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

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4 Reynolds/ Aquatic Ecosystem Health and Management 5 (2002) 3–17 external sources are truncated, owes principally to Sas (1989). In this instance, the restorative therapy, the reduction in external nutrient loading, is the forcing agent but the anticipated response of a reduced algal biomass response is neither immediate nor even linear. Several mechanisms are known to contribute to this behavior, including the buffer provided by unused supportive capacity and the recyclability of nutrient resources within the system (‘internal loading’). This buffering capacity is not inexhaustible but, in terms of management and of returns on investment, it can seem stubbornly persistent (Søndergaard et al., 1993). Supposing the capacity of the impacted system to have once supported lower algal and plant biomasses, the eutrophication and the protracted recovery are clearly parts of a strongly hysteretic behavior pattern. During the delay period, at least, the resilient system fails to regain a state of improved health. In almost every other ecological context in which the word has been used, resilience refers to the collective capacity for homeostasis that is acquired by developing ecosystems. Before the advent of a subdiscipline dealing with the large-scale properties of systems (‘macroecology’: Brown and Maurer, 1989), the manifestation of a functionally mature ecosystem was considered to be its stability. The notion of a sort of self-maintaining equilibrium condition, with very little fluctuation, was memorably challenged by Holling (1973). He recognized the scales and amplitude of normal, natural environmental variability that surviving systems have successfully to absorb. Their ability to do so is a measure of their resilience. Thus, resilience is the property of the system that keeps it at some recognizable steady state and permits it to return there following disturbances wrought by external forcing. It conforms to the role of a Lorenzian attractor of Chaos Theory in suppressing random behavior (Gleick, 1988). Thus, the supposed stability of a system is the manifestation either of its accommodation of mild environmental fluctuations or of the speed with which it is able to return to an erstwhile steady state. Provided the system retains the inocula of component species from which new populations may develop, it may well recover its former structure (Harrison, 1979). In other words, and in contrast with the earlier definition, the resilient system is one that is able to regain quickly a state of perceived health. It would be relatively simple to declare that these contrasted and seemingly opposed usages of resilience are unnecessarily confusing and that one of them should be rejected. Neither would it be too difficult to suggest that, because of the narrowness of its application and because the property it described is, in any case, closer to the understanding of resistance than to resilience, Sas’ (1989) usage should be the one that is discarded. Before that happens, however, the opportunity should be taken to explore the conceptual commonality of both usages to the adverse anthropogenic impacts on the health of aquatic ecosystems. More to the point, we need to assure ourselves of the mechanisms of ecosystem resilience, the factors that contribute to and achieve the so-called steady states and the critical levels of forcing that might move the system to some other steady state. Here, I seek to formulate a model that incorporates the assembly of aquatic systems, their robustness and the levels of forcing they may tolerate: the objective is a single view of ecosystem function that subsumes both understandings of system resilience. The structure of aquatic ecosystems Functional ecosystems comprise complex consortia of organisms with closely interrelated activities that together process the resources of geographicallydistinct segments of the planet into living biomass. Most of the driving energy for this processing comes from sunlight which has to be captured, stored chemically, and then released in a regulated manner to sustain the work involved in gathering and assembling resources into biological structures. At their most fundamental level, these exchanges are biochemical and they occur at the level of cytological structures—light harvesting centres, ribosomes, and mitochondria—the assembly of which is coordinated within cells, according to instructions copied in their DNA, each time it is replicated. Cells form tissues, the specialist functions of which contribute to the physiological workings of whole organisms, whose nature is to grow and reproduce. Organisms are, in turn, functionally differentiated among energy-capturing primary producers and resource-cycling heterotrophs. Their populations contribute to the ecological makeup of communities and their mutual interactions, most significantly in governing the trophic linkages of the food web, determine the eventual structure of the ecosystems. This much is well understood but there are two aspects that require emphasis. One is the continuous nesting of the relevant processes, from molecule and photon up to the material and energy budgets of whole ecosystems. In order to comprehend and quantify large-scale functions, it is justifiable to view ecosystems as fractal series, each level being
Reynolds/Aquatic Ecosystem Health and Management 5 (2002) 3–17 5 essentially quantifiable in common terms of resource processing and energy dissipation. Such series establish the basis for the achievable norm of a steady-state condition (Pielou, 1974). Second, while the ecosystem remains the sum of the component processes carried out by the aggregate of its constituent individuals, the highest level of directed, internal control is located in the genomic instructions of the individual component organisms. Every aspect of the behavior of structures involving populations of interacting species becomes contingent upon the most likely outcomes: the supposed ability of ecosystem structures to self-organize is a wholly emergent consequence of interactive responses to environmental variability (Reynolds, 2001). Aquatic ecosystems are scarcely less complex than are terrestrial ones. However, the traditional view of the interrelationships among the biota of large water bodies provides a convenient introduction to highorder behavior. They are supposed to be built and organized around a flow of organic carbon through a structured food web, which, as in the above ideal, involves the autotrophic primary producers (the phytoplankton), a series of phagotrophic consumers (zooplankton, planktivorous, and piscivorous fish), and heterotrophic micro-organisms (bacterioplankton) closing a material cycle. Carbon dioxide is fixed and assimilated into phytoplankton biomass, which, in turn, becomes the food of the pelagic phagotrophs and the substrate of the heterotrophs. It is emphasized that this simplistic view of the structure is no longer literally valid. In the first place, terrestrial producers fix much of the organic carbon arriving in freshwater systems. Some is particulate and its presence in small ponds and streams has long been recognized to support macroinvertebrate shredders and detritus feeders. Much arrives in the form of fulvic and humic soil leachates, derived ultimately decomposing plant material. Attention has been drawn by several authors (Salonen et al., 1992; Wetzel, 1995) to the fact that this dissolved fraction represents some 50 to 90% of the organic carbon in the open water of lakes, more than is represented by the aggregate of live biomass. The extent to which aquatic ecosystems are heterotrophic (Thomas, 1997) is not resolved and it is far from clear that this refractory organic carbon is readily converted to biomass. Even in the nutrient-deficient, ultra-oligotrophic oceans, containing up to 1 mg DOC l –1 , the heterotroph biomass is usually found to be carbonlimited (Bratbak and Thingstad, 1985). On the other hand, there is a rapid turnover of relatively low molecular weight photosynthetic alternatives (such as glycolate) produced by phytoplankton whose prospects for biomass increase are constrained by severe nutrient limitation and whose photosynthate and oxidant by-products cannot be stored. Among the green algae, glycolate is further oxidized (photorespired) to carbon dioxide and water, but, in the diatoms and Cyanobacteria, the glycolate is released into the medium, at reported rates equivalent to up to 92% of the contemporaneous carbon uptake rate of the cells (Geider and Osborne, 1992). Certainly at the slight, dilute producer concentrations obtaining, the bacteria constitute a more significant potential resource to the consumer network, though they are most efficiently gathered and progressively concentrated by phagotrophs working at the same nano- and micro-scales. The widespread demonstration of a microbial loop (Azam et al., 1983) of heterotrophic bacteria, nanoflagellates and ciliates linking photosynthesis to crustacean zooplankton emphasizes its pivotal importance to the transfer of autochthonous carbon within chronically nutrient-deficient systems. Although its role in oligotrophic systems is emphasized, microbial intervention is not unimportant in the structure of eutrophic food webs. Where a larger producer biomass is supportable, filter-feeding crustacea (especially daphniids) may feed very effectively on microplanktic algae (Lampert et al., 1986), at times reducing their standing biomass to trivial levels. Microplanktonic ciliates are capable of similar destruction of the main carbon resource (Dryden and Wright, 1987). However, in small and/or shallow lakes able to support a macrophytic vegetation, a parallel avenue of heterotrophically mediated carbon flow may be established. Mechanical disintegration of spent plant tissue, significantly assisted by bacterial decomposition, through the action of invertebrate shredders, collectors, and filterers, yield a food resource to fish largely independent of planktic producers. Detritus and bacteria may even sustain littoral populations of filterfeeding Cladocera (including daphniids) to the extent that they keep the water free from planktic microalgae (Scheffer and Jeppesen, 1998: Søndergaard and Moss, 1998). In this attainable steady state, the macrophytes are the dominant primary producers. A further paradigm shift from the traditional foodchain model concerns the production of fish biomass. With the exception of the true pelagic, where specialized feeding and filtering gill-raker adaptations for
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