High-nitrate,
low-chlorophyll (HNLC) ocean regions are characterized by low, stable
phytoplankton standing stocks which rarely deplete nitrate, phosphate or
silicate in the upper water column to growth limiting levels. These ‘balanced’
pelagic ecosystems have been studied in depth over past decades in order to
understand how they work and the ‘SUPER synthesis’ is now widely accepted with
supporting evidence: phytoplankton cell size is restricted by iron limitation,
the small cells are accessible to grazing by protists and the high growth rate
of these consumers ensures they will always overtake and suppress increases in
phytoplankton stock, therefore preventing blooms. However, without an understanding of the mechanisms setting the
lower limits to phytoplankton stock on both seasonal and short-term time
scales, our understanding of HNLC systems is incomplete. The main issue is the
means by which consumers avoid exterminating their prey and is a long standing
problem in ecology.
Removal processes hold the key to phytoplankton
biomass minima as long as phytoplankton cell division is occurring. This is
true regardless of the degree to which phytoplankton growth rates are limited
by resource availability. Grazers are the principal removers of phytoplankton
in the open ocean, therefore, grazer feeding rates and standing stocks must be
the primary determinants of phytoplankton biomass minima in oceanic systems. In
addition to grazing thresholds, possible explanations include spatial and
temporal inhomogeneity in the habitat, control of grazers by their predators,
and switching to alternative prey.
In
plankton dynamics models, the inclusion of one or more spatial dimensions can
generate patchy distributions which, through diffusional exchange, permit
persistence of prey populations that might otherwise be driven to extinction by
their predators. Small-scale patchiness, however, can only provide a refuge
from grazers (and hence determine lower limits of phytoplankton biomass) if phytoplankton
and grazers respond differently to, and can thus be uncoupled by, the patch
environment.
Control
of grazer populations by higher trophic level predators (carnivory) has been
proposed as a mechanism to provide ecosystem stability in the absence of feeding
thresholds. This indicates that top-down regulation of herbivores is a
necessary but not sufficient condition for control of both minimum and maximum
phytoplankton biomass in HNLC systems as predation control requires continuous
predation, which the life history and grazing behavior of protists does not
dictate.
Switching
behavior by predators (disproportionate grazing on the more abundant of multiple
prey types), has been suggested as a stabilizing influence on prey biomass. Switching
may occur between multiple phytoplankton taxa, or between phytoplankton and
other particle types such as bacteria and detritus. Prey types at low abundance
would then experience a refuge even though total feeding activity by the grazer
was not reduced.
This
paper brings together numerous proposed models used to predict the mechanics of
HNLC areas for comparison, highlighting the good and bad points of each and suggesting
elements of each that could be combined to produce a model that predicts both
the lower and upper limits of phytoplankton stocks in HNLC systems. The authors suggest that while our
knowledge of the upper limits is sufficient, until more work is done on the
lower limits we cannot be certain that our conclusions for the upper limits are
correct and that the ‘big picture’ is a long way from complete. They also
suggest that laboratory experiments need to mimic natural oceanic conditions a
lot better with the use of multiple prey and predator types etc. Overall, it’s
a nice paper with good critical analysis of all recent work and proposed models
on HNLC systems.
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