Development of an Ehux Bloom (Satellite Images)


Lasse H. Pettersson
Nansen Environmental and Remote Sensing Center (NERSC)
Edv. Griegs vei 3a
N-5037 Solheimsviken
NORWAY
Email: lasse.pettersson@nrsc.no


The NOAA AVHRR sensor has daily coverage of the Norwegian waters, and data may be provided in near real-time through the high latitude Tromso Satellite Station (TSS). Since the AVHRR ground resolution is at least 1 kilometer, this sensor is not optimal for local coastal and fjord studies of coccolithophorid blooms. However, the successfull use of AVHRR data for open ocean applications has been widely documented by others.

Five NOAA AVHRR scenes have been acquired for the 1993 Emiliania huxleyi blooming period in May/June 1993. High quality data have been obtained from the daytime passes on May 25th, and on June 2nd, 7th, 9th, and 10th.



25th: No particular high-reflectance signals are observed in the AVHRR data in the coastal region west of Bergen. The in situ observations from e.g Fauakangerpollen (too small to be resolved in the AVHRR data) confirm that the bloom has started, but not at a spatial scale and magnitude which can be resolved in the AVHRR data. Further south, near Stavanger, the shoreline was cloud covered and no conclusions can be made in this region.



2nd: Some increased surface reflection is observed in AVHRR channel 1. High reflection filaments are observed outside the Boknafjord, Bomlafjord, Selbjornsfjord and Korsfjord. A region of higher reflectance, presumably coccolithophore-laden, is also observed extending west off the Joren coast between Stavanger and Egersund and northward to the Boknafjord. Conclusions for this part of the image are limited by the fact that gradients in the cloud and haze layer may effect the remotely sensed data.



7th: clearest indication of the spatial structure of the algae bloom is evident in the data from this day. The boundaries of the bloom are well-evident, and the varying concentration of coccoliths within the bloom area are clearly shown in the enhanced channel 1 image. A comparison of the channel 1 image with the channel 4 thermal image reveals a high degree of correspondence in the thermal ocean front boundaries and the extension of the coccoliths, which is limited to the extension of the Norwegian Coastal Current. The colder water near the Joren coast is relatively algae-free, while the warmer water is associated with the bloom region. Alongcoast meandering in the off-shore extension is observed and, in particular, a wide extension is observed west off Karmoy.



9th: general structure seen in the June 7th data is evident in the June 9th data. Further, a part of the coccolithophore bloom is trapped southward outside Karmoy. This part forms a cyclonic eddy circulation pattern west of Boknafjorden. Close to the Joren coast the water has a lower reflectance, hence the origin of the coccolithophorids in this region may come from the off-shore North Sea region.



10th: the eddy west of Boknafjorden is still clear with a further southward extension of the thong of coccolithophorids trapped in the eddy.

Summary: the origin of the coccoliths observed in the AVHRR images (Channel 1) seem to have two sources of origin during the May/June bloom in 1993. One part originates from the fjords, as resolved in the data from June 2nd, and then transported out in the Norwegian Coastal Current from the fjords. The other part appears to originate from the North Sea and then interacts with the coastal current west of Joren. In this region, the boundary between the coccolithophorids are consistent with the temperature boundary between the two regimes.



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