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

More documents

Recommendations

Info

14 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
Reynolds/Aquatic Ecosystem Health and Management 5 (2002) 3–17 15 in the opening two paragraphs persists: one of them is supposed to be good (for stability, functional recovery, maintenance of diversity); the other seems rather bad (for cost-effective management, knowledge vindication, remediation return). Clear distinction between the two intents is still necessary: pending further discussion, I nominate as provisional terms, structural resilience” and “resourcing resilience”. Although the present forum was convened for resilience in the resourcing sense, there seems to be no good case to be made for continuing with this usage. As a scientific issue, it is no more than a hysteresis effect, brought about through the biological separation of the response (lower plant biomass) from the driving variable (reduced external nutrient loading). As an economic issue, a difficulty persists insofar as investment of large amounts of public or customer money in restoration projects must show results. Yet that has always been so—it is not sufficient to produce an obfuscation of limnetic insensitivity to restorative therapy by attributing it to resilience. Blaming delay on insensitivity might make a better case for researching, and quantifying, the sensitivity of systems before the therapy is applied so that some better anticipation of its likely efficacy and value can be given at the start. Against this, structural resilience is a variable property of all ecosystems and, I suggest, provides a dimension for checking on their condition, or the state of their ecosystem health. If I cannot satisfy the forum’s wishes, perhaps it would support the organizers’ cause to fit structural and resourcing resilience into a general view of the diagnostics of ecosystem health. The term is still quite young and, to many ecologists, lacks a precise and widely-accepted meaning. For some, healthy is synonymous with oligotrophic and pristine; lakes that comply with the latter two adjectives may well be healthy but a presupposition that everything else is therefore unhealthy is erroneous and dangerous. The criterion of ecosystem health should be its sustainability—that is, its ability to maintain its structure and function in the face of (a) (b) Figure 5. (a) Axes for representing ecosystem health as proposed by Costanza and Mageau (1999), corresponding to resilience (x axis), structure (y axis), and productivity (z axis). The axes are used in (b) to distinguish assembly tracks of an ascendant ecosystem with increasing resilience, structure and productivity from one undergoing rapid eutrophication, where productivity and resilience increase, but structural information is deficient.
Page 1 and 2: Resilience in aquatic ecosystems—
Page 3 and 4: Reynolds/Aquatic Ecosystem Health a
Page 11: Reynolds/Aquatic Ecosystem Health a
Page 15: Reynolds/Aquatic Ecosystem Health a

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

Create successful ePaper yourself

Delete template?

Save as template?