Impact of currents and turbulence on turbidity dynamics at the time series station Spiekeroog (Wadden sea, Southern North sea)

Abstract

Shallow coastal areas with depths less than 20 m are high-energy regions where strong mixing processes occur mainly influenced by currents and turbulence. To investigate these mixing processes, it is necessary to obtain a high number of field observations of mixed variables as well as their interactions with other processes and dynamics. In this context, our research platform (Time Series Station Spiekeroog) in the East Frisian Wadden Sea, southern North Sea, offers the opportunity to measure hydrodynamic and meteorological parameters over long time periods. This allows us to get insights into the natural processes of an intertidal ecosystem under various environmental conditions such as storm surges or algae blooms. The objective of this paper was to investigate the impact of currents and turbulence on turbidity dynamics based on measurements at the Time Series Station Spiekeroog. Current velocities are continuously recorded in three dimensions from a bottom-mounted Acoustic Doppler Current Profiler (ADCP, RDI Workhorse Sentinel, 1200 kHz) with a high temporal and spatial resolution. These high-resolution data sets are used to estimate turbulence using the production rate of turbulent kinetic energy. To determine turbidity, we used an ECO FLNTU sensor (WETlabs) which measured optical scattering at a wavelength of 700 nm while being mounted 12 m above the sea floor close to the sea surface. Comparing the turbidity data set with the backscatter signal from the ADCP we could estimate the vertical structure of the turbidity. Further we compared the turbidity data set with current speed, production rate and shear velocity. Our results showed a link between the production rate and turbidity at slack water times. Although the research area is an ebb dominated system, we observed a opposite behaviour of the turbidity and backscatter signals. Since current speeds, shear velocities and turbulence signals all show ebb dominance, we concluded that the production rate holds the turbidity in the water column at high tides. At times with high current speeds the shear velocities and therefore the current velocities are responsible for the turbidity dynamics: At flood, the shear loads the turbidity up into the water column; In contrast, at ebb tide the shear prevents the transport to the sea surface area.