INTENSE PLANKTON BLOOMS AND SARGASSUM DETECTED BY MERIS

Jim Gower and Stephanie King

Fisheries and Ocean Canada, Institute of Ocean Sciences, PO Box 6000, Sidney BC, Canada V8L 4B2, Email: gowerj@pac.dfo-mpo.gc.ca

ABSTRACT/RESUME The search for intense plankton blooms which can be detected using the peak in the spectrum of water-leaving radiance at about 705 nm has been extended to cover the globe using the 7-day rolling archive of MERIS image data. We present here examples of bloom events in waters off India, South Africa, Chile and the USA. In several cases the peak, detected by the 709 band of MERIS is the dominant feature in the spectrum, and the bloom event would be nearly invisible to sensors lacking this band. Observations of blooms in the Gulf of Mexico led to the chance detection of Sargassum lines extending over large areas of the Gulf, showing that in this area and season, Sargassum represents a significant fraction of marine productivity. 1. INTRODUCTION As reported at the previous Envisat Symposium [1], MERIS can be used to detect a peak in the optical spectrum of water-leaving radiance at about 705 nm which provides a signature of intense plankton blooms. Such blooms (colloquially called “red tides”) are becoming an increasingly important phenomenon in coastal waters, affecting coastal communities and causing significant financial losses to fisheries, aquaculture and tourism. The MERIS band at 709 nm, which is an essential part of this detection scheme, is not present in either Seawifs or MODIS. At the high concentrations of surface chlorophyll present in the blooms, the fluorescence peak at 685 nm [2,3] is masked by the 705 nm peak caused by a combination of backscatter and absorption [1,4,5]. In many cases the higher spatial resolution (300 m) provided in the FR mode of MERIS data is important for detecting small area events. The combination of wide area coverage, 300m spatial resolution, and appropriate spectral bands makes MERIS a unique and potentially important tool for global, intense plankton bloom monitoring. We have reported on bloom detection in Canadian waters [1,5] and now extend the search to other areas of the world using the 7-day rolling archive of MERIS image data. We present here examples of blooms in waters off India, South Africa, Chile and the USA. In several cases the peak detected by the 709 band of MERIS, is the dominant feature in the spectrum, and the bloom event would be nearly invisible to sensors lacking this band, such as SeaWiFS and MODIS. Floating and semi-submerged vegetation are possible sources of “false alarms” for this type of bloom detection, but could also be the basis of important applications of the MCI signal. We report here on detection of an extensive area of Sargassum lines in the Gulf of Mexico in the spring and summer of 2005. 2. MERIS IMAGES AND SPECTRA OF BLOOMS The 7-day rolling archive of MERIS image data makes it possible for us to search the coastal waters of the globe for examples of intense blooms which show the peak in radiance at 709 nm. Image data must be down-loaded within the 7-day window. This can be a severe limitation, since it is not usually possible to make use of in-situ reports of events for selecting files. Also, all imagery on this archive is so far at the reduced resolution of 1200 m, so that in most cases we cannot exploit MERIS full, 300 m capability. Fig. 1 shows the fluorescence Maximum Chlorophyll Index (MCI) image for the south west coast of India for 17 September 2004. The image is computed from bands 8, 9 and 10 (681, 709 and 753 nm) of the level 1 RR (1200 m resolution) MERIS data using a linear baseline algorithm [4,5] designed to detect the radiance peak near 705 nm above an assumed linear baseline interpolated between radiance values at 681 and 753 nm. In [5] we used a simple model of water reflectance to demonstrate that this peak is characteristic of high chlorophyll a concentrations, in the range 30 to 300 mg.m-3 and higher. Figure 1 also shows spectra derived from all 15 bands of MERIS. The data are at level 1, that is, measured radiances uncorrected for atmospheric scattering and absorption. The red spectra are for pixels with high MCI values. The green spectra are for nearby “clear water” spectra. The differences show the radiance increases due to the plankton bloom. In all cases, these show a strong peak at the 709 band. The height of this peak is probably underestimated, since the spectra shown are based on only 3 radiance measurements in the range 670 to 760 nm (MERIS bands centred at 681, 709 and 753 nm with widths of 7.5, 10 and 7.5 nm respectively).

Fig. 1. Maximum Chlorophyll Index (linear baseline MCI) from bands 8, 9 and 10 of MERIS reduced resolution, level 1 data for the SW coast of India on 17 September 2004. Clouds are masked to black using a threshold in band 13 (865 nm). Small areas of high MCI (yellow, red, white) can be seen along the coast and offshore. Spectra are shown for selected areas with high MCI and for nearby “clear water.” The differences between these pairs of spectra show a peak at 709 nm.

Figures 2, 3 and 4 show examples from coastal waters in other parts of the world, where the MCI data have detected high chlorophyll conditions. We have no surface data to confirm high chlorophyll concentrations or to provide quantitative values to compare with the spectra. However, all these areas are known to be affected by blooms that have caused problems to local wild fisheries or aquaculture operations.

Fig. 2. The west coast of South Africa imaged by MERIS (reduced resolution) on 25 April 2005 showing high MCI indicating plankton blooms along the coast of St. Helena Bay. True colour image (left) compared to images of FLH (linear baseline, bands 7, 8, 9), MCI, linear baseline, bands 8, 9 and 10) and the Algal 1, chlorophyll concentration product. The 3 left images are computed from MERIS level 1 data. The Algal 1 product is derived from atmospherically corrected, level 2 data.

Fig 3. Bloom conditions in coastal waters of Chile on 29 March 2005 shown by MERIS level 1 reduced resolution data. True colour image (left) compared to images of FLH (linear baseline, bands 7, 8, 9) and MCI, (linear baseline, bands 8, 9 and 10). Spectra show radiances in areas with high MCI and in nearby “clear water.” The differences between these pairs of spectra show a peak at 709 nm.

Fig. 4. Maximum Chlorophyll Index (linear baseline MCI) from bands 8, 9 and 10 of MERIS FR level 1 data for the north-eastern Gulf of Mexico on 5 September 2005. Clouds are masked to black. Small areas of high MCI (yellow, red, white) can be seen off the Florida coast and round the Mississippi delta. The spectra show radiances in areas with high MCI and in nearby “clear water.” In all cases, the difference spectra show a peak at 709 nm.

3. MERIS IMAGES OF SARGASSUM In the case of Figure 4, high chlorophyll areas are seen offshore in the days after the passage of hurricane Katrina (August 28 to 30 2005). Other areas of high MCI are seen in the shallow waters near shore, where the spectra are strongly affected by suspended sediment. Similar spectra in coastal lagoons in the western Gulf of Mexico were detected earlier in 2005 (Figure 5). However this image also showed the extensive pattern of lines of high MCI offshore in the Gulf. The difference spectra shows the red edge characteristic of floating vegetation, and visual observations from a number of sources allows us to identify the lines as Sargassum, which was unusually abundant in the spring and summer of 2005.

Figure 5. MERIS MCI image of the western Gulf of Mexico for 2 June 2005, showing the spectra of the radiance increases in Sargassum lines and coastal blooms compared to nearby “clear water” areas. Sargassum lines show the characteristic “red edge” spectrum of vegetation, while coastal bloom spectra show the peak near 705 nm. High sunglint covers the right side if the image, with scattered cloud (black) over the Western Gulf.

This appears to be the first report of a satellite image of Sargassum. It is usually associated with the area of the North Atlantic (20 to 35 N and 30 to 70 W) known as the Sargasso Sea after the Sargassum encountered there by early explorers. However, it is also found in the Gulf of Mexico as well as in other parts of the world. In 1983, Stoner [6] summarized data on Sargassum concentration determined by ship sampling in the 1930s, and compared it with his own measurements from 1977 to 1981. He reported a major decrease in biomass in both the Sargasso Sea and the Gulf of Mexico over this time interval, from an average of about 1 g/m2 of biomass to below 0.1 g/m2. He suggested oil pollution as a possible reason for the decrease, but there appears to have been no further work on this question.

Figure 6. Sargassum lines in the western Gulf of Mexico, observed with the MERIS imager in full resolution mode (300 m, left) on 23 May 2005, and compared with the equivalent reduced resolution image (right).

Compared to ship sampling, satellite imagery provides improved spatial coverage for deriving statistics of Sargassum biomass averaged over large areas. The major uncertainty will be in deriving quantitative biomass from the observed radiances. Since weed lines are commonly observed to vary from a few meters to a few hundred meters wide, the higher, full resolution mode of MERIS (300 m) is preferred. In fact, however, Figure 6 shows that although the full resolution data provides more detail, the pattern is also well preserved in the reduced resolution imagery. The statistics of the MERIS full-resolution image of 23 May 2005 (Figure 6) show that of 430,000 “clear-sky” pixels (300 by 300 m) in a test area measuring about 200 km square, 45 show an MCI signal greater than 5 mW/(m2.nm.sr.) and 3500 show a signal greater than 0.5 mW/(m2.nm.sr.) (compare with spectra on Figure 5). We estimate that for the solar illumination conditions on that day, a 5 mW/(m2.nm.sr.) MCI signal corresponds to about 16 % coverage of the pixel by exposed biomass. Maximum observed MCI signal in the test area was 12.7 mW/(m2.nm.sr.), corresponding to 40% coverage of the pixel by exposed biomass. Computation from the histogram of the MCI values in the FR image of Figure 6 gave an average coverage fraction for the whole area of 0.0006, equivalent to 1 in 1700 pixels being completely covered in Sargassum. If the Sargassum layer has a thickness of 3 cm, then this coverage fraction implies a biomass of about 20 g/m2, significantly greater than the biomass values of the 1930s (1 g/m2) noted by Stoner. It appears that Sargassum may have recovered from the drop he reported in the 1970s. Carpenter and Cox [7] used the biomass estimate of about 1 g/m2 to conclude that Sargassum productivity is only 0.5% of that for phytoplankton in the Sargasso Sea. The present observations suggest that Sargassum productivity can make a more significant contribution to total productivity, in this case near 10%. 4. CONCLUSIONS The 709 band (band 9) in the baseline spectral band-set of MERIS is an important tool for detection of intense plankton blooms, and also has a role in the detection and mapping of floating vegetation. Additional spectral information in the MERIS data can be used to distinguish between the two cases. To the best of our knowledge, MERIS is here providing the first examples of Sargassum patterns imaged from space. Analysis of Figures 5 and 6 can improve estimates of Sargassum biomass, and its contribution to ocean productivity. Since MERIS spectral bands are in principle programmable, the instrument has the potential to gather more detailed information on spectral signatures between 650 and 720 nm. So far the mission has been frozen in the “baseline” band

configuration to simplify processing and to ensure data continuity. This represents a missed opportunity. There is a significant danger that MERIS will fail before its full spectral capability has been demonstrated. 5. ACKNOWLEDGEMENTS Funding for this project came from the Canadian Government (Fisheries and Oceans Canada and the Canadian Space Agency’s GRIP program). Image data were provided by ESA under the MERIS AO program. 6. REFERENCES [1] Gower, J.F.R., King, S., Borstad, G. and Brown, L., 2004, Use of the 709 nm band of MERIS to detect intense plankton blooms and other conditions in coastal waters, Proceedings of Envisat and ERS Symposium, Salzburg, Austria, September 2004, http://earth.esa.int/workshops/salzburg04/papers_posters/2p07_6_gower_212.pdf.

[2] Gower, J.F.R., and King, S., 2004, Validation of chlorophyll fluorescence derived from MERIS on the west coast of Canada, Proceedings of the Envisat and ERS Symposium, Salzburg, Austria, September 2004, http://earth.esa.int/workshops/salzburg04/papers_posters/2b2_gower_211.pdf [3] Gower, J.F.R., L. Brown and G.A. Borstad, 2004, Observations of chlorophyll fluorescence on the west coast of Canada using the MODIS satellite sensor, Canadian Journal of Remote Sensing, 30, 17-25.

[4] Gower, J.F.R., R. Doerffer, and G.A. Borstad, 1999, Interpretation of the 685 nm peak in water-leaving radiance spectra in terms of fluorescence, absorption and scattering, and its observation by MERIS, Int. J. Remote Sensing, 9, 1771-1786.

[5] Gower, J.F.R., King, S., Borstad, G.A., Brown, L., 2005, “Detection of intense plankton blooms using the 709 nm band of the MERIS imaging spectrometer,” International Journal or Remote Sensing, Volume 26, pages 2005-2012. [6] Stoner, A.W., 1983, Pelagic Sargassum: Evidence for a major decrease in biomass, Deep Sea Research, 30, 469-474. [7] Carpenter, E.J. and J.L. Cox, 1974, Production of pelagic Sargassum and a blue green epiphyte in the western Sargasso Sea, Limnology and Oceanography, 19 429-436.