestrialization stages. Our results showed that the aquatic macrophyte biomass generally increased with the increasing terrestrialization stage. In addition, the biomass in September was lower than that in May. On the community scale, the biomass of Phragmites australis, Phragmites australis – Ceratophyllum demersum, and Nelumbo nucifera - Ceratophyllum demersum - Typha orientalis communities was significantly higher than that of Potamogeton pectinatus and Potamogeton pectinatus - Nelumbo nucifera communities. Terrestrialization exhibited lower impacts on plant biomass in May, whereas a greater impact of terrestrialization was observed in September. Generally, single-species communities had higher plant biomass than mixed communities. The target management years for different terrestrialized zones were established to calculate the safe operating space of macrophyte biomass. The current biomass values were much higher than those calculated for the safe operating space, especially in September and for the P. australis and P. australis - C. demersum communities, indicating that there is high pressure to manage these communities. The approach proposed in the study provides a scientific reference for macrophyte management to control the terrestrialization status of grass-type shallow lakes.

Introduction

Shallow lakes provide a range of ecosystem services, such as controlling floods, regulating the regional climate, and supplying drinking water and agricultural water (Liu et al., 2006; Zhao et al., 2005). However, multiple-lake ecosystems, which are among the most threatened aquatic ecosystems, have been threatened by the expansion of aquatic macrophytes and the increase in bioaccumulation (Li et al., 2016; Sharp et al., 2013). Therefore, lake terrestrialization has become the main problem for shallow lakes (Li et al., 2014; Zimmer et al., 2016), and more attention should be paid to lake restoration and control (Wu et al., 2000).

Terrestrialization is the succession process from a lake to a terrestrial ecosystem due to the accumulation of sediment and detritus deposition in the lake bottom (Asaeda et al., 2000; Tallis, 1973; Zhang et al., 2012). Specifically, the lake aquatic macrophytes continuously grow and die, and lakes are gradually filled with sediments and plant detritus, resulting in lower water depth; then, macrophytes spread from the littoral zone to the center. Finally, the whole lake evolves into a terrestrial ecosystem (Niska and Kołodziej, 2015; Hall et al., 2018). Lake deposition includes chemical deposition, sediment deposition, and biological deposition, of which biodeposition is the key factor influencing lake terrestrialization (Zhao et al., 2011). Therefore, aquatic macrophytes are considered as reliable indicators for detecting lake or pond status because of their sensitive response to environmental change (Mahaney et al., 2004; Sass et al., 2010).

The growth and spread of macrophytes highly depends on lake sediments and nutrient abundance in the water (Lu et al., 2018). Sediments that are rich in nutrients create a favorable habitat for macrophyte growth; in turn, macrophytes can also immobilize sediment and promote its deposition. Furthermore, lake sediment layers continue to rise due to high plant detritus input, which stimulates the overgrowth of macrophytes. Meanwhile, the dramatic increase in the biomass of macrophytes and a decreased water level can greatly damage aquatic life habitats and accelerate lake terrestrialization (Horppila and Nurminen, 2005; Li et al., 2010, 2013).

Although previous studies have focused on the causes (Crushell et al., 2011; Hunter et al., 2016) and evolution processes (Cressler et al., 2010; Kenrick et al., 2012; Vecoli et al., 2010) of lake terrestrialization, research on ecological controlling thresholds for lake terrestrialization is still scarce, making it difficult to provide a scientific basis for terrestrialization control. Therefore, it is necessary to identify the safe operating space for lakes that are subject to local stresses and the effects of climate change, which collectively erode ecosystem resilience (Green et al., 2017; Popp and Maquat, 2015). Biological control is a promising, cost-effective, and environmentally friendly approach for slowing lake terrestrialization (Arroita et al., 2019; Hazelton et al., 2017; Xu, X. et al., 2014a). Direct mechanical harvesting of aquatic macrophytes combined with ecological excavation is widely utilized to reduce the accumulation of organic matter, especially on a large scale, and can effectively alleviate the accumulation and release of inner nutrient loadings; this technique has been carried out in recent years (Borin and Salvato, 2012; Wu et al., 2011). Nevertheless, there is a considerable uncertainty in ecological control of the terrestrialization of shallow lakes as well as precise threshold values. This uncertainty is considered by defining the boundary of the safe operating space at a reasonable threshold range. Therefore, a better understanding of the safe operating space is important for the effective management and ecological control of the terrestrialization of shallow lakes (Green et al., 2017; Kelly et al., 2014).

Multiple lake ecosystems have been threatened by the expansion of aquatic macrophytes and the resulting bioaccumulation, which greatly accelerates the terrestrialization process of grass-type lake ecosystems. Plant biomass is the key parameter for managing macrophytes although few studies has proposed the safe operating space for plant biomass to responde to lake terrestrilization control. Therefore, in this study, we innovatively developed a suitable method to calculate the safe operating space of macrophyte biomass based on the target management years for different terrestrialized zones of a large grass-type shallow lake. Therefore, the objectives of this work were (1) to identify the relationships between lake terrestrialization and macrophyte biomass at the plant species and community scales and (2) to develop a suitable method for identifying the safe operating space of aquatic macrophyte biomass for terrestrialization control in different terrestrialized zones of Baiyangdian Lake, the largest grass-type shallow lake in the North China.