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Long-Term Effects of the Harvesting of Trapa Natans on Local Water Quality and Aquatic Macrophyte Community in Lake Erhai, China

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Long-Term Effects of the Harvesting of Trapa Natans on Local Water Quality and Aquatic Macrophyte Community in Lake Erhai, China ORIGINAL RESEARCH published: 09 July 2021 doi: 10.3389/fenvs.2021.706746 Long-Term Effects of the Harvesting of Trapa natans on Local Water Quality and Aquatic Macrophyte Community in Lake Erhai, China Changbo Yuan 1†, Xiaohu Bai 2,3,4,5†, Tianshun Zhu 6, Zihao Wen 1, Te Cao 1, Xiaolin Zhang 1* and Leyi Ni 1 1Donghu Experimental Station of Lake Ecosystems, State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China, 2Xinjiang Academy of Environmental Protection Science, Urumqi, China, 3Xinjiang Key Laboratory for Environmental Pollution Monitoring and Risk Warning, Urumqi, China, 4Xinjiang Engineering Technology Research Center for Cleaner Production, Urumqi, China, 5Junggar Desert-oasis Ecotone Station for Scientific Observation and Research of National Environmental Protection, Urumqi, China, 6College of Life Sciences, Zaozhuang University, Edited by: Zaozhuang, China Peng Bao, Institute of Urban Environment (CAS), China Trapa natans is one of the main species causing the swamping in the littoral zones of Erhai Reviewed by: Lake. It commonly forms a dense canopy on the water surface in the growing season Guorong Zhu, (June–September), which hampers the local water quality and habitat of submerged Henan Normal University, China Yibo Wu, macrophytes, and releases nutrients to the water after death in autumn and winter, Ningbo University, China resulting in the deterioration of local water quality. At present, there are many and positive *Correspondence: research studies on the short-term effects of harvesting water chestnut on water quality Xiaolin Zhang [email protected] and aquatic plants, but long-term observation results are lacking. In response to the above †These authors have contributed problems, we studied responses of water quality and aquatic plant community to the equally to this work and share first removal of Trapa in littoral zone of a northern bay in Erhai from August 2014 to January authorship 2017. This could be the first attempt to discover the long-term effects of floating-leaved fi Specialty section: vegetation management in the freshwater ecosystem. The results showed that the arti cial This article was submitted to removal of Trapa significantly improved the local water quality in the growing season, for Freshwater Science, example, the concentrations of total nitrogen (TN), dissolved nitrogen (DN), total a section of the journal Frontiers in Environmental Science phosphorus (TP), and dissolved phosphorus (DP) in the non-Trapa zone (NTZ) were Received: 08 May 2021 much lower than the concentrations of those in the adjacent Trapa zone (TZ). And the Accepted: 18 June 2021 biomass of aquatic macrophyte community (BAMC) was significantly increased in the NTZ, Published: 09 July 2021 up to the maximum value of about 21 kg/m2 in fresh weight. However, the diversity indexes Citation: of the community in the NTZ declined. Therefore, we suggested that although the removal Yuan C, Bai X, Zhu T, Wen Z, Cao T, Zhang X and Ni L (2021) Long-Term of Trapa improved the water quality and increased the productivity of the submerged Effects of the Harvesting of Trapa aquatic plant community, it reduced the species diversity of the aquatic plant community in natans on Local Water Quality and Aquatic Macrophyte Community in the long run. This is another issue that we need to pay attention to in the later management Lake Erhai, China. in Erhai Lake. Front. Environ. Sci. 9:706746. doi: 10.3389/fenvs.2021.706746 Keywords: harvesting, water quality, aquatic plant community, Lake Erhai, Trapa natans Frontiers in Environmental Science | www.frontiersin.org 1 July 2021 | Volume 9 | Article 706746 Yuan et al. Long-Term Effects of Vegetation Harvesting INTRODUCTION actual aquatic plant management because the stabilization of aquatic plant community composition requires a long-term Trapa species are typical floating-leaved aquatic plants. The process. As Scheffer et al. (2003) indicated by the model floating-leaved plants are obviously superior for light approach, the competition relationship between floating and competition than submersed plants, and the only limited submersed plants can cause alternative attractors. However, factor which could prevent their growth is nutrients. As such a hypothesis still needs a useful complementary type of aggravation of the water eutrophication, floating-leaved species evidence through large-scale field experiments (Scheffer et al., would quickly outcompete other aquatic plants and become 2003). dominant species in many shallow water bodies (Kociˇ c´ et al., Therefore, in order to explore the long-term response of water 2009; Chen and Wang, 2019). As the water temperature and the quality and aquatic vegetation to harvesting of the Trapa input of external nutrients increase, their biomass will further population, we conducted a 30-month study in the northern increase, leading to the deepening of the eutrophication of the bay of Erhai Lake. Our research could be the first attempt to ecosystem and threatening the ecological balance of the habitat experimentally evaluate the ecological functions of floating- (Markovic et al., 2015). leaved plant management through long-term and large-scale On the one hand, Trapa has serious negative effects on water field experiment. And our results will provide a valuable and bodies during the growing season and the decline season. In the practicable management strategy in the large-scale ecological growing season, Trapa can reach a high biomass, for example, restoration. 504 ± 91 g dry Weight/m2 in Lake Lanca di Po (Bolpagni et al., Based on the limited research studies (Scheffer et al., 2003; Xu 2007), thereby adsorbing a large amount of nutrients, most of et al., 2014), it is reasonable to expect that submersed macrophyte which is from the sediment (Watanabe et al., 2010). At the same community will gradually recover after the removal of Trapa time, it forms an underwater environment of high total populations. But the further community succession and phosphorus concentration (Akabori et al., 2015). In addition, ecosystem management still need to be studied. In this most of the nutrients absorbed by Trapa are released to the water research, we hypothesized that 1) the removal of the Trapa column after their death and decomposition in autumn and population will significantly improve the local water quality, 2) winter, hampering the local water quality (Choi et al., 2016). the submersed macrophytes will reassemble soon after the On the other hand, the existence of Trapa species inhibits the removal of Trapa, and 3) the local biodiversity will be propagation and spread of submerged plants, and threatens the improved dramatically. living environment of other organisms (Caraco and Cole, 2002) by forming water environment of low oxygen concentration (Hummel and Findlay, 2006; Goodwin et al., 2008; Kato et al., MATERIALS AND METHODS 2016; Klancnikˇ et al., 2017), low light intensity (30% coverage can block 74% of the light) (Netten, 2011), and so on, which in turn Study Area affects the service functions of lake ecosystems (Markovic et al., This research was conducted in Haichao Bay, which is in the 2015). The low oxygen content and low underwater light intensity northern part of Erhai Lake (25°52′N, 100°06′E) in subtropical in local waters caused by Trapa species (Caraco and Cole, 2002; Yunnan Plateau, China. It was adjacent to the breeding base of Kara et al., 2010; Klancnikˇ et al., 2017) severely inhibit Erhai aquatic plant seedlings without seasonal pollutant inputs. photosynthesis intensity of submerged macrophytes (Barko Erhai Lake is a mesotrophic lake with a moderate water depth and Smart, 1981). Once the abundance of the floating-leaf (max. 20.5 m, mean. 10.5 m) and has an area of about 250 km2.In plants develops rapidly through its asymmetric competitive the last decades, with the increased eutrophication of the local advantage over submerged macrophytes and exceeds a certain watershed, the littoral zone of Haichao Bay was dominated by T. threshold, submerged macrophytes will face the threat of decay, natans, and the submerged vegetation was seriously threatened which will seriously weaken the health and stability of the lake (Fu et al., 2013). ecosystem (Scheffer et al., 2003; Kagami et al., 2019). At present, there are a few research studies on the harvesting Experiment Design management of Trapa, and they mainly focus on the short-term The experiment was conducted in shallow waters near Baima (within 1 year) effects of reducing the nutrient load of water village on the west bank of Haichao Bay. It was divided into two bodies (Galanti et al., 1990; Xu et al., 2014; Lawniczak and experimental zones (Figure 1). One was enclosed by a Achtenberg, 2018), lacking results of long-term monitoring of polyethylene mesh to an area of about 20,000 m2. In this zone, harvesting Trapa on water quality and the plants. These short- T. natans was repeatedly removed with a sickle from August 16, term results showed that the harvesting of Trapa significantly 2014 to August 25, 2014 as the non-Trapa zone (NTZ) treatment. reduced the concentrations of nitrogen and phosphorus in the The second (control zone) without harvesting was adjacent to the water body, and promotes the growth of some subdominant NTZ with the area of about 10,000 m2 and was treated as the aquatic plants. The removal of Trapa will definitely improve the Trapa zone (TZ) treatment (Figure 1). Monitoring sites were submerged plant community to a certain extent (Xu et al., 2014), deployed as below and were sampled monthly (except December but the recovery of aquatic plant community after Trapa 2016 for the lacking of personnel and sampling tools) from harvesting is a long run, which has also an important effect on August 15, 2014 to January 15, 2017, for measurements of local water quality. Thus, these results are difficult to apply to water quality parameters and aquatic plant community Frontiers in Environmental Science | www.frontiersin.org 2 July 2021 | Volume 9 | Article 706746 Yuan et al.
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