Doctoral defence: Mirjam Uusõue “Suspended particles dynamics and characteristics in optically complex waterbodies”

On 10 May at 12:15 Mirjam Uusõue will defend her doctoral thesis “Suspended particles dynamics and characteristics in optically complex waterbodies” for obtaining the degree of Doctor of Philosophy (in Environmental Technology).

Supervisors:
Associate Professor Martin Ligi, University of Tartu
Professor Tiit Kutser, University of Tartu
Lecturer François Bourrin, University of Perpignan (France)

Opponent:
Research Fellow Therese Harvey, Norwegian Institute for Water Research (Norway)

Summary
Clean water is necessary for all living organisms, but human activities and climate change have deteriorated water quality in many regions. It is important to monitor water bodies to take the necessary measures. Conventional contact measurements have been carried out for decades. However, they are time-consuming and expensive and can cover only a small number of water bodies. This is insufficient as changes in waterbodies are often rapid and occur over large areas. Remote sensing methods have been introduced to fill these gaps in temporal and spatial coverage. On the other hand, there is a need to study parameters that impact the formation of water-leaving signal in order to increase the accuracy of remote sensing products. One such substance that requires further investigation is suspended particulate matter. It consists of mineral and organic particles suspended in the water column due to natural processes (waves, erosion, river transport) and human activities (trawling, dredging, construction). The high content of suspended particles increases the water turbidity, consequently reducing the light availability. In turn, this worsens aquatic organisms' living conditions and alters physical processes (like radiative heating) in the water environment. Suspended particles can contain harmful substances such as organic pollutants and heavy metals. We studied the behaviour of heavy metal-rich sediments in Portman Bay (one of the most polluted areas of the Mediterranean), in southern Spain, after trawling the sea bottom. Coarse and heavy particles and flocs settled quickly. Fine particles remained in suspension for a longer time. Therefore, the content of heavy metals in the water increased. From an optical point of view, the suspended particles mainly scatter light. The optical properties of suspended particles (light absorption, scattering, backscattering/scattering ratio) were studied in several Estonian coastal areas. The backscattering ratio, which is used as a constant in bio-optical remote sensing algorithms, was highly variable, depending on the conditions (algal bloom, storm). Such variability was also observed for other optical properties. It was found that small particles resuspended by a storm aggregated into larger flocs (> 30 μm), affecting the particles' optical properties in the Pärnu Bay, which has very low salinity and little organic content. This study shows that using an assumption that the backscattering ratio is a fixed constant reduces the accuracy of remote sensing products.

Defence can be also followed in Zoom: https://ut-ee.zoom.us/j/96224907046?pwd=d21FY2NvWGpuSHhYWUJGMjdDZEo1dz09. Meeting ID: 962 2490 7046, Passcode: 196606.