Internal Dynamics of Satellite-Observed Algal Blooms
11
useful where province boundaries can be induced to respond to the month-to-month
variability of regions such as GFST or ARAB, where mesoscale, chlorophyll-enhanced
eddies dominate the pattern. In other regions, little change in location of boundaries
is induced. Note that this routine for obtaining dynamic, rather than static, boundaries
differs from that described by Platt et al. (2005) that is discussed in Chapter 7.
Internal Dynamics of Satellite-Observed
Algal Blooms
Because one of the most useful kinds of observations now made by satellite-borne sensors
is changes in the location and concentration of regions of relatively high chlorophyll
concentration, it will be useful briefly to review what we may reasonably infer from these
data. Because the images risk being interpreted by those not entirely familiar with the
complex dynamics of phytoplankton, it may be useful to emphasize once again that there
is not necessarily a simple relationship between changes in chlorophyll concentration and
changes in phytoplankton growth rate.
What do we really mean by “algal blooms”? This is one of the most frequently used
terms in biological oceanography and one that is often given rather little thought although
it always refers to an increase in the standing stock of chlorophyll. As biological oceanographers, our thinking about algal blooms is probably still colored by the title of Sverdrup’s
(1953) classical paper “On the Conditions for Vernal Blooming of the Phytoplankton”
and by terms such as the “spring outburst” or “vernal flowering” of diatoms commonly
used not so long ago. To examine this phenomenon, Sverdrup returned to Gran and
Braarud’s concept of a critical depth of mixing that retains algal cells within sufficient
irradiance as to permit net growth (see Chapter 4). We should remember that Sverdrup
discussed the critical depth concept by analysis of those oceanographic conditions that
would permit “an increase in the phytoplankton population” to occur—adding only,
and somewhat as an afterthought, that “if grazers are present the phytoplankton
population may remain small in spite of heavy production.” Despite this caution, it has
been all too easy to interpret an increase or decrease in chlorophyll as having some
proportionality to production rate. It is also very easy to interpret a change in the sign of
biomass change as necessarily reflecting a similar change in sign of the production rate.
None of which, as we should know, is necessarily so.
One of the earliest findings in the study of plankton dynamics was that even large
diatoms can be consumed almost as rapidly as they are produced. In 1935, Harvey and
others thought that 98% of the production in the English Channel of the spring bloom
was grazed down by copepods in a few weeks, and later workers, who included Fleming,
Riley, Gauld, and Cushing, reached similar conclusions for coastal waters. By 1942, Hart
had extended such calculations to the Southern Ocean, where he found that the standing stock of phytoplankton represented only about 2% of its daily production. From
these early observations, the dynamic balance between production and disappearance
of phytoplankton cells should subsequently have been the central theme of biological
oceanography but, as Banse (1992) pointed out, this has, most disconcertingly in retrospect, not been the case. Perhaps, he suggests, because of the difficulty of research on
consumers (great functional diversity and dimensional range) and the easier access to
simple demonstration of biological principles in phytoplankton research, teachers have
given priority to the latter: consequently, research on production and consumption has
been only very loosely coupled. So it may be instructive to ask what satellite imagery can
tell us about the dynamics of algal blooms.
11
useful where province boundaries can be induced to respond to the month-to-month
variability of regions such as GFST or ARAB, where mesoscale, chlorophyll-enhanced
eddies dominate the pattern. In other regions, little change in location of boundaries
is induced. Note that this routine for obtaining dynamic, rather than static, boundaries
differs from that described by Platt et al. (2005) that is discussed in Chapter 7.
Internal Dynamics of Satellite-Observed
Algal Blooms
Because one of the most useful kinds of observations now made by satellite-borne sensors
is changes in the location and concentration of regions of relatively high chlorophyll
concentration, it will be useful briefly to review what we may reasonably infer from these
data. Because the images risk being interpreted by those not entirely familiar with the
complex dynamics of phytoplankton, it may be useful to emphasize once again that there
is not necessarily a simple relationship between changes in chlorophyll concentration and
changes in phytoplankton growth rate.
What do we really mean by “algal blooms”? This is one of the most frequently used
terms in biological oceanography and one that is often given rather little thought although
it always refers to an increase in the standing stock of chlorophyll. As biological oceanographers, our thinking about algal blooms is probably still colored by the title of Sverdrup’s
(1953) classical paper “On the Conditions for Vernal Blooming of the Phytoplankton”
and by terms such as the “spring outburst” or “vernal flowering” of diatoms commonly
used not so long ago. To examine this phenomenon, Sverdrup returned to Gran and
Braarud’s concept of a critical depth of mixing that retains algal cells within sufficient
irradiance as to permit net growth (see Chapter 4). We should remember that Sverdrup
discussed the critical depth concept by analysis of those oceanographic conditions that
would permit “an increase in the phytoplankton population” to occur—adding only,
and somewhat as an afterthought, that “if grazers are present the phytoplankton
population may remain small in spite of heavy production.” Despite this caution, it has
been all too easy to interpret an increase or decrease in chlorophyll as having some
proportionality to production rate. It is also very easy to interpret a change in the sign of
biomass change as necessarily reflecting a similar change in sign of the production rate.
None of which, as we should know, is necessarily so.
One of the earliest findings in the study of plankton dynamics was that even large
diatoms can be consumed almost as rapidly as they are produced. In 1935, Harvey and
others thought that 98% of the production in the English Channel of the spring bloom
was grazed down by copepods in a few weeks, and later workers, who included Fleming,
Riley, Gauld, and Cushing, reached similar conclusions for coastal waters. By 1942, Hart
had extended such calculations to the Southern Ocean, where he found that the standing stock of phytoplankton represented only about 2% of its daily production. From
these early observations, the dynamic balance between production and disappearance
of phytoplankton cells should subsequently have been the central theme of biological
oceanography but, as Banse (1992) pointed out, this has, most disconcertingly in retrospect, not been the case. Perhaps, he suggests, because of the difficulty of research on
consumers (great functional diversity and dimensional range) and the easier access to
simple demonstration of biological principles in phytoplankton research, teachers have
given priority to the latter: consequently, research on production and consumption has
been only very loosely coupled. So it may be instructive to ask what satellite imagery can
tell us about the dynamics of algal blooms.