Satellite observations of the Southern California

“red tide”


Mati Kahru,


·        We can use MODIS 250 m image data from Terra and Aqua satellites to detect the red tide. The resolution of typical ocean color imagery at 1 km is too coarse and even 250 m pixel size is not enough to map the bloom near-shore blooms and in smaller bays (e.g. the San Diego bay). The methods of analysis are described here and in Kahru et al., MODIS detects a devastating algal bloom in Paracas Bay, Peru, EOS, Trans. AGU, Vol. 85, N45, p. 465-472, 2004.

·        The Terra-MODIS quasi true color image shown here is from June 17, 2005 at 1:00 PM PST. The image has some technical problems (banding of individual scans are well visible due to not perfect calibration) as well as natural problems (too hazy and cloudy offshore). It does not show the near-shore red tide well. It does show that the highly absorbing (i.e. dark on this image) areas that are probably rich in phytoplankton pigments and CDOM extend quite far into the ocean. The alongshore band of highly absorbing water north of the La Jolla bay is approximately 10 km wide.  Dark surface slicks due to kelp beds (K, off La Jolla and Point Loma) are visible. The increased scattering area offshore (C) looks like a coccolithophore bloom.

·        The grayscale image shows "turbidity" which is caused by highly scattering sediment particles (e.g. during river run-off). It picks up only a few slight patches of turbidity (San Diego bay, Ensenada bay and south of Ensenada) – this is in contrast with the large and intense turbid areas during the winter runoff events (e.g. here ).

·        To see the full resolution images, please click on one of the images below to open a new window, then maximize the image inside the window.

  • The following images were created with Wimsoft ( software. A more detailed tutorial is available here.




Left: Quasi-true color image from Terra-MODIS of June 17, 2005, 1:00 PM PST. C – coccolithophore bloom?, K – kelp beds


Right: Turbidity map of the same image. Only a few slightly increased turbidity areas detected.


Next morning, on June 18, 2005 at 10:30 PST MODIS-Terra covered the same area. A stretched true-color image of the San Diego area is shown on the left. Coastlines are overlaid in blue. The band of brownish water is seen along the coast north of La Jolla. Brownish water is also seen near the mouth of the Tijuana river.