Krista Alikas defended her doctoral thesis about monitoring optically complex waters with MERIS/ENVISAT data

Tiia Lillemaa | 15.06.2016

Krista Alikas defended her doctoral thesis titled "From research to applications-monitoring optically-complex waters with MERIS/ENVISAT data" on 15 June 2016 in the University of Tartu in the field of environmental technology.

Krista kaitsmine.jpg

Supervisors Anu Reinart, PhD, Director of Tartu Observatory and Marko Vana, PhD, Research Fellow of Institute of Physics, University of Tartu.

Opponent Sampsa Koponen, PhD (Finnish Environmental Institute).


Lakes and coastal areas provide a wide range of essential ecosystem services. Various directives and regional conventions have been established to ensure the monitoring and assessment of the ecological status of the aquatic ecosystems. Since water quality can have rapid changes in temporal and spatial scale, remote sensing can provide a cost-effective approach to assess the current and derive historic water quality information also for waterbodies that have not been part of conventional monitoring programmes.

This thesis presents research about applications for MERIS/ENVISAT data in order to monitor optically-complex aquatic environment such as inland and coastal waters on the basis of bio-optical data from five North European lakes and two Baltic Sea coastal sites. The validation of MERIS standard water quality products indicated their unsuitability for waters with high amounts of chlorophyll a and humic substances. To map the phytoplankton parameters (CHL, cyanobacterial biomass, phytoplankton biomass) a spectral index which operates on red and near-infrared bands was used and calibrated to local conditions. So, this index allows derivation of the water quality parameters quantitatively in case of highly scattering cyanobacterial blooms, which is not possible with standard algorithms.

To estimate underwater light field via transparency, an empirical combined band ratio algorithm was developed which switches from various band ratios based on the water transparency and determines the diffuse attenuation coefficient Kd(490) with high accuracy. Additionally, Secchi depth can be also estimated reliably via satellite derived inputs of diffuse and total beam attenuation coefficients and reflectance over visible wavelengths.

The developed algorithms were applied on the MERIS archive from 2002 to 2011 to estimate the ecological status in lakes as required by the EU Water Framework Directive which showed that remote sensing products could be used as an additional source of information for assessment and reporting purposes. Despite the study in this thesis is based on the MERIS/ENVISAT data, the developed algorithms and methods can be applied on new Sentinel3/OLCI data that will provide EO data over optically complex waters at least until 2029.