10 Ocean Colour Remote Sensing of Harmful Algal Blooms in the Benguela System
199
10.5 Summary and Outlook
Ocean colour products show great potential for enhancing our ability to both detect
and quantify HABs, in particular for the very high biomass blooms typically encountered in the Bengeula. It should be stressed that many toxic algal species can
have harmful effects at low biomass (e.g. McGillicuddy et al. 2005); the algorithms
presented here, and arguably perhaps even ocean colour data in general, may not be
effective for such bloom types. Combined with an understanding of ecosystem dynamics, satellite derived products can be used to detect elevated biomass and indicate
potential for occurrence of toxic species at appropriate temporal and spatial scales.
This work highlights the importance of band selection on satellite-mounted sensors,
particularly when wishing to exploit pigment-related effects and fluorescence-based
products. It is apparent that accurate atmospheric correction in turbid waters must
be a first priority to fully utilise these spectrally dependent algorithms. Alternatively,
top of atmosphere approaches such as the MPH algorithm used here can circumvent
this requirement. The frameworks provided in this study should be adaptable for
use in other systems where HABs occur. For systems where species have broadly
similar inherent optical properties (IOPs) to those in the Benguela, a semi-analytical
inversion approach such as that used in the EAP algorithm, demonstrated with the
case study in Sect. 4.1, should be able to provide improved chlorophyll estimates
and an indication of the effective diameter of the assemblage. Where mono-specific
blooms are prevalent, this parameter is most useful in detecting changes in dominance. This method may prove less successful where other constituents dominate the
optical signal, e.g. in severely sediment loaded waters. In cases where species are
present which have obscure spectral signatures, resulting from accessory pigments
or ultrastructure, new IOPs would need defining, however empirical, spectral ratio
techniques such as those used here to detect Myrionecta rubra in Sect. 4.2, could
also be applied, given sufficient biomass.
References
Alvain S, Loisel H, Dessailly D (2012) Theoretical analysis of ocean color radiances anomalies and
implications for phytoplankton groups detection in case 1 waters. Opt Express 20(2):1070–1083
Barlow R, Sessions H, Balarin M, Weeks S, Whittle C, Hutchings L (2005) Seasonal variation
in phytoplankton in the southern Benguela: pigment indices and ocean colour. Afr J Mar Sci
27:275–287
Barlow RG (1982) Phytoplankton ecology in the southern Benguela Current. I. Biochemical
composition. J Exp Mar Biol Ecol 63(3):209–227
Behrenfeld MJ, Westberry TK, Boss ES, O’Malley RT, Siegel DA, Wiggert JD, Franz BA, McClain CR, Feldman GC, Doney SC, Moore JK, Dall’Olmo G, Milligan AJ, Lima I, Mahowald
N (2009) Satellite-detected Fluorescence reveals global physiology of Ocean Phytoplankton.
Biogeosciences 6:779–794
Bernard S (2005). The bio-optical detection of harmful algal blooms. PhD Thesis. University of
Cape Town
199
10.5 Summary and Outlook
Ocean colour products show great potential for enhancing our ability to both detect
and quantify HABs, in particular for the very high biomass blooms typically encountered in the Bengeula. It should be stressed that many toxic algal species can
have harmful effects at low biomass (e.g. McGillicuddy et al. 2005); the algorithms
presented here, and arguably perhaps even ocean colour data in general, may not be
effective for such bloom types. Combined with an understanding of ecosystem dynamics, satellite derived products can be used to detect elevated biomass and indicate
potential for occurrence of toxic species at appropriate temporal and spatial scales.
This work highlights the importance of band selection on satellite-mounted sensors,
particularly when wishing to exploit pigment-related effects and fluorescence-based
products. It is apparent that accurate atmospheric correction in turbid waters must
be a first priority to fully utilise these spectrally dependent algorithms. Alternatively,
top of atmosphere approaches such as the MPH algorithm used here can circumvent
this requirement. The frameworks provided in this study should be adaptable for
use in other systems where HABs occur. For systems where species have broadly
similar inherent optical properties (IOPs) to those in the Benguela, a semi-analytical
inversion approach such as that used in the EAP algorithm, demonstrated with the
case study in Sect. 4.1, should be able to provide improved chlorophyll estimates
and an indication of the effective diameter of the assemblage. Where mono-specific
blooms are prevalent, this parameter is most useful in detecting changes in dominance. This method may prove less successful where other constituents dominate the
optical signal, e.g. in severely sediment loaded waters. In cases where species are
present which have obscure spectral signatures, resulting from accessory pigments
or ultrastructure, new IOPs would need defining, however empirical, spectral ratio
techniques such as those used here to detect Myrionecta rubra in Sect. 4.2, could
also be applied, given sufficient biomass.
References
Alvain S, Loisel H, Dessailly D (2012) Theoretical analysis of ocean color radiances anomalies and
implications for phytoplankton groups detection in case 1 waters. Opt Express 20(2):1070–1083
Barlow R, Sessions H, Balarin M, Weeks S, Whittle C, Hutchings L (2005) Seasonal variation
in phytoplankton in the southern Benguela: pigment indices and ocean colour. Afr J Mar Sci
27:275–287
Barlow RG (1982) Phytoplankton ecology in the southern Benguela Current. I. Biochemical
composition. J Exp Mar Biol Ecol 63(3):209–227
Behrenfeld MJ, Westberry TK, Boss ES, O’Malley RT, Siegel DA, Wiggert JD, Franz BA, McClain CR, Feldman GC, Doney SC, Moore JK, Dall’Olmo G, Milligan AJ, Lima I, Mahowald
N (2009) Satellite-detected Fluorescence reveals global physiology of Ocean Phytoplankton.
Biogeosciences 6:779–794
Bernard S (2005). The bio-optical detection of harmful algal blooms. PhD Thesis. University of
Cape Town