Suse Kratzer and Therese Harvey, Department of ecology, environment and plant sciences, Stockholm University
Suse Kratzer and Therese Harvey, Department of ecology, environment and plant sciences, Stockholm University

How would you describe this project in a few sentences?

Bio-optics can be described as the interaction of light with phytoplankton, suspended particulate matter (SPM) and coloured dissolved organic material (CDOM) within the water environment. Bio-optical variables can also be measured form space using remote sensing data. In our group we work on methods that can be used to monitor coastal and open sea areas from space, with the aim to improve adaptive management.

What are your most important results, and for whom are they particularly useful?

In Harvey et al. we have studied how different concentrations of the bio-optical parameters affect the variations in the Secchi depth in different Baltic Sea regions. Secchi depth is one of the most common water quality parameters, and is an integrated measure of the water transparency. We found that the variations in the Secchi depth were governed by different proportions and ranges of the main optical components (chlorophyll-a, suspended and dissolved matter) in the different regions. We also developed a new algorithm for turbidity that can be applied to MERIS data over the Baltic Sea in a paper by Kari et al. The algorithm only had about 5% bias when compared to in situ data. This will make the monitoring of suspended particles in the coastal zone more reliable and cost efficient. In Kyryliuk et al. suspended sediment retrieved from the MERIS satellite sensor were used as an indicator for coastal waters, which showed that the coastal processes extent much further in the southern Baltic proper (up to ~ 40-100 km), than in the NW Baltic proper (~20-40 km). This could be explained by difference in land run-off, sediment type and wind forcing. The paper by Beltran-Abaunza et al., on using MERIS data to evaluate the spatio-temporal changes of phytoplankton in Himmerfjärden with remote sensing data, allows to derive phytoplankton phenology charts that are representative for each sub-basin in Himmerfjärden. We also deployed a radiometer close to the oceanographic buoy at B1 during June 2015. The radiometer recorded the diel changes in the diffuse attenuation of light, and thus also in Secchi depth. We are planning to re-deploy the instrument during May 2016 for Sentinel-3 validation.

Our results are important for management of the water quality, especially with regards to eutrophication assessment within both the European Water Framework Directive and the Marine Strategy Framework Directive. The use of bio-optical data and remote sensing are becoming a more common source of information within monitoring programs and for authorities working with issues related to water quality.

How can it assist an ecosystem-based management of the marine environment?

Chlorophyll-a concentration is often used as a direct link to changes in the water transparency, and to often wrongly assumed to be the only optical parameter affecting the Secchi depth. The Secchi depth paper contributes to a broader view of the use of Secchi depth within monitoring and management of the Baltic Sea. Other optical parameters besides chlorophyll-a concentration need to be considered within management for e.g. setting the target goals for Secchi depth. The results showed that by improving the chlorophyll-a concentration it was still not possible to reach the Secchi depth for Good Ecological/Environmental Status (GES) within the directives target in some water bodies. There is already a high background absorption of coloured dissolved organic matter (CDOM) that has a major effect on the Secchi depth. In some coastal areas the scatter from suspended material also has a strong influence on the Secchi depth. The other papers demonstrate how remote sensing can be included in both coastal zone and basin-wide management of the Baltic Sea. The methods have shown to be reliable, cost-efficient and also have a better spatio-temporal resolution than data measured from ship. The satellite data can be used to identify areas that should be monitored more frequently by ship.

It must be noted, though, that ship monitoring is very important as it allows to sample and evaluate many more ecological variables. The satellite method should be therefore seen as a complement to ship monitoring and buoy measurements.

Members of the Marine Remote Sensing Group at DEEP:

  • Susanne Kratzer (project leader), Associate Professor
  • Therese Harvey, Researcher
  • Jose Beltran-Abaunza, PhD student
  • Elina Kari, PhD student
  • Dmytro Kyryliuk, PhD student


  • Jakob Walve, Researcher, DEEP (SU)
  • Helena Höglander, DEEP (SU)
  • Steve Lyon, Professor, INK, (SU)
  • Michelle McCrackin, Baltic Eye (SU)
  • Barbara Deutsch, Researcher (Askö Laboratory)
  • Mattias Murphy, Technician (Askö Laboratory)
  • Petra Philipson, Brockmann Geomatics Sweden AB
  • Agneta Andersson, Professor (UU)
  • Bengt Karlson, Researcher, (SMHI)
  • Matti Leppäranta, Professor, University of Helsinki, Finland
  • Ioanna Merkouriadi, University of Helsinki, Finland
  • Diana Vaičiūtė, Researcher, Klaipeda University, Lithuania
  • Gerald Moore, Researcher, Bio-Optika, UK