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

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Reynolds/ Aquatic Ecosystem Health and Management 5 (2002) 3–17
collecting dilute calanoid zooplankton are evident
among their (typically) clupeoid representatives
(shads, smelts, coregonids, and salmonids), fish biomass
and growth are often demonstrably unsustainable
on the basis of production in the zooplankton.
Elsewhere, littoral and benthic foraging is required to
sustain the resource requirements of more intensive
and more diverse production of adult fish. The underpinning
primary products may emanate from sedimenting
phytoplankton or be augmented, substantially
in the case of smaller water bodies, from aquatic
macrophytes and/or inwashed organic matter. Among
these small lakes, feeding activities of fish populations,
including recruiting juveniles, “cascade”
(Carpenter et al., 1985) through the substantial cropping
of zooplankton to mitigate the impact of grazing
on the phytoplankton (Carpenter and Kitchell, 1993).
There now seems little doubt that the complexity of
interactions arising through resource exploitation and
cropping contributes to the richness of fish species
that may co-exist in a lake (Hairston et al., 1960;
Vander Zanden et al., 1999).
This present appreciation of the structure of the
carbon pathways in freshwaters recognizes the enduring
contribution of autotrophs (aquatic or otherwise),
a series of phagotrophs and, indeed, of heterotrophic
microorganisms. There remains a need to investigate
the provenance of the particulate organic carbon fluxes
in selected lakes, and to determine how much is
actually autochthonous. Jónasson’s (1996) comparisons
of the sedimentary fluxes in various lake types
covered a range between 90 and >300 g C m –2 y –1 . On
the other hand, it is difficult to argue for a supporting
flux of atmospheric carbon dioxide, entering exclusively
across the air-water interface of much more
than 50 to 70 g C m –2 y –1 (Reynolds, 1996, 1999).
The shortfall may be met by dissolved carbon dioxide
supplied in inflow streams (Maberly, 1996), but its
photosynthetic fixation would be aquatic. On the
other hand, inflows to particular lakes have been
shown to bring in a substantial load of particulate
organic carbon that is either sedimented or consumed
by the lake’s heterotrophs. Organic loads are not necessarily
diagnostic of pollution but may contribute to
the normal carbon dynamics of a lake. The direct consumption
by zooplankton of catchment-derived
organic debris in Loch Ness (Jones, 1992) may not be
at all unusual.
The building of aquatic ecosystems
For the present, the concern is not the precision of
the structure of the trophic network described, nor
even whether the fund of organic carbon is generated
in the water body itself or on the land. The preceding
section identifies the central flow of organic carbon,
from its synthesis in primary producers, through the
food web to its final dissipation by the heterotrophs.
The present section seeks a comparably reductionist
view of how such organization comes about. In so
doing, we should keep in mind a general picture of the
above structure but we must not simply try to explain
the assemblages we observe. Even the issue of where
the essential control resides—at the bottom, among
the producers, or at the top, among the consumers, or
on the way down again, among the decomposers—
is avoided. What is required is an understanding of the
powering mechanisms and processing constraints and
the nature of the outcomes.
The strength of the ecosystem lies in its power.
Thus, the capture and chemical fixation of energy in
photosynthesis is the foundation of the building
ecosystem. In order to develop the gathering potential,
it is necessary to be able to deploy photosynthate into
assembling further structures for photointerception
and photoconversion. Growth and replication provide
the means of harnessing more of the solar power
available and siphoning its energy into the biological
components. These include the phagotrophs and the
decomposers that exploit the energy store into building
their own biomass, once they have covered the
costs of gathering it.
Common to each of these component steps is the
ability for growth and replication to occur, provided
the energy and resources are available, and that it can
be realized provided that the energetic costs of their
accumulation and the losses to other levels are
exceeded or, at least, balanced by the energetic
income. This makes it sound like a simple profit and
loss account, and that is just what it is. Indeed, it is this
logic which underpins harvest theory for forestry and
fishery management. The growth-reproduction curves
developed by Ricker (1954) and the stock-recruitment
models considered by Gulland (1983) extrapolate the
potential biomass losses and fishery exploitation
against the capacity of the population to recruit subsequent
generations and to increase the standing