Munich: Researchers from the Technical University of Munich (TUM) have developed a remote sensing method for regularly monitoring plant populations in lakes.
Aaquatic plants better known as western waterweed have been spreading rapidly in German water bodies in recent years. Plants like spiny naiad and waterweed have the potential to change the ecosystem of water body in the long term. They could displace other species or change the habitats of different organisms like fish. Traditionally, divers have been mapping such “vegetation blankets” at different depths in lakes. The process does produce highly detailed information, but it requires a lot of effort. Researchers from TUM’s Limnological Research Station have replaced the diving effort with high-resolution aerial and satellite images. “In order to draw conclusions on plant growth from the imagery produced, we measure reflectance. Each plant species reflects the incident light in a specific way, depending on its pigmentation and structure,” explains project supervisor Dr. Thomas Schneider. The researchers developed a digital library with the spectral characteristics of plants to help them evaluate the aerial and satellite images. It took them two years to photograph the plants from a boat and measure their reflectance. In order to capture the plants from a suitable angle and avoid shadow, the cameras and sensors were submerged using an extension arm. Factors like dissolved matter, sediment type, light refraction and different depths of water made it hard to assess plant populations. That is why the researchers developed mathematical algorithms to “factor out” the image errors in combination with the measurement data from the boat. Since every body of water has its own distinct characteristics, a different algorithm was developed for each lake.