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Response of Submerged Aquatic Vegetation to Water Depth in a Large Shallow Lake After an Extreme Rainfall Event

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Response of Submerged Aquatic Vegetation to Water Depth in a Large Shallow Lake After an Extreme Rainfall Event water Article Response of Submerged Aquatic Vegetation to Water Depth in a Large Shallow Lake after an Extreme Rainfall Event Jinge Zhu, Jiancai Deng, Yihui Zhang, Zhaoliang Peng and Weiping Hu * State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China; [email protected] (J.Z.); [email protected] (J.D.); [email protected] (Y.Z.); [email protected] (Z.P.) * Correspondence: [email protected] Received: 30 September 2019; Accepted: 15 November 2019; Published: 17 November 2019 Abstract: Submerged aquatic vegetation (SAV) is an important part of lake ecosystems, and a proper SAV community structure is the key factor in keeping a clear-water state. Although the response of SAV to water depth has been widely studied in different aquatic environments, little is known about the response of the SAV community to changes in water depth of a large lake after an extreme rainfall event. To examine this question, 780 samples were collected from Lake Taihu, China, between 2013 and 2017 to analyze the variations in SAV and water depth. The water level of the lake ranged from 2.75 to 4.87 m, and the water depth at sampling sites ranged from 1.07 to 3.31 m. The SAV biomass at the sampling sites ranged from 0 to 17.61 kg/m2. The influence of water depth on SAV biomass and frequency of occurrence differed by seasons and by species. The adaptation of SAV species to increasing water depth is a key element for community dynamics, which in turn contributes to water level regulation. A new method was proposed to identify the optimal water depth for SAV biomass accumulation based on calculation of the cumulative probability and probability density. Keywords: submerged aquatic vegetation; water depth; eutrophic lake; vegetation biomass; Lake Taihu 1. Introduction Submerged aquatic vegetation (SAV) affects the physical environment (light extinction, temperature, hydrodynamics, substrate), chemical environment (oxygen, inorganic and organic carbon, nutrients), and the biota (epiphytes, grazer, detritivores, fishes) of aquatic ecosystems, thereby playing important roles in their structure and function [1]. Areas with dense SAV in shallow lakes typically have very clear water and low concentrations of nutrients and phytoplankton [2,3]. However, a variety of studies have described large fluctuations in SAV biomass and communities due to variable water depths [4,5], which may have different consequences for ecosystem processes in lakes. Many studies have addressed how water depth affects the growth and regrowth of SAV individuals and the community [6–10]. For example, dense SAV is usually found only in water ecosystems of <2 m depth [11]. Increasing water depth decreased the growth of Potamogeton perfoliatus, Myriophyllum spicatum, and Chara fragilis in terms of biomass, number of shoot nodes, and shoot length [5]. In a field experiment conducted under controlled conditions, plant length, root length, root number, and biomass of Vallisneria natans decreased as water depth increased, and the plant was absent at a water depth of 200 cm regardless of substrate type and wave exposure conditions [12]. The situation was different in a natural lake ecosystem, where a feedback mechanism was induced with the increase in water depth [13]. Compared with the experiments conducted in a laboratory or pond, macrophyte communities in large lakes and reservoirs are often diverse because water exchange and plant seed drift are less restricted Water 2019, 11, 2412; doi:10.3390/w11112412 www.mdpi.com/journal/water Water 2019, 11, 2412 2 of 12 owing to considerable water flow and wave disturbance. Furthermore, the temporal and spatial scaling is much higher in lake ecosystems than in controlled experiments, and the intra- and interspecific competition among aquatic plants is continuous in the former. Water level fluctuations in lakes can be driven by natural fluctuations in rainfall and runoff, as well as by water abstraction and flood control. Species that respond quickly to changes and have high morphological and physiological plasticity may suffer more than slow-response or non-plastic species under changing water levels, ultimately resulting in altered SAV [8,14]. Lake Taihu is a large shallow lake with a surface area of 2338 km2 and mean depth of 1.8 m. It is dominated by subtropical summer monsoon, with an average annual rainfall about 1177 mm in its basin [15]. The lake is located in the southern part of the Yangtze River Delta, one of the most densely populated regions in China. Lake Taihu is known to simultaneously harbor both macrophyte- and phytoplankton-dominated regions [16–18]. Macrophytes are especially abundant in the eastern part of the lake, such as Guangfu Bay, Linhu Bay, and Dongtaihu Bay [19–22], and phytoplankton is mainly dominant in Zhushan Bay, Meiliang Bay, and the western part of the lake [23–25]. However, the distribution of macrophytes in Lake Taihu has changed dramatically in recent years, according to in situ investigation and remote sensing research [22,26]. Nutrient enrichment and light limitations were reported as important factors affecting the distribution of macrophytes in Lake Taihu [27]. Changes in water depth are another key factor critical for the variation in biomass and community structure of macrophytes. A previous study documented a “natural experiment” about the impact of lake drawdown and severe drought on the SAV community in Lake Okeechobee [4]. The present study investigated a different “natural experiment” in a large shallow eutrophic lake—a dramatic increase in water level after a heavy rainfall in the lake basin. This event had a remarkable impact on the SAV community. There was an ongoing extensive SAV monitoring program already in place prior to the dramatic changes in water level, which documented seasonal and year-to-year variations in SAV. Therefore, it was possible to (1) examine submerged plant biomass in relation to changes in water depth across different seasons; (2) quantify the effects of water depth fluctuations on the development of the submerged community; and (3) provide suggestions for water depth regulation that will benefit SAV biomass in large shallow lakes. 2. Methods 2.1. Aquatic Vegetation Biomass and Water Depth Survey A total of 39 sites that covered the whole macrophyte-dominated area were used to monitor aquatic plants (Figure1). Sampling sites were selected with the aim of sampling the aquatic plants distribution area as evenly as possible. At each site, water depth was measured with a calibrated plastic rod. Considering that Secchi depth has been reported as a limit factor for SAV species re-growth and secession [6], Secchi depth transparency was measured with a black and white disk measuring 0.25 m in diameter. The SAV was manually extracted using weighted stainless-steel frames with an area of 0.2 m2 and a nylon mesh bag. The SAV was harvested underwater by breaking the stems just above the sediment surface and collecting the aboveground materials in the nylon mesh bag. The bag was rinsed in the lake water to dislodge loosely attached periphyton, and the plant material was identified to the species level. The biomass of each species was measured as wet weight expressed in kilogram per square meter (kg/m2). Water 2019, 11, 2412 3 of 12 Water 2019, 11, x FOR PEER REVIEW 3 of 12 FigureFigure 1.1. Aquatic plants samplingsampling sites.sites. 2.2.Aquatic Calculation vegetation of Proper was Water sampled Depth infor February, SAV May, August, and November from 2013 to 2017. The daily water level data for the period 2013 to 2017 were collected from Taihu Basin Authority of the MinistryIncreasing of Water Resources. the SAV biomass is an important factor in restoration and management of eutrophic lakes. Therefore, the water depth at which SAV biomass experiences the maximum increasing rate 2.2.was Calculation determined of Proper and Waterthe most Depth optimal for SAV water depth for SAV biomass accumulation was proposed based on the calculation of cumulative probability distribution (CPD) and probability density of SAV Increasing the SAV biomass is an important factor in restoration and management of eutrophic biomass for all 780 groups. First, CPD of the SAV biomass with water depth was calculated as: lakes. Therefore, the water depth at which SAV biomass experiences the maximum increasing rate was determined and the most optimal water depth for SAV biomass accumulation was proposed based on = ∑ , the calculation of cumulative probability distribution (CPD ) and probability density of SAV biomass forwhere all 780 groups. is the First,waterCPD depth,of the SAV is biomass the CPD with of waterthe SAV depth biomass was calculated at wateras: depth ( =), is SAV biomass when = according to the monitoring data, and is Xi Biomassi the sum of all SAV samples (780 CPDgroups)i = collected during the, study period. CPD was calculated for i=0 Total Biomass each water depth at which monitoring data were sampled. The maximum CPD is 1 when is the wheremaximumi is the water depth,depth. CPDi is the CPD of the SAV biomass at water depth i (WD = i), Biomassi is SAV biomassSecond, when theWD CPD= andi according water depth to the monitoringdata were fitted data, andto aTotal curve Biomass to examineis the sum the ofrelationship all SAV samplesbetween (780 the groups) independent collected variable during thewater study depth period. (x) andCPD thewas response calculated variable for each CPD water (y). depthThe curve at whichfitting monitoring equation datawas expressed were sampled. as = The maximum. CPD is 1 when i is the maximum water depth. Second,Third, the theCPD derivativeand water of depth was data calculated were fitted as to = a curve, towhere examine is the the relationship probability between density of theSAV independent biomass. variableThe maximum water depth of ( xis) andthe highest the response probability variable ofCPD SAV( ybiomass).
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