Liisa Metsamaa

Mapping coloured dissolved organic matter concentration in coastal waters

Optical properties of the Baltic Sea are dominated by coloured dissolved organic matter (CDOM). High concentration of CDOM is probably one of the reasons why standard chlorophyll-retrieval algorithms fail badly in the Baltic Sea. Our aim was to test can CDOM be mapped by remote sensing instruments in coastal waters of relatively CDOM-rich environments like the Baltic Sea. The results show that sensors with high radiometric resolution, such as Advanced Land Imager (ALI), can be used for mapping CDOM in a wide concentration range. The ALI image also showed that optical properties of coastal waters are extremely variable. CDOM concentration may vary 4–5-fold in two adjacent 30 m pixels. This indicates a need for relatively high spatial resolution (30 m or less) remote sensing data in monitoring coastal environments.

On suitability of MODIS satellite chlorophyll products for the Baltic Sea conditions

The amount of phytoplankton, usually expressed as concentration of chlorophyll-a, is one of the most important parameters in describing state of water bodies. The amount of chlorophyll-a in coastal waters is extremely variable both spatially and temporally. Therefore, the classical monitoring programs, which are based on infrequent measurements in a few fixed stations, or ships of opportunity cannot provide spatial and/or temporal coverage needed to determine the amount of chlorophyll-a. Remote sensing could be the tool to provide necessary information. Chlorophyll retrieval algorithms proposed by different authors are used by space agencies to provide satellite derived chlorophyll products. These products are relatively accurate in Case I (oceanic) waters. However, they often fail in Case II (coastal) waters. We have compared MODIS chlorophyll product with in situ measurements carried out in the frame of Estonian National Monitoring Program. The results indicate that correlation between satellite derived chlorophyll estimates an in situ measurement results is very pure in Estonian coastal waters and there is strong need to develop local chlorophyll retrieval algorithms. This is a task our group is currently working on.

Mapping Seagrass Biomass with Photo-Library Method

Validation of benthic habitat maps produced from remote sensing imagery is quite time consuming and expensive. Validating maps of seagrass biomass is even more sophisticated and time consuming as the seagrass has to be collected, dried and weighed in the laboratory. We developed a method for estimating the dry weight of the seagrass based on photo transect data and a photo library of quadrats with known seagrass biomass. For seagrass biomass estimation we selected 13 different bottom classes. A photo of each 25 × 25 cm quadrat was taken prior to collecting the samples for each class. Seagrass (and macroalgae, if present) dry weight for each class was measured in the laboratory. These photos were then used to estimate seagrass biomass in 100 m long geolocated photo transects. Seagrass dry weight estimated from the photo transects using the photo-library method was compared with QuickBird satellite radiances. Preliminary results show that QuickBird imagery may be used for mapping seagrass biomass even in highly variable environment such as the Ngederrak Reef in Palau.

Accordance of MERIS Standard Products over the Gulf of Finland to the Parameters Measured Under Regular Monitoring Program

The Medium Resolution Imaging Spectrometer (MERIS/Envisat) was launched in March 2002 to provide a dedicated mission of coastal zone monitoring. A full reprocessing of the MERIS products using the latest set of algorithms was finished in March 2006. The main objective of the present study was to evaluate performance of the new MERIS Level 2 products in such optically complex waters like those found in the Gulf of Finland, the Baltic Sea. Large temporal and spatial variability in the concentrations of chlorophyll and suspended sediments is characteristic of these waters. Potentially toxic cyanobacterial blooms occur every year in the middle of summer in the Baltic Sea. Aggregations of cyanobacteria often form dense subsurface blooms or even surface scum, which are spatially extremely patchy. Coloured dissolved organic matter has a significant influence on the optical properties of the Baltic Sea and is the major contributor to the optical properties of coastal waters. MERIS products are compared with available in situ data obtained regularly by an Estonian monitoring programme. Our results indicate that the satellite sensors give consistent results, which are in the same range as in situ data. Chlorophyll concentrations may exceed standard processing limits during heavy bloom conditions.

Baltic Sea from space. An Estonian experience

The paper summarizes remote sensing studies carried out by Estonian scientists in investigation of the Baltic Sea. Mainly satellite remote sensing results obtained in different applications are shown.