340
Chapter 11: The Pacific Ocean
In recent years, we have come to assume that protistan microzooplankton are the
principal consumers of these small autotrophs and thus control their population biomass
rather than, as was previously assumed, the large copepods. However, as discussed by
Rivkin et al. (1999), there is other evidence to suggest that, on an annual basis, this
consumption could account for no more than 50% of potential autotrophic production;
dilution experiments demonstrated that protists really consume more heterotrophic bacteria than either Synechococcus or organisms containing chlorophyll-a. Grazing losses to
protists of eukaryotic phytoplankton was less than 50% of the growth, leaving these cells
with the potential to double their biomass every 3–6 days.
As Rivkin et al. comment, their results “do not support the paradigm” concerning the
effects of protists grazing. Nevertheless, this remains the current textbook explanation
of the OWS P phytoplankton cycle (e.g., Miller, 2004), as taught today in schools of
oceanography. It is also the basis of recent models of processes at OWS P (e.g., Denman
and Peña, 1999), which simulates NO 3 flux and utilization, the physiological effects of Fe
limitation, and zooplankton consumption. I find more convincing the construct of Vézina
and Savenkoff (1999) that follows the increasing food-web diversity that occurs at OWS
P in late summer when chlorophyll is maximal and when the microbial loop food web is
most strongly developed. It emphasizes the role of ammonium regeneration and hence
the suppression of nitrate uptake by autotrophs, as well as the strong coupling between
microzooplankton (dinoflagellates and ciliates) and mesozooplankton (copepods). Much
of the autotrophic and bacterial production in late summer is not consumed and enters
the detrital pool. This model exposes serious gaps in our knowledge, especially of the
larger microzooplankton and the smaller mesozooplankton, including naupliar stages.
Wheeler and Kokkinakis (1990) had earlier asked if ammonium recycling might not
inhibit nitrate uptake. By careful budgeting, and investigating temporal variability in NH 4
and NO 3 concentrations, they concluded that this might be a significant factor that would
impose an upper limit on NO 3 uptake in nutrient-rich, grazing-balanced ecosystems.
Results such as these, it seems to me, illustrate the dangers of jumping to conclusions on
the basis of limited observations, as we all did in the early years of studies at OWS P: what
are we to make, for instance, of the observations of Wong and Matear (1999) of episodic
silicate limitation in long time series (1970–1980) of proxies for OWS P productivity? In
the summers of 1972, 1976, and 1979, SiO 3 at OWS P was depleted to <10 M liter
−1 .
There was also unusual depletion of nitrate in some years, and the ratio of NO 3 //SiO 3
for 1979 showed that diatoms dominated the nutrient draw-down, whereas in 1972
this was not the case, and the phytoplankton community was probably similar to those
years when anomalous draw-down does not occur. Sediment trap data during the 1980s
showed that in years of abnormally large flux, the opal (and hence diatom) content was
not unusually high. This is indeed a cautionary tale.
None of this negates the possibility that Fe limitation may establish a phytoplankton
community deficient in large cells, as suggested by Martin et al. (1989), who outlined a
correlation between regional aeolian Fe inputs, nitrate depletion, and community cell size
in the Gulf of Alaska. It may be, as suggested by Banse (1990a), that Fe limitation applies
only to large cells (small surface area/volume ratio in the presence of low concentrations
of Fe), which normally can outcompete nanophytoplankton for nitrate. If aeolian Fe
limitation is eventually proved to be the critical factor in determining the composition
of the dominant autotroph community in this province, it will no doubt have a more
significant effect in the Alaska gyre than further west because aeolian dust from the
exposed loess deposits of eastern Asia must be attenuated eastward (Duce and Tindale,
1991; Duce et al., 1991).
Despite close coupling between autotrophic and heterotrophic microbiota, the subarctic ecosystem also supports an abundant population of several species of large herbivorous
copepods, though their life histories do not resemble those of Calanus finmarchicus of the
Chapter 11: The Pacific Ocean
In recent years, we have come to assume that protistan microzooplankton are the
principal consumers of these small autotrophs and thus control their population biomass
rather than, as was previously assumed, the large copepods. However, as discussed by
Rivkin et al. (1999), there is other evidence to suggest that, on an annual basis, this
consumption could account for no more than 50% of potential autotrophic production;
dilution experiments demonstrated that protists really consume more heterotrophic bacteria than either Synechococcus or organisms containing chlorophyll-a. Grazing losses to
protists of eukaryotic phytoplankton was less than 50% of the growth, leaving these cells
with the potential to double their biomass every 3–6 days.
As Rivkin et al. comment, their results “do not support the paradigm” concerning the
effects of protists grazing. Nevertheless, this remains the current textbook explanation
of the OWS P phytoplankton cycle (e.g., Miller, 2004), as taught today in schools of
oceanography. It is also the basis of recent models of processes at OWS P (e.g., Denman
and Peña, 1999), which simulates NO 3 flux and utilization, the physiological effects of Fe
limitation, and zooplankton consumption. I find more convincing the construct of Vézina
and Savenkoff (1999) that follows the increasing food-web diversity that occurs at OWS
P in late summer when chlorophyll is maximal and when the microbial loop food web is
most strongly developed. It emphasizes the role of ammonium regeneration and hence
the suppression of nitrate uptake by autotrophs, as well as the strong coupling between
microzooplankton (dinoflagellates and ciliates) and mesozooplankton (copepods). Much
of the autotrophic and bacterial production in late summer is not consumed and enters
the detrital pool. This model exposes serious gaps in our knowledge, especially of the
larger microzooplankton and the smaller mesozooplankton, including naupliar stages.
Wheeler and Kokkinakis (1990) had earlier asked if ammonium recycling might not
inhibit nitrate uptake. By careful budgeting, and investigating temporal variability in NH 4
and NO 3 concentrations, they concluded that this might be a significant factor that would
impose an upper limit on NO 3 uptake in nutrient-rich, grazing-balanced ecosystems.
Results such as these, it seems to me, illustrate the dangers of jumping to conclusions on
the basis of limited observations, as we all did in the early years of studies at OWS P: what
are we to make, for instance, of the observations of Wong and Matear (1999) of episodic
silicate limitation in long time series (1970–1980) of proxies for OWS P productivity? In
the summers of 1972, 1976, and 1979, SiO 3 at OWS P was depleted to <10 M liter
−1 .
There was also unusual depletion of nitrate in some years, and the ratio of NO 3 //SiO 3
for 1979 showed that diatoms dominated the nutrient draw-down, whereas in 1972
this was not the case, and the phytoplankton community was probably similar to those
years when anomalous draw-down does not occur. Sediment trap data during the 1980s
showed that in years of abnormally large flux, the opal (and hence diatom) content was
not unusually high. This is indeed a cautionary tale.
None of this negates the possibility that Fe limitation may establish a phytoplankton
community deficient in large cells, as suggested by Martin et al. (1989), who outlined a
correlation between regional aeolian Fe inputs, nitrate depletion, and community cell size
in the Gulf of Alaska. It may be, as suggested by Banse (1990a), that Fe limitation applies
only to large cells (small surface area/volume ratio in the presence of low concentrations
of Fe), which normally can outcompete nanophytoplankton for nitrate. If aeolian Fe
limitation is eventually proved to be the critical factor in determining the composition
of the dominant autotroph community in this province, it will no doubt have a more
significant effect in the Alaska gyre than further west because aeolian dust from the
exposed loess deposits of eastern Asia must be attenuated eastward (Duce and Tindale,
1991; Duce et al., 1991).
Despite close coupling between autotrophic and heterotrophic microbiota, the subarctic ecosystem also supports an abundant population of several species of large herbivorous
copepods, though their life histories do not resemble those of Calanus finmarchicus of the