Long-Term Monitoring of Emiliania huxleyi Blooms

Objective

Introduction

High concentrations or "blooms" of the coccolithophore Emiliania huxleyi can significantly affect a region by acting as a source of organic sulfur (i.e. dimethyl sulfide, DMS) to the atmosphere and calcium carbonate to the sediments, and by altering the optical properties of the surface layer. Documenting the occurrence of blooms in time and space, is consequently essential in characterizing the biogeochemical environment of a region. Furthermore, their distribution pattern can be employed to define the environmental conditions favorable for their occurrence.

E. huxleyi blooms have been recorded to achieve cell concentrations of up to 115 million cells per liter (Berge, 1962). Determing how prevalent these blooms are in the global ocean will help to estimate the magnitude of bloom produced CaCO3 and DMS in the ocean relative to other sources and assess their affect on regional CO2 dynamics and planetary albedo. In addition, the interannual variability of the blooms could conceivably be used, when correlated to physical parameters, to understand the conditions favorable for the bloom's initiation and growth.

Several investigators have observed coccolithophore blooms in visible satellite imagery (Ackleson and Holligan, 1989; Balch et al., 1991; Brown and Yoder, 1993; Brown and Yoder, 1994a; Brown and Yoder, 1994b; Fukushima, 1991; Holligan and Groom, 1986; Holligan et al., 1983). The spatial and temporal variability of their blooms during 1978 to 1986 has recently been determined on both a regional (Brown and Yoder, 1994b) and global (Brown and Yoder, 1994a) scale. This was accomplished by classifying pixels of Coastal Zone Color Scanner (CZCS) imagery, based on the pixel's normalized water-leaving radiances, into bloom and non-bloom classes using a supervised, multispectral scheme. Much of the variability observed in these two studies, however, could be attributed to the variability of CZCS image coverage.

The greater global coverage of the dedicated ocean color missions of the SeaStar Sea-view Wide Field-of-view Sensor (SeaWiFS) will improve our knowledge of the distribution pattern of coccolithophore blooms, on both an annual and long-term basis.

Approach

The approach is similar to that followed by Brown and Yoder (Brown and Yoder, 1994a; Brown and Yoder, 1994b). E. huxleyi blooms in the surface waters of the world oceans will be mapped by classifying pixels of weekly global composites of available ocean color data, such as those from SeaWiFS, into bloom and non-bloom classes based on their mean normalized water-leaving radiances using a supervised, multispectral classification scheme.

The classification algorithm used in (Brown and Yoder, 1994a) will be updated and refined to incorporate these new decision boundary values.

The surface area of classified blooms will be calculated from the classified images by multiplying pixel area by the frequency of bloom pixels. Crude estimates of inorganic CaCO3 and DMS produced by the blooms can be calculated from their areal dimensions given assumptions of the depth and the concentration of CaCO3 and DMS in the surface mixed- layer. Alternatively, Gordon et al. (1988) present a model to estimate the concentrations of chlorophyll and detached coccoliths from the normalized water-leaving radiances. Together with values of the backscattering coefficient of coccoliths (Balch et al., 1991) and DMS per cell biomass (Keller et al., 1989), the model can be used to estimate CaCO3 and DMS in blooms. This method, though not yet confirmed by in situ sampling, will also be pursued.

Significance

The results of this study will extend and improve our observations of coccolithophore bloom occurrence and its variability in the world's oceans. This study will also resume the compilation of an interannual record of coccolithophore bloom occurrence initiated by Brown and Yoder (Brown and Yoder, 1994a). In addition to monitoring the general health of the marine environment, this time-series may be used in ascertaining the environmental conditions favorable to E. huxleyi blooms and to test the Charleston et al. (1987) hypothesis of a feedback mechanism operating between the abundance of DMS producing phytoplankton species and climate.

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Literature Cited

Ackleson, S. G., and Holligan, P. M. (1989), AVHRR observations of a Gulf of Maine coccolithophorid bloom, Photogramm. Eng. Remote Sensing 55:473-474.

Balch, W. M., Holligan, P. M., Ackleson, S. G., and Voss, K. J. (1991), Biological and optical properties of mesoscale coccolithophore blooms in the Gulf of Maine, Limnol. Oceanogr. 36:629-643.

Berge, G. (1962), Discoloration of the sea due to Coccolithus huxleyi "bloom", Sarsia 6:27-40.

Brown, C. W., and Yoder, J. A. (1993), Coccolithophorid blooms in surface waters of the Nova Scotian Shelf and the Grand Bank, J. Plankton Res.

Brown, C. W., and Yoder, J. A. (1994a), Coccolithophorid blooms in the global ocean, J. Geophys. Res. 99:7467-7482.

Brown, C. W., and Yoder, J. A. (1994b), Distribution pattern of coccolithophorid blooms in the western North Atlantic, Cont. Shelf Res. 14:175-197.

Fukushima, H. (1991), High-reflectance water mass in Off-Sanriku area observed by CZCS, Japan,

Gordon, H. R., Brown, O. B., Evans, R. H., Brown, J. W., Smith, R. C., Baker, K. S., and Clark, D. K. (1988), A semianalytic radiance model of ocean color, J. Geophys. Res. 93:10909-10924.

Holligan, P. M., and Groom, S. B. (1986), Phytoplankton distributions along the shelf break, Proc. Roy. Soc. Edinburgh 88B:239-263.

Holligan, P. M., Viollier, M., Harbour, D. S., Camus, P., and Champagne-Philippe, M. (1983), Satellite and ship studies of coccolithophore production along a continental shelf edge, Nature 304:339-342.

Last Revised: October 31, 1997