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

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10 Reynolds/ Aquatic Ecosystem Health and Management 5 (2002) 3–17 exergy gain but that it is not a guarantee of income. For our Chlorella-based system, for example, seasonal changes in water temperature and daylength have deep-reaching impacts upon anabolic capacity. Even day-to-day changes in the intensity of the incident radiant flux, owing to atmospheric absorption and cloud backscattering, will affect the output of new Chlorella carbon. While the biomass is small and the immediate energy requirements of Chlorella are easily saturated, the assembly potential is hardly at risk, but a developing biomass raises the absolute maintenance demand and, so, increases its sensitivity to fluctuating light income. Of even greater importance to aquatic phototrophs is the fact that water absorbs light (even in the clearest lakes and oceans, light scarcely penetrates to beyond 120 m from the surface). This means that the depth to which phytoplankton are entrained beyond the water surface by turbulent diffusivity subtracts exponentially from the supportive capacity. Given the absorption coefficient in pure water averages about 0.17 m –1 over the visible spectrum and exceeds 0.4 m –1 in the spectral region of chlorophyll excitation (Kirk, 1994), it is easy to appreciate the diminution in chlorophyll-carrying capacity represented by increasing water-column depth. Deep entrainment has long been recognized to be the major limitation to the initiation of vernal phytoplankton blooming in temperate latitudes (Sverdrup, 1953), a finding which modern treatments have continued to uphold (Szeligiewicz, 1998; Huisman et al., 1999). Models using general calculations of photosynthetic potential, corrected for respiration, are available which predict the chlorophyll carrying capacity of water columns as a function of their entrainment depth and background light extinction (see Reynolds, 1997, 1998; Steel and Duncan, 1999). Even under the assumptions of incoming irradiance used to construct Figure 1, the supportive capacity of a water column of 80 m depth is less than 80 mg chlorophyll m –2 (i.e.,
Reynolds/Aquatic Ecosystem Health and Management 5 (2002) 3–17 11 result. If the conditions are sufficiently persistent, that is, for the equivalent of two to four generation times (see Reynolds et al., 1993), the species composition alters significantly, heralding a community response (called “shift”; Reynolds, 1980) to weather-forced environmental change, and from which ready reversion is impossible. A new structure establishes, around a new attractor, pending at least another externally forced shift. The crucial issue is that while a structural response, or disturbance, is often clearly related to a critical forcing event or threshold, the lack of a significant reaction to a forcing should be seen as a positive result of its accommodation by resilient buffering. Either the difference between harvest and cost is such to absorb the forcing; if it is, the harvesting ability provides sufficient exergy to resist the variability in the flux. If it does not, harvesting rate suffers but the apparatus remains sufficiently intact to recover its full function in the event of suspension of the forcing. If the forcing persists, the buffering resilience moves to the adaptability of the cells. If this too is exceeded, resort moves to the population resilience. Ultimately, once this too is exhausted, the species composition of the phytoplankton itself becomes liable to alteration. These various responses to external forcing are represented in Figure 2. It is emphasized that the intensity and duration of forcing have to be viewed separately from the response. The severity of forcing must be measured against the scale of the exergy buffer and the resilience this provides to the biological structure to allow it to bounce back, once the forcing abates. However, if the forcing persists beyond the capacity of the exergy, then the non-sustainability of energetic outgoings in excess of harvested income leads to a rapid and downward revision in the biomass that can be carried. Mass is lost, information is sacrificed—the clear symptoms of disturbance (sensu Pickett et al., 1989). It is also plain that frequent alternation (say, at the scale of four to eight generations (a) (b) Figure 2(a). Representation of ideal system ascendency, in which community assembly works at the limit of the exergy flux. (b) In variable systems, ascendency is regulated by fluctuations in the harvestable energy income, which can be followed through time (broad arrow) and from time to time, falls below a level that sustains the maintenance demand; the necessary restructuring constitutes a disturbance, beyond the acquired structural resilience.
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