14
Chapter 1: Toward an Ecological Geography of the Sea
forced flux of nitrate across the pycnocline as to result in a predictable and characteristic
concentration of mixed-layer nitrate and chlorophyll observed in data fields. The only
stable and predictable balance would appear to derive from the case in which nitrate
or another limiting nutrient is driven to undetectable levels, as indeed over most of
the ocean.
The answer to this riddle must lie in the dynamics of grazing organisms, including
protists. For copepods, there is a wide literature on the response of grazing rate to the
concentration and size of available phytoplankton cells and much attention has been
given to grazing thresholds and the general response of grazers to increasingly dense
or increasingly attenuated concentrations of food particles. The questions, somewhat
simplified, have been the following: do they eat until they burst, or do they stop when
satisfied and, on the other hand, can food be so scarce that they quit and go on a fast?
It is the lower threshold that is of particular interest here. Plankton ecosystem modelers
have found it necessary to introduce such a threshold to stabilize their models, and one
has been demonstrated experimentally for copepods on some occasions (Conover, 1981).
I have been informed (Lessard, personal communication) that protists in an oligotrophic
sea have a threshold for successful feeding that is close to the average chlorophyll value
outside the brief winter bloom. If this is so, writes Karl Banse (1992), perhaps small cells
are kept near this concentration by protistan grazers, whereas the macronutrients that
are measurable in the oligotrophic phase do not control plant growth but are—in his
words—“left over.” In fact, Karl Banse has returned profitably to this same theme on a
number of occasions, most recently in 2002, in a reminder that Steeman Neilsen—he of
the
14 C technique—had, already in the 1930s, commented that the abundance of algal
cells that we observe is a product of the balance between production, consumption, and
sinking. We now have the tools to tell us how tight that balance is in most seas, most of
the time.
One of the conclusions that I draw from these musings is that it is no longer very
useful to examine only one of the two terms in the production/consumption equation
as, for example, was done in the study by Obata et al. (1996). Their analysis, based
on the global CZCS data and the Levitus mixed-layer depth climatology, appears to
confirm some aspects of Sverdrup’s model—that the North Atlantic and northwest Pacific
spring blooms occur after “euphotic conditioning” (i.e., after the mixed layer becomes
shoaler than the critical depth). They also suggest that some blooms are terminated by
the opposite condition. However, I suggest that this can tell us very little about what
causes the observed chlorophyll accumulation: simple inferences that we may make from
maps of change of biomass can turn out to be entirely erroneous. We should all have
understood that a long time ago.
Our New Understanding of the Role
of Very Small Organisms
During the very recent past, one emergent novelty is so fundamental to our understanding
of the characteristic functioning of the pelagic ecosystem in different regions of the oceans
that it is imperative to discuss it, at least superficially, before proceeding to a synthesis of
oceanic and shelf phytoplankton. This novelty is the role of extremely small organisms
in all pelagic ecosystems.
We may visualize two realms in the pelagic ecosystem, and both are intimately
connected—so that the one cannot get along without the other—but, until very recently,
almost everything you read about, and most of what you cared about, belonged to only
one of the two. These realms are those in which, respectively, viscous and inertial forces
Chapter 1: Toward an Ecological Geography of the Sea
forced flux of nitrate across the pycnocline as to result in a predictable and characteristic
concentration of mixed-layer nitrate and chlorophyll observed in data fields. The only
stable and predictable balance would appear to derive from the case in which nitrate
or another limiting nutrient is driven to undetectable levels, as indeed over most of
the ocean.
The answer to this riddle must lie in the dynamics of grazing organisms, including
protists. For copepods, there is a wide literature on the response of grazing rate to the
concentration and size of available phytoplankton cells and much attention has been
given to grazing thresholds and the general response of grazers to increasingly dense
or increasingly attenuated concentrations of food particles. The questions, somewhat
simplified, have been the following: do they eat until they burst, or do they stop when
satisfied and, on the other hand, can food be so scarce that they quit and go on a fast?
It is the lower threshold that is of particular interest here. Plankton ecosystem modelers
have found it necessary to introduce such a threshold to stabilize their models, and one
has been demonstrated experimentally for copepods on some occasions (Conover, 1981).
I have been informed (Lessard, personal communication) that protists in an oligotrophic
sea have a threshold for successful feeding that is close to the average chlorophyll value
outside the brief winter bloom. If this is so, writes Karl Banse (1992), perhaps small cells
are kept near this concentration by protistan grazers, whereas the macronutrients that
are measurable in the oligotrophic phase do not control plant growth but are—in his
words—“left over.” In fact, Karl Banse has returned profitably to this same theme on a
number of occasions, most recently in 2002, in a reminder that Steeman Neilsen—he of
the
14 C technique—had, already in the 1930s, commented that the abundance of algal
cells that we observe is a product of the balance between production, consumption, and
sinking. We now have the tools to tell us how tight that balance is in most seas, most of
the time.
One of the conclusions that I draw from these musings is that it is no longer very
useful to examine only one of the two terms in the production/consumption equation
as, for example, was done in the study by Obata et al. (1996). Their analysis, based
on the global CZCS data and the Levitus mixed-layer depth climatology, appears to
confirm some aspects of Sverdrup’s model—that the North Atlantic and northwest Pacific
spring blooms occur after “euphotic conditioning” (i.e., after the mixed layer becomes
shoaler than the critical depth). They also suggest that some blooms are terminated by
the opposite condition. However, I suggest that this can tell us very little about what
causes the observed chlorophyll accumulation: simple inferences that we may make from
maps of change of biomass can turn out to be entirely erroneous. We should all have
understood that a long time ago.
Our New Understanding of the Role
of Very Small Organisms
During the very recent past, one emergent novelty is so fundamental to our understanding
of the characteristic functioning of the pelagic ecosystem in different regions of the oceans
that it is imperative to discuss it, at least superficially, before proceeding to a synthesis of
oceanic and shelf phytoplankton. This novelty is the role of extremely small organisms
in all pelagic ecosystems.
We may visualize two realms in the pelagic ecosystem, and both are intimately
connected—so that the one cannot get along without the other—but, until very recently,
almost everything you read about, and most of what you cared about, belonged to only
one of the two. These realms are those in which, respectively, viscous and inertial forces