Estimating the biomass of unevenly distributed aquatic vegetation in a lake using the normalized water-adjusted vegetation index and scale transformation method
Graphical abstract
Introduction
There are 304 million natural lakes with an area greater than or equal to 0.1 ha on all continents. These natural lakes cover more than 2.8% of the land surface, not including temporary water bodies and wetlands (Downing and Duarte, 2009). Aquatic vegetation is an essential element in the life systems of most lakes and performs a number of useful functions in maintaining the productivity (Brothers et al., 2013) and biogeochemical cycles (Carpenter and Lodge, 1986) in lakes. It has been reported that macrophytes provide microhabitats for zooplankton (e.g., space and food resources, yielding positive relationships with zooplankton diversity) and that microhabitat structure determines the diversity and abundance of zooplankton communities (i.e., dry weight, species number, and plant type) (Folt and Burns, 1999, Padial et al., 2009, Choi et al., 2014). Macrophytes also provide habitat areas for aquatic insects, fish, and other resident aquatic and semiaquatic organisms and provide structure and food for mammals, birds, reptiles, and amphibians. Aquatic macrophytes can stabilize clear water conditions in shallow lakes (Scheffer et al., 1993, Hilt et al., 2011). Aquatic plants reduce the resuspension of sediment particles (Barko and James, 1998, Vermaat et al., 2000), the water turbidity (Sachse et al., 2014), the light availability for phytoplankton (Pokorný et al., 1984), the phosphorus and the nitrogen (Zuo et al., 2015). In addition, aquatic vegetation serves to anchor soft sediments, stabilize underwater slopes and soft lake bottoms and minimize shoreline erosion by dampening the effect of waves; macrophyte uptake from the interstitial waters is responsible for a significant loss of phosphorus from the sediment (Gudimov et al., 2015). Aquatic vegetation produces and stores significant amounts of carbon as biomass (Means et al., 2016) and represents a major component of the carbon balance due to its fast growth rates and high productivity (Piedade et al., 1991, Costa, 2005, Engle et al., 2008, Silva et al., 2009, Silva et al., 2010). All of these factors emphasize the importance of a healthy, natural aquatic plant community.
Information on the areal biomass and distribution of aquatic vegetation is necessary for the monitoring, management and understanding of lake aquatic ecosystems (Vis et al., 2003). However, determining the properties of aquatic plants such as biomass is difficult at both small and large spatial scales because of the spatial heterogeneity of aquatic plant communities (Duarte and Kalff, 1990, Vis et al., 2003, Silva et al., 2008). Traditional field-based mapping and monitoring of aquatic vegetation biomass at large spatial extents presents several challenges, including the inaccessibility of some areas for field sampling; rapid changes in aquatic plant location, extent, and density; and high costs and travel time. Furthermore, such mapping and monitoring is labor intensive and destructive to sensitive lake ecosystems (Zhang et al., 1997, Jakubauskas et al., 2002), resulting in reduced sampling effort and/or incomplete data sets with limited spatial and temporal coverage (Vis et al., 2003). Given these limitations, biomass estimation through satellite remote sensing is a feasible alternative (Costa, 2005, Silva et al., 2010, Goetz and Dubayah, 2011, Byrd et al., 2014). Remote-sensing methods offer the ability to continuously monitor growth and phenology on the same individuals and can reduce the effort involved in biomass harvesting, but these methods depend on strong relationships between the predictor variables and plant biomass (Armstrong, 1993, Peñuelas et al., 1993, Daoust and Childers, 1998, Zhang, 1998, Silva et al., 2010). Estimating the biomass of aquatic vegetation using remote sensing and statistical regression involves two important parameters: the biomass values of the field sampling-site quadrats and the spectral data. These two parameters directly determine the accuracy and precision of the biomass estimation of aquatic vegetation obtained through satellite remote sensing. Due to the large spatial heterogeneity and uneven distribution of aquatic vegetation in lakes, areas of continuous and uniformly distributed aquatic vegetation that are 30 × 30 m or larger are uncommonly found in field investigations. Thus, the biomass values of field sampling-site quadrats cannot be regarded as identical to those of field sampling-site pixels. If the biomass values of field sampling-site quadrats were directly used to build a model, serious errors or large deviations would result in the biomass estimation based on remote sensing. Therefore, it is necessary to apply a scale transformation to the biomass values of the field sampling-site quadrats before building a statistical model of biomass based on remote sensing. The other parameter used for biomass estimation is the spectral data of the remote-sensing imagery, which mainly include the bands or band combinations (ratios, indices) (Armstrong, 1993, Peñuelas et al., 1993, Zhang, 1998, Silva et al., 2008). To date, published studies (e.g., Peñuelas et al., 1993, Zhang, 1998, Payton, 2001, Schweizer et al., 2005, Ma et al., 2008, Robles, 2009, Byrd et al., 2014, Massicotte et al., 2015) on the use of vegetation indices to estimate the biomass of aquatic vegetation are limited to the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), and the normalized difference green index (NDGI). However, these vegetation indices were established mainly for terrestrial vegetation, and the question remains as to whether a new aquatic vegetation index can be established that is more suitable for the remote-sensing estimation of aquatic vegetation biomass in lakes.
The remote-sensing estimation of aquatic vegetation biomass in lakes remains rare, and few studies have used scale transformation and the aquatic vegetation index in the remote-sensing estimation of aquatic vegetation (including floating-leaf and submergent vegetation and excluding emergent vegetation, as described below) biomass in a lake. In the following, we proposed a scale transformation (CT) method of converting field sampling-site biomass from the quadrat to the pixel scale for remote-sensing biomass estimation and a new normalized water-adjusted vegetation index (NWAVI) based on remote-sensed imagery for the biomass estimation of aquatic vegetation. Then, we tested whether it was possible to estimate the biomass of aquatic vegetation in a lake using the proposed CT- and NWAVI-based method and evaluated the performance of the biomass estimation. Finally, we mapped the spatial distribution and temporal change of the aquatic vegetation biomass in a test lake in different months.
Section snippets
Study area
Lake Taihu, the third largest freshwater lake in China, is located in the south of Jiangsu Province, China (119°53′49″–120°35′25″E and 30°55′32″–31°32′50″N) (Fig. 1). It has a surface area of 2338 km2 and average and maximum water depths of 1.89 m and 2.6 m, respectively. Before the 1960s, Lake Taihu had a large area of aquatic vegetation (Nanjing Institute of Geography, Chinese Academy of Sciences, 1965). According to recent field surveys, the distribution of aquatic vegetation was widest for
Field data: quadrat- and pixel-scale biomass
In May 2013, the biomass values of the thirty-five field sampling-site quadrats ranged from 0.17 to 6.64 kg/m2 with an average of 1.75 kg/m2 and a standard deviation of 1.34 kg/m2; the biomass values of the thirty-five field sampling-site pixels ranged from 0.03 to 3.29 kg/m2 with an average of 0.73 kg/m2 and a standard deviation of 0.75 kg/m2. Compared with the values from the corresponding quadrats, the biomass values of the thirty-five field sampling-site pixels were reduced from − 34.59% to −
Importance of field biomass-scale transformation
In most studies estimating the biomass of aquatic vegetation using remote sensing (e.g., Peñuelas et al., 1993, Zhang, 1998, Payton, 2001, Schweizer et al., 2005, Ma et al., 2008, Robles, 2009, Byrd et al., 2014, Massicotte et al., 2015), measurements of aboveground biomass of aquatic vegetation made during field surveys were used to model the relationship between the field biomass and the spectral data of the remote-sensing imagery. Byrd et al. (2014) averaged the biomass from multiple plots
Data accessibility
The remote-sensing data used in this manuscript have been archived on the website of the China Center For Resources Satellite Data and Application (http://www.cresda.com).
Acknowledgments
This research was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment (Grant numbers 2012ZX07501-001-03 and 2012ZX07506-001). The authors would like to thank the China Center for Resources Satellite Data and Application (CCRSDA) for their significant efforts in developing and distributing the remotely sensed satellite data and products and for their kindness in providing online accessibility to download this information.
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