10 Ocean Colour Remote Sensing of Harmful Algal Blooms in the Benguela System
187
and autumn as seasonal stratification strengthens. Wind relaxation and consequent
downwelling force the inshore accumulation of these blooms. Here bloom decay,
driven by the exhaustion of nutrients, creates a very high oxygen demand though
the entire water column (Pitcher and Probyn 2011). The resulting onset of hypoxia
and anoxia may cause major perturbations to the diversity, structure and functioning
of the coastal environment as witnessed on occasions by mass mortalities of tons of
rock lobster and other marine life (Cockcroft et al. 2000). Other ecosystem impacts
may be more subtle and difficult to quantify, and may include altered food web interactions and habitats. The poor food source provided by blooms of the pelagophyte
Aureococcus anophagefferens, owing either to their small size or to a toxic entity,
has impacted food chains in this way, with dramatic reductions in the growth rates
of filter feeding bivalves (Probyn et al. 2001). Further impacts associated with high
biomass blooms, often dominated by a single species, include unsightly discolouration of the water, which may negatively influence the important economic sectors of
tourism and recreation.
Globally, more HABs are recorded now than in the past placing a greater need
for resource managers and public health officials to be provided with better tools to
monitor and possibly forecast imminent HAB events. The association of HABs in
upwelling systems with elevated biomass allows spaceborne surveillance of ocean
colour to contribute to such an operational capability through the provision of rapid
and spatially broad information relating to the development and progression of HABs
(Bernard et al. 2006). Ocean colour may therefore contribute fundamental information for the establishment of early warning systems allowing better assessment of
the incidence of blooms, and better planning and management options in different
coastal regions.
10.2 Aims of Ocean Colour Application to Harmful
Algal Blooms
Ocean colour radiometry offers routine synoptic data pertaining to the phytoplankton
biomass and assemblage type in the upper optical depths, and thus offers systematic
observations from the event to decadal time scales. On the event scale ocean colour
derived observations in upwelling systems offer enhanced, even unique, capabilities
in several domains: freely-available, near real time synoptic data for operational
bloom monitoring; the detection of precursive and formative bloom conditions; an
enhanced capacity to determine whether a bloom is of a potentially harmful nature;
an accessible facility to monitor bloom growth, movement and decay; and a greater
understanding of the bio-physical dynamics underlying bloom formation (Bernard
et al. 2006, Pitcher et al. 2008a). Over longer time scales, such observations offer
insight into upwelling system functionality, inter- and intra-seasonal variability and
perhaps even environmental regime shifts (Kahru et al. 2009, Pitcher and Weeks
2006, Weeks et al. 2006). However it must be realised that ocean colour data, dependent on the gross bio-optical characteristics of surface water constituents, is by
187
and autumn as seasonal stratification strengthens. Wind relaxation and consequent
downwelling force the inshore accumulation of these blooms. Here bloom decay,
driven by the exhaustion of nutrients, creates a very high oxygen demand though
the entire water column (Pitcher and Probyn 2011). The resulting onset of hypoxia
and anoxia may cause major perturbations to the diversity, structure and functioning
of the coastal environment as witnessed on occasions by mass mortalities of tons of
rock lobster and other marine life (Cockcroft et al. 2000). Other ecosystem impacts
may be more subtle and difficult to quantify, and may include altered food web interactions and habitats. The poor food source provided by blooms of the pelagophyte
Aureococcus anophagefferens, owing either to their small size or to a toxic entity,
has impacted food chains in this way, with dramatic reductions in the growth rates
of filter feeding bivalves (Probyn et al. 2001). Further impacts associated with high
biomass blooms, often dominated by a single species, include unsightly discolouration of the water, which may negatively influence the important economic sectors of
tourism and recreation.
Globally, more HABs are recorded now than in the past placing a greater need
for resource managers and public health officials to be provided with better tools to
monitor and possibly forecast imminent HAB events. The association of HABs in
upwelling systems with elevated biomass allows spaceborne surveillance of ocean
colour to contribute to such an operational capability through the provision of rapid
and spatially broad information relating to the development and progression of HABs
(Bernard et al. 2006). Ocean colour may therefore contribute fundamental information for the establishment of early warning systems allowing better assessment of
the incidence of blooms, and better planning and management options in different
coastal regions.
10.2 Aims of Ocean Colour Application to Harmful
Algal Blooms
Ocean colour radiometry offers routine synoptic data pertaining to the phytoplankton
biomass and assemblage type in the upper optical depths, and thus offers systematic
observations from the event to decadal time scales. On the event scale ocean colour
derived observations in upwelling systems offer enhanced, even unique, capabilities
in several domains: freely-available, near real time synoptic data for operational
bloom monitoring; the detection of precursive and formative bloom conditions; an
enhanced capacity to determine whether a bloom is of a potentially harmful nature;
an accessible facility to monitor bloom growth, movement and decay; and a greater
understanding of the bio-physical dynamics underlying bloom formation (Bernard
et al. 2006, Pitcher et al. 2008a). Over longer time scales, such observations offer
insight into upwelling system functionality, inter- and intra-seasonal variability and
perhaps even environmental regime shifts (Kahru et al. 2009, Pitcher and Weeks
2006, Weeks et al. 2006). However it must be realised that ocean colour data, dependent on the gross bio-optical characteristics of surface water constituents, is by