Resilience in aquatic ecosystems - hysteresis, homeostasis, and ...

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Reynolds/ Aquatic Ecosystem Health and Management 5 (2002) 3–17
Figure 4. Diagram to show the environmental growth tolerances
of three species of phytoplankton (Microcystis aeruginosa,
Planktothrix agardhii, Synura petersenii) with respect to photo
flux, phosphorus and carbon availability.
also be noted that the exergy buffer narrows at higher
levels of biomass and, so, increases the vulnerability
of enriched, productive systems to disturbance by
external forcing. In the relatively recent instances of
anthropogenic eutrophication (within the last 30–50
years), there has been rapid filling of the added capacity
by the larger biomass of the existing biota—or
more of the same. Primary biomass is added ahead of
development of the new information represented by
an appropriately modified consumer community. In
the terminology of Ulanowicz (1986), eutrophication
is a form of ascendancy in which added throughflow
exceeds the addition of new information in the flow
network. It could be said that the biological system is
not yet sufficiently organized to allocate the new
resources in the most efficient structures. Until the
additional resource can be absorbed into the biomass
of long-lived, resource-efficient (classically K-selected)
organisms, it remains labile and exchangeable
among the storages provided mainly by short-lived
(classically r-selected) organisms. Much of the additional
resource oscillates among water, sediment and
transient biomass, whereas the more desirable alternative
is for it to be invested in the flora and fauna of
a macrophyte-dominated system (Scheffer and
Jeppesen, 1998). In its planktonic state, the luxury
labile capacity buffers the resource fluctuations in a
manner analogous to the exergy riding on the energy
exchanges of the system.
Considering now the impact of a lake therapy
achieved by the abrupt reduction of the external loading
of the limiting resource, the desired result is a
similarly abrupt system response represented by the
leftward shift of the resource barrier. Of course, this is
not achievable while resource capacity is still being
freely exchanged among the system components. As
has been pointed out in Sas’ (1989) analyses of
schemes based upon reduced phosphorus loading, the
truncation of resources is effective in deeper (mainly
>5 m) systems where the sediment phosphorus is generally
closed to effective recycling. That part of the
extrabiotic pool which does not remain in the sediment
store is progressively flushed from the lake: the
generality that this takes the equivalent of three
hydrological retention times before a new steady state
is achieved is probably well-testified (author’s unpublished
assessments). However, in shallow lakes
wherein most of the sediment is reached by wind- and
wave-generated shear stress, recent sedimentary fractions
are retained in the extant internal cycling and,
thus, the resource buffering role is prolonged.
Eventual rundown is predictable but the time scale
depends both on the margin of excess and the recycling
frequency. Not all lakes have proved as
intractable as Søbygaard (Søndergaard et al.,
1993) but this merely represents one of the fullest
studied and most impressive examples of resilience to
limnotherapy designed to reverse the effects of
eutrophication.
By extension of the analogy to the exergy buffer,
resilience to restoration measures is explicable as a
function of the buffer provided by extrabiotic resource
pools in the system. Variations in the resource constraint
through what is, in effect, a short period in the
life span of a water body are damped by the geochemical
exchanges involving parts of the system
other than the respondents in question.
Conclusion: Resilience
and ecosystem health
However far we attempt to compare and to interrelate
the analogies between two distinct limnological
connotations of resilience, the dilemma identified