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

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12 Reynolds/ Aquatic Ecosystem Health and Management 5 (2002) 3–17 times) between instances of severe forcing and with the more quiescent interludes that provide opportunities for restructuring, hampers the progress of an ecosystem towards energetic steady state. The deduction is no more than a restatement of Connell’s (1978) Intermediate Disturbance Hypothesis. The effect of resources on community assembly The second main impediment to the achievement by a developing ecosystem of its energy-determined potential is the premature exhaustion of the supply of one or other of the component elements. The effects of nutrient sparsity are well-recognized in aquatic systems, where sharp physical boundaries, across which exchange rates are defined or measurable, together with a well-developed understanding of the nutrient chemistries and a suite of sensitive analytical techniques, all contribute to the relative ease of determining availability and demand. Moreover, the composition of protoplasm is broadly similar among bacteria, protists, plants and animals: to be able to assemble biomass requires the delivery of up to twenty elements, some of which are much harder to attain, relative to requirement, than the others. For instance, this consistent cell stoichiometry indicates that in order to build as much as 50 g cell C m –2 requires the assimilation of an average of 1.2 g P, 9 g N and 0.05 g Fe m –2 . For various reasons (electrochemical binding to oxides, chemical inertia, insolubility), natural hydraulic exchanges are rarely able to maintain appropriate fluxes of all three elements. The affinity of the uptake mechanisms of the primary producers is usually very well-developed, and able to reduce the bioavailable sources to very low levels, indeed. In the plankton, at least, if bioavailable nutrient can be measured, primary production is not limited. Among temperate freshwaters, phosphorus exhaustion is a frequent limiting constraint, whereas nitrogen is generally supposed to be relatively the scarcest in lakes of tropical and arid regions, as well as in the coastal and shelf waters of the sea. In the Pacific Ocean, where concentrations of N and P are also habitually quite low, experiments have nevertheless revealed that iron maintains the critical limit (Martin and Fitzwater, 1988). One or other of these nutrients then sets a maximum carrying capacity on the system, which in terms of active biomass, can be marked as an offset on the x axis of Figure 1 in order to re-define the shape of the exergy buffer (now reminiscent of the sail of a dhow or, perhaps, a windsurfer; see Figure 3a). The new boundary reduces the limits within which biomass assembly occurs but leaves the supportable limit with a margin of exergy. As is true of the other two margins, proximity of the system coordinates to the resource limit is indicative of strong selection for appropriate adaptive traits. However, the identity of the species to which any advantage may accrue will depend on just which resource it is that is deficient. Exhaustion of bioavailable silicon (which is not a problem for most non-diatoms) would favour a different outcome from exhaustion of bioavailable nitrogen compounds (which is, supposedly, no bar to the growth of dinitrogen-fixing Cyanobacteria). However, for other resources, such as phosphorus, the requirement is universal and the dependence upon an appropriate source is common. The crucial property in which planktonic algae vary is in their uptake affinity for the resource at very low concentrations. In essence, a high affinity permits one species to be able to gain more of its immediate resource requirement than its competitors, thereby permitting it to sustain its growth for longer, to add to its own potential exergy flux and, in all probability, to build a larger standing population than its rivals. Crucially, this may also mean an enhanced ability to recruit future populations when suitable conditions return, because a relatively greater number of propagules are poised to exploit the next growth opportunity. So it is that dominant organisms frequently match, and are indicative of, the habitats in which they occur preferentially. For examples of superior affinity, one need look no further than the classic experiments of Tilman and Kilham (1976) or of Rhee (1978). These revealed how much “further” certain algal species perform than others when low external nutrient concentrations obtain. Interestingly, the adaptations tend to be nutrient-specific, so that being able to operate on lower phosphorus concentrations than can other species is in no sense a guarantee of being able to work also at low concentrations of, for example, silicon or nitrogen. In fact, there is often an element of “trade-off ” with specialists in, say, silicon uptake having a relatively weak affinity for phosphorus, and vice versa. The key to this behavior is presumably the number of ion-specific acceptor sites per unit of biomass and the level to which the internal transport and assimilation can fall before the cell recognizes its own nutrient deficiency (see Mann, 1995). In practical terms, the behavior is revealed in the specific values of the vectors characterizing the Michaelis-Menten description of uptake dynamics. The higher is the value of maximum uptake
Reynolds/Aquatic Ecosystem Health and Management 5 (2002) 3–17 13 Figure 3(a). The imposition of limited resource availability on the upper attainable biomass. (b) Eutrophication is the abrupt rightward displacement of this limit; conversely, restoration measures are designed to bring about an equally abrupt leftward move: a delay in this response is perceived as a form of resourcing resilience. rate V U max and the lower is the concentration K U required to half-saturate it (both properties favoured by having a large number of receptor sites), then the lower is the external concentration, S, at which V U can continue to meet the demands of growth. As more has been discovered about the needs of plankton autotrophs, or, rather, about how well they are adapted to secure their needs under conditions of severe supply difficulties, quite remarkable differences in affinity have been diagnosed. The sketch in Figure 4 is purely illustrative, purporting to represent the extremes of tolerance of Microcystis aeruginosa to very low concentrations of carbon, of Planktothrix agardhii to very low light doses and of the mixotroph, Synura petersenii, to very low external concentrations of bioavailable phosphorus. There are more benign conditions of light and nutrients where all three are able to coexist with each other and, also, with a large number of more generalist species. The point that must be emphasized is that specialist adaptations are likely to be advantageous to a species when the environment does become highly selective when one or other of these deficiencies begins to impose severe restrictions upon the abilities of most species to operate at all. As with carbon dynamics, the principle of resource limitation is not confined to the primary producers. The importance of the concentration of adequately nutritious and edible planktonic foods (including suspended fine detritus and bacteria as well as phytoplankton) to the ability to support a primarily filterfeeding zooplankton, rather than one of microprotists and calanoid raptors, is generally recognized (about 0.1 g C m –3 : Thompson et al., 1982). The dietary requirements of facultatively planktivorous fish are also sufficiently well-known (e.g, Elliott and Hurley, 1999) for it to be deduced that concentrations of zooplankton equivalent to
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