For many water management issues of shallow lakes with non-consolidated sediments hydrographic surveys of the open water area and reed belt areas are required. In the frame of water management strategy for the steppe lake Neusiedler See, located between Austria and Hungary, a hydrographic survey was conducted. In the open water area (water depth ≥1 m) a sediment echosounder was used. To validate these measurements and to distinguish between water, mud, and sediment layers in the shallow lake and reed belt area additional measurements were needed. As no common standard methods are available yet, we developed a measurement system based on two commonly applied soil physical measurement techniques providing reproducible physical values: a capacitive sensor and a cone penetrometer combined with GNSS-positioning enable dynamic measurements of georeferenced vertical water-mud-bedsediments profiles. The system bases on site-specific calibrated sensors and allows instantaneous, in situ measurements. The measurements manifest a sharp water-mud interface by a sudden decline to smaller water content which is a function of the dielectric permittivity. A second decline indicates the transition to compacted mud. That is concurrently the density where the penetrometer starts registering significant penetration resistance. The penetrometer detects shallow lakebed-sediment layers. Within the lake survey this measurement system was successfully tested.
Abstract. Lake sediment characterization, a prerequisite for the vulnerability assessment of lake ecosystems, demands reliable in situ methods for the characterization of the sediment layer composition. A unified characterization of lake sediments within lake ecotopes (open water, open water patches within the reed, and the reed) is still a challenge. Each ecotope is covered by different classical scientific disciplines (hydrography and terrestrial remote sensing to soil physics) with their specific characterization methods. Recently, a complementary tool that bridges the gap between land and hydrographic surveying methods was introduced. It is a non-acoustic device that combines two soil physical sensors (a capacitive sensor and a cone penetrometer) and GNSS-positioning in a measuring system (CSPS). The CSPS enables rapid in situ delineation of water-mud-consolidated lakebed interfaces. The system was successfully applied across ecotopes at the Neusiedler See, a well-mixed shallow lake rich in fine-grained sediments. The geo-referenced vertical CSPS profiles show ecotope-specific layer composition. The effects of wind-induced turbidity, particle size, and electrical conductivity were analysed. The water-mud interface was precisely delineated at the open water due to a persistent high water content gradient, equivalent to a lutocline. The penetration resistance (PR) for open water showed either a shallow and highly compacted consolidated lakebed or a consolidated lakebed with a partially compacted layer above, while in the reed the PR smoothly increased until reaching the deepest penetration depths.
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