Many enclosed former estuaries experience water quality problems. These problems are related to stratification, which inhibits the downward mixing of dissolved oxygen to the deep former estuarine channels and pits, leading to hypoxia. Former estuaries like Lake Veere and Grevelingenmeer have also seen events of massive fish mortality, presumably related to the wind-driven upwelling of deep water. To unravel these dynamics, we require more insight into the spatiotemporal behaviour of the stratification.

Stratification is currently measured through fortnightly temperature-salinity-oxygen transects with measurement points every few kilometres. These often do not capture all relevant dynamics, such as mixing and internal oscillations of isopycnals (e.g., tilting, seiching). Echosounding, which overcomes these issues, has been used extensively in the ocean to detect stratification but has seen limited use in lakes, where scatterers are often scarce. Impedance gradients by stratification are an established source of backscatter but are often overshadowed by other potential scatterers (i.e., air bubbles, biota, turbulence). The current generation of acoustic Doppler current profilers (ADCPs) allows for multi-frequency echosounding in combination with the usual velocity measurements, providing new opportunities to acoustically monitor stratification by covering a large frequency bandwidth.

In this study, we aimed to continuously monitor stratification and infer internal processes through narrowband multi-frequency echosounding. In the summer of 2024, we collected measurements with two up-looking ADCPs, three underwater moorings equipped with thermistor chains, and a high-resolution CTD casting instrument in the northwest of Lake Veere. The ADCPs measured backscatter at three frequencies: 250, 500, and 1000 kHz, a wide bandwidth that we expected to facilitate the distinction between backscatter mechanisms at a high spatial resolution. We compared direct temperature measurements by moorings, high-resolution conductivity-temperature-depth (CTD) casts, and frequency-dependent acoustic backscatter to infer backscatter mechanisms. We then used the acoustic backscatter to monitor stratification by tracking (gradient) maxima and compared acoustically derived thermocline heights to directly inferred thermocline heights from the temperature moorings. Finally, we used the monitored stratification to characterize internal seiching by combining data from several locations. To that end, we analysed the time series of thermocline heights using the continuous wavelet and wavelet coherence transforms to identify periods of internal seiching and validated those with current profiles measured by the ADCPs.

We observed stratification of the upper water column in the acoustic data during extended warm periods. The upper and lower layers were separated by a gradient in backscatter, with increased backscatter in the upper water column. This gradient coincided with the directly measured main thermocline, demonstrating the potential of monitoring the thermocline height through acoustic backscatter. We hypothesize that the large backscatter gradient (as opposed to a local backscatter maximum) was caused by a difference in phytoplankton concentration, which remained above the thermocline through buoyancy and bloomed during extended warm periods (end of July and August). Consequently, stratification was more visible in the acoustic data during these warm periods, despite not necessarily being strongest at those times. We did not observe impedance gradients due to stratification; instead, we attribute local backscatter maxima to suspended matter aggregating on isopycnals. As such, we continuously observed a secondary layer near the bed throughout June and July.

We computed phase velocities of internal seiching using CTD casts and estimated the internal seiching bandwidth between 1–10 hours. We found events of cross-lake and along-lake internal seiching—with respective periods of 4 and 6 hours—in thermocline heights through increased wavelet coherence and out-of-phase behaviour between measurement locations on opposing sides of the lake. Additionally, most thermocline height time series sporadically contained a daily component, which was likely forced by wind rather than heat input. The secondary layer near the bottom contained both a daily and semi-daily component, with the latter likely related to tidal forcing. We validated the observed internal seiching using velocity data to confirm that velocities above and below the thermocline were out-of-phase (i.e., reversed). Finally, we found that velocity shear was not a consistent indicator of thermocline height, but was often either concentrated at or bounded by the thermocline, thus providing a general measure of the average thermocline height.