A method for extracting the spatial distribution of submerged vegetation in lakes based on topography measurements

Abstract

Aquatic vegetation in lakes provides various ecosystem services, identification of its spatial distribution has great importance in lake ecosystem management and conservation. Monitoring of submerged aquatic vegetation is limited due to plants being under the water surface. Based on topography measurements, a new method was proposed to identify the distribution of submerged vegetation. With submerged vegetation occurred in lakes, water depth measured by an echo sounder could be influenced by vegetation and differ from its true value. Vegetation distribution can be extracted by comparing the topographic maps measured with and without vegetation presence. In present study, identification for Potamogeton crispus distribution in Dongping Lake (China) was taken as an example to show the procedures. Errors and accuracy of this method were analyzed, and a discussion was provided about the usefulness and limitations of the proposed method. In this study, measurements were conducted in March (with P. crispus occurrence) and September (without P. crispus occurrence), 2021. Five interpolation methods with ten resampling intervals were used and compared for each measurement, and the minimal mean absolute errors (MAEs) for interpolated topographies in Mar. and Sep. measurements were 0.63 m and 0.46 m, respectively. MAE for the subtraction of raster data between two topographic maps over the entire lake was 0.59 m. Besides, accuracy of this method was validated by comparing the extracted distribution of P. crispus with its distribution of biomass. Taking the P. crispus distribution with dry biomass density larger than 26 g/m2 as reference, the identification accuracy of this method reached its maximum (78%). In regions with P. crispus sparsely distributed, the measured water depth was seldom influenced by vegetation, leading to the differences between two maps ignorable in these regions.