Our New Understanding of the Role of Very Small Organisms
17
Chlorophyll biomass seems more tightly regulated in smaller than in larger cell fractions because the rapid reaction of ultraplankton consumers to an increase in numbers
of their picoplanktonic food induces greater population stability than is characteristic
of the realm of larger organisms. Although, as we shall see, large cells characteristically
dominate algal blooms, Li (in press) points out that microbial size spectra conform in
shape and slope with those characteristic of larger plankton: partial size overlap between
components results in smooth ensembles. Li suggests that such observations tend to
support previous speculation that plankton ecosystems tend to self-organize into holistic
states characteristic of each oceanic environment.
Li et al. (2002, 2004) have examined some of the basic relations between small planktonic organisms and between them and their environment, using macroecological analysis
of very large data sets, some obtained pixel-by-pixel from satellite images. These studies show that we may no longer use a straight-line model for the ecological coupling
between heterotrophic bacterioplankton and phytoplankton biomass. Li et al. point out
that the relationship between these two trophic groups is different at high and low levels
of phytoplankton biomass, suggesting a difference between bottom-up resource control
and top-down mortality control. This is seen when all data for the North Atlantic are
consolidated, but when a region-by-region approach is taken, region-specific differences
in slopes and intercepts in the relationship become apparent.
The differential distribution of heterotrophic bacteria and autotrophic cells within the
viscous realm between regions or in relation to (say) temperature is complex; for one
thing, bacterial biomass and numbers are known to be relatively constant everywhere,
while the biomass of autotrophs varies strongly: how, then, ask Li and Harrison (2001),
is balance achieved? Where phytoplankton productivity is low, they suggest, bacteria “are
respiring a great deal and not growing very much,” while where it is high “bacteria channel
their resources more equitably between respiration and growth.” Of course, as they also
remind us, the proportion of metabolically active bacteria in the heterotrophic population
is quite small in oligotrophic waters but increases as phytoplankton productivity increases;
such a mechanism appears capable of accounting for the apparent invariance in total
heterotrophic bacterial numbers in all surface waters.
It may be useful to remind ourselves very briefly at this point that benthic ecology has
recently undergone at least one sea change in its acknowledgement of the diversity of the
very small biota that inhabit the interstices between sand grains and other particles in
the substrate. This was progressively exposed to science during the second half of the last
century and (as Tom Fenchel commented in 1978) “next to the astonishing morphological
adaptations the next most remarkable thing about the meio- and micro-fauna is
perhaps its richness in species.” He refers to the extraordinary manner in which organisms
like holothurians, polychaetes, or mollusks manage to fit their complex anatomy into
dimensions of only 0.5–5.0 mm. For instance, the tiny medusa Halamohydra is <10 mm
long and lives between sand grains. These interstitial organisms comprise both protists and
metazoans and their interest for us is the complexity of their ecological relations, and how
these change with the nature of the substrate. Although mostly investigated along the shore
and in shallow water, the interstitial fauna is known to extend to great depths and even
into anoxic, reducing sediments of the shelf where ciliates and zooflagellates dominate.
For such reasons, the interstitial fauna has become a paradigm for how species within
ecosystems partition the available resources, and for the existence of specialized food
niches. Rather than the generalized type of food gathering that we might anticipate,
specialists abound for both food particle size and type. We shall have to bear the existence
of this fauna in mind when considering the dynamics of benthic–pelagic coupling in
continental shelf regions.
17
Chlorophyll biomass seems more tightly regulated in smaller than in larger cell fractions because the rapid reaction of ultraplankton consumers to an increase in numbers
of their picoplanktonic food induces greater population stability than is characteristic
of the realm of larger organisms. Although, as we shall see, large cells characteristically
dominate algal blooms, Li (in press) points out that microbial size spectra conform in
shape and slope with those characteristic of larger plankton: partial size overlap between
components results in smooth ensembles. Li suggests that such observations tend to
support previous speculation that plankton ecosystems tend to self-organize into holistic
states characteristic of each oceanic environment.
Li et al. (2002, 2004) have examined some of the basic relations between small planktonic organisms and between them and their environment, using macroecological analysis
of very large data sets, some obtained pixel-by-pixel from satellite images. These studies show that we may no longer use a straight-line model for the ecological coupling
between heterotrophic bacterioplankton and phytoplankton biomass. Li et al. point out
that the relationship between these two trophic groups is different at high and low levels
of phytoplankton biomass, suggesting a difference between bottom-up resource control
and top-down mortality control. This is seen when all data for the North Atlantic are
consolidated, but when a region-by-region approach is taken, region-specific differences
in slopes and intercepts in the relationship become apparent.
The differential distribution of heterotrophic bacteria and autotrophic cells within the
viscous realm between regions or in relation to (say) temperature is complex; for one
thing, bacterial biomass and numbers are known to be relatively constant everywhere,
while the biomass of autotrophs varies strongly: how, then, ask Li and Harrison (2001),
is balance achieved? Where phytoplankton productivity is low, they suggest, bacteria “are
respiring a great deal and not growing very much,” while where it is high “bacteria channel
their resources more equitably between respiration and growth.” Of course, as they also
remind us, the proportion of metabolically active bacteria in the heterotrophic population
is quite small in oligotrophic waters but increases as phytoplankton productivity increases;
such a mechanism appears capable of accounting for the apparent invariance in total
heterotrophic bacterial numbers in all surface waters.
It may be useful to remind ourselves very briefly at this point that benthic ecology has
recently undergone at least one sea change in its acknowledgement of the diversity of the
very small biota that inhabit the interstices between sand grains and other particles in
the substrate. This was progressively exposed to science during the second half of the last
century and (as Tom Fenchel commented in 1978) “next to the astonishing morphological
adaptations the next most remarkable thing about the meio- and micro-fauna is
perhaps its richness in species.” He refers to the extraordinary manner in which organisms
like holothurians, polychaetes, or mollusks manage to fit their complex anatomy into
dimensions of only 0.5–5.0 mm. For instance, the tiny medusa Halamohydra is <10 mm
long and lives between sand grains. These interstitial organisms comprise both protists and
metazoans and their interest for us is the complexity of their ecological relations, and how
these change with the nature of the substrate. Although mostly investigated along the shore
and in shallow water, the interstitial fauna is known to extend to great depths and even
into anoxic, reducing sediments of the shelf where ciliates and zooflagellates dominate.
For such reasons, the interstitial fauna has become a paradigm for how species within
ecosystems partition the available resources, and for the existence of specialized food
niches. Rather than the generalized type of food gathering that we might anticipate,
specialists abound for both food particle size and type. We shall have to bear the existence
of this fauna in mind when considering the dynamics of benthic–pelagic coupling in
continental shelf regions.