194
S. Bernard et al.
the algal assemblage. In the case of mono-specific blooms this will equate approximately to the average cell size of the dominant species. Imagery from 30 March
and 5 April 2005 processed with the EAP algorithm allows for detection of the elevated biomass levels within the greater St Helena Bay region through this period
(in situ values between 172.5 and 39.6 mg m
−3 ). In addition, a significant change in
effective diameter of the assemblage is detected (Fig. 10.2e, f). Concurrent in situ
measurements (microscopy and coulter counter derived size distributions) suggest
that this change in size represents a change in dominance of this bloom from the
small-celled dinoflagellate Prorocentrum triestinum to the large-celled dinoflagellate
Ceratium furca.
Comparisons between in situ (Satlantic H-TSRB) and satellite (MERIS) derived
radiometry (Fig. 10.2a, b) show that the satellite captures both the magnitude and
shape of the spectra in each case. The significant signal in the NIR bands seen here
triggers the use of the bright pixel atmospheric correction (BPAC) over large portions
of the images, particularly in areas associated with high chlorophyll concentrations.
10.4.2 Use of Fluorescence and an Assessment of Pigment
Detection Using Hyperspectral and Multispectral
Radiometry Using the EAP Algorithm and Spectral Flags
Whilst detection of cell size provides a useful ecosystem parameter and potential
indicator of taxonomic shifts, the assessment of HAB impact often requires species
identification, particularly where toxic effects are being considered. As previously
mentioned, a shift from diatom to dinoflagellate dominance forms a key part of
many conceptual frameworks for phytoplankton succession in upwelling systems
(Margalef 1978). An ability to detect this shift using satellite data may help provide
early prediction for the onset of HABs, as well as for comparison to oceanographic
data to validate these succession schemes.
Though differentiating between diatoms and dinoflagellates is useful, an ability to
identify particular species would be optimal in cases where toxicity is suspected. The
existence of accessory pigments with specific absorption characteristics provides a
way that individual species can be identified from radiometric data. However the
extent to which these spectral characteristics can be exploited is dependent on both
the level of biomass and the specifications of instruments used, particularly in the
case of satellite derived radiometry. Figure 10.3a shows match-up in situ and MERIS
reflectance associated with a relatively low biomass (Chl a = 3.5 mg m
−3 ) bloom in
the St Helena Bay area containing Myrionecta rubra. Figure 10.3b displays a variety
of in situ R rs spectra from a 3-week period during the same bloom event; spectra
chosen to show naturally occurring admixtures of diatoms and M. rubra at a range
of biomass. Peaks related to phycorethyrin absorption can be seen in the data from
the in situ Satlantic H-TSRB between 500 and 620 nm. However, from satellite it is
difficult to fully resolve these peaks due to a paucity of bands in this region of the
spectra (Fig. 10.3a), leading to difficulties during the inversion procedure. In lower
S. Bernard et al.
the algal assemblage. In the case of mono-specific blooms this will equate approximately to the average cell size of the dominant species. Imagery from 30 March
and 5 April 2005 processed with the EAP algorithm allows for detection of the elevated biomass levels within the greater St Helena Bay region through this period
(in situ values between 172.5 and 39.6 mg m
−3 ). In addition, a significant change in
effective diameter of the assemblage is detected (Fig. 10.2e, f). Concurrent in situ
measurements (microscopy and coulter counter derived size distributions) suggest
that this change in size represents a change in dominance of this bloom from the
small-celled dinoflagellate Prorocentrum triestinum to the large-celled dinoflagellate
Ceratium furca.
Comparisons between in situ (Satlantic H-TSRB) and satellite (MERIS) derived
radiometry (Fig. 10.2a, b) show that the satellite captures both the magnitude and
shape of the spectra in each case. The significant signal in the NIR bands seen here
triggers the use of the bright pixel atmospheric correction (BPAC) over large portions
of the images, particularly in areas associated with high chlorophyll concentrations.
10.4.2 Use of Fluorescence and an Assessment of Pigment
Detection Using Hyperspectral and Multispectral
Radiometry Using the EAP Algorithm and Spectral Flags
Whilst detection of cell size provides a useful ecosystem parameter and potential
indicator of taxonomic shifts, the assessment of HAB impact often requires species
identification, particularly where toxic effects are being considered. As previously
mentioned, a shift from diatom to dinoflagellate dominance forms a key part of
many conceptual frameworks for phytoplankton succession in upwelling systems
(Margalef 1978). An ability to detect this shift using satellite data may help provide
early prediction for the onset of HABs, as well as for comparison to oceanographic
data to validate these succession schemes.
Though differentiating between diatoms and dinoflagellates is useful, an ability to
identify particular species would be optimal in cases where toxicity is suspected. The
existence of accessory pigments with specific absorption characteristics provides a
way that individual species can be identified from radiometric data. However the
extent to which these spectral characteristics can be exploited is dependent on both
the level of biomass and the specifications of instruments used, particularly in the
case of satellite derived radiometry. Figure 10.3a shows match-up in situ and MERIS
reflectance associated with a relatively low biomass (Chl a = 3.5 mg m
−3 ) bloom in
the St Helena Bay area containing Myrionecta rubra. Figure 10.3b displays a variety
of in situ R rs spectra from a 3-week period during the same bloom event; spectra
chosen to show naturally occurring admixtures of diatoms and M. rubra at a range
of biomass. Peaks related to phycorethyrin absorption can be seen in the data from
the in situ Satlantic H-TSRB between 500 and 620 nm. However, from satellite it is
difficult to fully resolve these peaks due to a paucity of bands in this region of the
spectra (Fig. 10.3a), leading to difficulties during the inversion procedure. In lower