Hydroacoustic analysis of spatial and temporal variability of bottom sediment characteristics in Lake Kinneret in relation to water level fluctuation

Rapid alterations of aquatic ecosystems associated with changing climate and growing anthropogenic stress require the development of effective and low-cost acoustic methods of bottom sediment characterization and mapping. Acoustic reflectance and scattering properties of surface bottom sediment (SBS) were characterized by energetic, statistical, spectral, wavelet, and fractal parameters of single-beam echo envelopes. A common feature of the chosen parameters is indication of energy distribution in the frequency domain and fractal description of the echo-envelope shape. Data collected with a 120-kHz single-beam echosounder along 14 regular transects were used to determine the acoustical properties of SBS, and study their spatial and temporal changes in Lake Kinneret, and their relation to sedimentological characteristics of the bottom. Special post-processing procedures were applied to remove the instability of the acoustical signal (caused by angular movement of the transducer), and the impact of the bottom slope. The influence of changing depth on the shape of the echo signal was corrected by the depth normalization procedure. Sediment grain size in the upper few-centimeter layer was studied based on grab samples. Some acoustical parameters showed close association with granulometric variables and organic matter content in the upper sediments. The observed effect of water level fluctuation on several echo parameters was apparently associated with changes in gas content in the SBS. Spatial distributions of some echo parameters were very stable with time; other parameters showed large changes in the central part of the lake, but were nearly unchanged at the lake periphery. Overall, the suggested approach is a useful tool for investigation of fine modifications of SBS properties in aquatic ecosystems.