12
Chapter 1: Toward an Ecological Geography of the Sea
For each province subsequently to be described and for each month, we have mean
values that specify surface chlorophyll, integrated chlorophyll, and integrated primary
production rate. By comparing rates of actual and potential increase of phytoplankton
from these, it is simple to propose a first-order plankton calendar for each province that
integrates both production and loss of cells. This can be done by computing a value for
the difference between the algal biomass in each province between successive months.
The sign of this value indicates loss or gain. From the rate of change of algal biomass
and the rate of primary production, both on a monthly basis, it is then possible calculate
how much of the production of algal biomass does not accumulate but must be lost to
consumption, sinking, advection, or entrainment.
The result of this simple investigation is shown in Figure 1.1, where I have computed
the relationship in as simple terms as I can devise: monthly accumulation of biomass in
terms of the daily production rate and standing stock in terms of the monthly production
rate. In each case, you may be surprised by the result if you have never thought seriously
about the relationship. Look first at the open ocean areas lying under the trades and
westerlies—monthly increase of biomass usually represents less than a quarter of one
day’s primary production, and the standing stock represents less than one-tenth of the
production of a whole month. These values are higher in the polar regions, but not by very
much. If we examine individual months, rather than their climatological mean values,
we consistently get the same result. The bloom induced by the southwest monsoon in
the Arabian Sea in June and July accumulates no more than 3.0% and 2.3%, respectively,
of the production that occurs during each of these months. The spring bloom in the
temperate North Atlantic in April and May accumulates even less: 0.26% and 0.58%,
respectively. These percentages are almost matched in May and June in the Atlantic
subarctic regions.
The series of seasonal graphs for production rate and chlorophyll standing stock presented in Chapters 9–12 illustrates the lack of confidence we should have in interpreting
underlying ecological dynamics from indications of plant biomass alone. Phytoplankton
standing stock may track seasonal trends in primary production rate quite closely, or
may significantly diverge from them. Therefore, the graphs show that chlorophyll may
accumulate when the production rate is decreasing or may decline when the primary
production rate is increasing. Such deviations should be welcomed as implying something about the dynamic biology that forces the observed pattern. Where possible, I have
suggested in Chapters 9–12 what the reasons may be for each important example shown.
Associating Chl sat data with global maps that relate the critical depth to the mixed layer,
Obata et al. (1996) assume that a doubling of the surface chlorophyll concentration must
indicate a bloom. However, as I shall suggest later, perhaps this idea may not be quite as
useful as it seems to be.
What are we to make of all this? Obviously, these observations support the concept
that the production and loss rates for plant material in the ocean are fiercely coupled
and rarely resemble those pertaining to the spring outburst of terrestrial plants, where
caterpillars in no way keep up with the growth of oak leaves. Perhaps it is only in
some circumstances, as in the North Atlantic spring bloom south of Iceland, that the
latter model would be appropriate. Loss of observed chlorophyll cannot be accurately
partitioned between sinking and grazing, but the former term is usually accepted to be
the smaller. Diatoms, after all, unless aggregated, have sinking rates of no more than
1 m d
−1 , whereas the small cells that inhabit the viscous realm, and that comprise the
largest biomass component over much of the surface of the ocean, sink even more
slowly—if at all. Further, the measured precipitation of organic material from the photic
zone is usually observed to comprise carbon that has already been consumed; diatom
frustules obtained from the sediments are mostly broken as if by copepod grazing, and
sediment traps capture mostly fecal pellets. The sinking flux (the j-flux of Berger et al.,
Chapter 1: Toward an Ecological Geography of the Sea
For each province subsequently to be described and for each month, we have mean
values that specify surface chlorophyll, integrated chlorophyll, and integrated primary
production rate. By comparing rates of actual and potential increase of phytoplankton
from these, it is simple to propose a first-order plankton calendar for each province that
integrates both production and loss of cells. This can be done by computing a value for
the difference between the algal biomass in each province between successive months.
The sign of this value indicates loss or gain. From the rate of change of algal biomass
and the rate of primary production, both on a monthly basis, it is then possible calculate
how much of the production of algal biomass does not accumulate but must be lost to
consumption, sinking, advection, or entrainment.
The result of this simple investigation is shown in Figure 1.1, where I have computed
the relationship in as simple terms as I can devise: monthly accumulation of biomass in
terms of the daily production rate and standing stock in terms of the monthly production
rate. In each case, you may be surprised by the result if you have never thought seriously
about the relationship. Look first at the open ocean areas lying under the trades and
westerlies—monthly increase of biomass usually represents less than a quarter of one
day’s primary production, and the standing stock represents less than one-tenth of the
production of a whole month. These values are higher in the polar regions, but not by very
much. If we examine individual months, rather than their climatological mean values,
we consistently get the same result. The bloom induced by the southwest monsoon in
the Arabian Sea in June and July accumulates no more than 3.0% and 2.3%, respectively,
of the production that occurs during each of these months. The spring bloom in the
temperate North Atlantic in April and May accumulates even less: 0.26% and 0.58%,
respectively. These percentages are almost matched in May and June in the Atlantic
subarctic regions.
The series of seasonal graphs for production rate and chlorophyll standing stock presented in Chapters 9–12 illustrates the lack of confidence we should have in interpreting
underlying ecological dynamics from indications of plant biomass alone. Phytoplankton
standing stock may track seasonal trends in primary production rate quite closely, or
may significantly diverge from them. Therefore, the graphs show that chlorophyll may
accumulate when the production rate is decreasing or may decline when the primary
production rate is increasing. Such deviations should be welcomed as implying something about the dynamic biology that forces the observed pattern. Where possible, I have
suggested in Chapters 9–12 what the reasons may be for each important example shown.
Associating Chl sat data with global maps that relate the critical depth to the mixed layer,
Obata et al. (1996) assume that a doubling of the surface chlorophyll concentration must
indicate a bloom. However, as I shall suggest later, perhaps this idea may not be quite as
useful as it seems to be.
What are we to make of all this? Obviously, these observations support the concept
that the production and loss rates for plant material in the ocean are fiercely coupled
and rarely resemble those pertaining to the spring outburst of terrestrial plants, where
caterpillars in no way keep up with the growth of oak leaves. Perhaps it is only in
some circumstances, as in the North Atlantic spring bloom south of Iceland, that the
latter model would be appropriate. Loss of observed chlorophyll cannot be accurately
partitioned between sinking and grazing, but the former term is usually accepted to be
the smaller. Diatoms, after all, unless aggregated, have sinking rates of no more than
1 m d
−1 , whereas the small cells that inhabit the viscous realm, and that comprise the
largest biomass component over much of the surface of the ocean, sink even more
slowly—if at all. Further, the measured precipitation of organic material from the photic
zone is usually observed to comprise carbon that has already been consumed; diatom
frustules obtained from the sediments are mostly broken as if by copepod grazing, and
sediment traps capture mostly fecal pellets. The sinking flux (the j-flux of Berger et al.,