Spatial-temporal observations of thermocline dynamics in an enclosed former estuary

In the recent decades, throughout coastal areas all around the world, estuaries have been closed off to improve coastal safety. An example is Lake Veere, located in the Southwest of the Netherlands. However, this lake experiences water quality issues, strongly related to the two-layer stratification that develops in summer, inhibiting downward mixing of oxygen. In combination with benthic oxygen demand, this results in anoxic conditions in the lower layer, most prominently in the western part of Lake Veere. It is hypothesized that upwelling of this lower anoxic layer has caused mass fish mortality events. This study investigates the spatial-temporal behaviour of the thermocline, studying how the stratification arises and develops, and what the effects of environmental factors such as wind and rainfall are on the stratification intensity and the movements of the thermocline.During the summer of 2024, field observations were conducted in Lake Veere. From June till August (92 days), three moorings with arrays of temperature sensors with a vertical resolution of 0.4 m measuring at 1 Hz were deployed in the northwestern part of the lake. The moorings were placed in a triangle, with two moorings at the ends of a previous tidal channel, and the third at a shallower locationto obtain a spatial image. In addition, 2 ADCP’s were placed between the moorings to obtain flow velocity data. Data on wind and precipitation was obtained from the Dutch weather institute (KNMI) and Rijkswaterstaat (RWS), and RWS also provided measurements of the oxygen levels in the lake, co-located with our temperature measurements.Oxygen measurements at depths above the thermocline show decreasing levels of oxygen during certain wind events. The temperature profiles clearly show stratification as it arises from warm weather and then develops until it is disturbed. Correlating the wind velocity and direction with the temperature results, it can clearly be seen that increasing wind speeds in along-channel direction have a strong effect on the stratification, ranging from tilting of the thermocline to completely mixing the water column. The spatial image of the data shows that during certain wind events, tilting occurs. Comparing the temperature data with the measured oxygen levels, it can be observed that the tilting of the thermocline results in movement of the anoxic water, causing it to well up at the end of the lake.Due to climate change, period of warm weather are more extreme and longer in duration. This results in stronger stratification, deteriorating the water quality in these lakes. Studying the fluid dynamics in these systems contributes to finding solutions to improve the water quality and the local ecosystem.