Radar Detection of Algae Bloom with Radar. Experiment and Model.
Ermakov, Stanislav¹; Kapustin, Ivan¹; Sergievskaya, Irina¹; daSilva, Jose²; Lazareva, Tatjana¹; Shomina, Olga^1
1Institute of Applied Physics RAS, RUSSIAN FEDERATION; ²Porto University, PORTUGAL

Intensive algae bloom is a serious threat for the ecology of inland waters and shelf areas. Satellite optical and IR systems widely used nowadays to monitor algae bloom areas, have strong limitations in their operation in the night time or for cloudy sky. A very effective all-weather and day and night instrument of remote sensing of the Earth is microwave radars, in particular, satellite SAR. A first evidence that algae bloom can be detected by radar was given by Alpers et al (2003) from the analysis of optical and radar images f the sea surface. However, there was no direct proof of the relation between radar backscattering, biogenic films and phytoplankton, and the very physical mechanisms of radar backscatter depression were not well understood. This paper is aimed to understand better the physics of the action of algae on radar backscattering and possibilities of quantitative characterization of algae bloom areas from radar observations. The influence of algae bloom on the damping of gravity-capillary wave was studied experimentally in field experiment on the Gorky Water Reservour and in laboratory. Samples of phytoplankton from the upper water layer and of biogenic film from the water surface were collected, and simultaneous and co-located X-band scatterometer measurements from a boat were carried out. Concentrations of main phytoplankton constituents as well as total algae concentration and their variability in the studied area were obtained. Laboratory studies of damping of gravity-capillary waves on the water/surface film samples were performed, and an effective water viscosity and the film elasticity values were retrieved as functions of phytoplankton concentration. It was obtained that the intensity of X-band scaterometer signal decreased with phytoplankton concentration. It was concluded that the increased water viscosity and the film elasticity in the presence of algae were the main physical reasons of enhanced wave damping, the contributions of the two effects in total wave damping and correspondingly in radar backscatter depression appeared to be comparable to each other. Since both the effective viscosity and the film elasticity depend on algae concentration the latter can be estimated from measurements of radar backscatter. Model calculations of contrasts in the spectrum of short wind waves and of the radar backscatter using the effective viscosity and film elasticity retrieved from experiment were performed, and the theory was shown to be in good agreement with observations. Some measurements of phytoplankton concentration, wind velocity and currents from a boat were carried out nearly simultaneously and co-located with TerraSAR-X imaging of the water surface and it has been concluded that depressions in the SAR images after correction on wind speed variations corresponded well to enhanced phytoplankton concentration.
The work has been supported by the Russian Foundation of Basic Research (Projects RFBR
(projects 12-05-31237, 11-05-00295), the Program RAN Radiophysics, and by the Russian Government (Grants No. 11.G34.31.0048 and 11.G34.31.0078).