tree vol. 7, no. 8, august 1992 · lewi stone is at the yigal allon kinneret limnologi- cal...

TREE vol. 7, no. 8, August 1992

Documentation, The Netherlands 6 Grime, 1. (1973) /. Environ. Manage. I, 151-167 7 Tilman, D. ( 1982) Resource Competition and Community Structure, Princeton University Press 8 Connell, 1.H. (1978) Science 199, 1302-1310 9 Huston, M. (1979) Am. Nat. 113, 81-101 IO Hubbell, S.P. and Foster, R.B. (19861 in Community Ecology (Diamond, 1. and Case, T.I., edsl, pp. 314-329, Harper & Row II Huston, M. (1980) /. Biogeogr. 7, 147-157 12 Gentry, A.H. and Emmons, L.H. (1987) Biotropica 19, 216-227 I3 Ashton, P.S. (1988) Annu. Rev. Eco/. Syst. 19, 347-370 I4 Gentry, A.H. (1988) Ann. MO. Bot. Card. 75, l-34 I5 Faber-Langendoen, D. and Gentry, A.H. (I991 I Biotropica 23, 2-l I I6 Hall, I.B. and Swaine, M.D. (1981) Distribution and Ecology of Vascular Plants in a Tropical Rain Forest, Dr W. lunk I7 Gentry, A.H. and Dodson. C.H. (1987)

Ann. MO. Bot. Card. 74, 205-233 I8 Windsor, D.M. (19901 Smithson. Contrib. Earth Sci. 29, I-145 I9 wright, S.f. (1991) Ecology 72, 1643-1657 20 Becker, P., Rabenold, P.E., Idol, 1.R. and Smith, A.P. (1988) /. Trap. Ecol. 4, 173-184 21 Vitousek, P.M. and Denslow, I.S. ( 1986) j. Eco/. 74, 1167-l I78 22 Smith, A.P. (1987) Rev. Bio/. Trap. 35 (Suppl. I ), I I l-l I8 23 Mulkey, S.S., Smith, A.P. and Wright, S.J. ( I99 I ) Oecologia 88, 263-273 24 Wright, SJ., Machado, I.L., Mulkey, S.S. and Smith, A.P. (1992) Oecologia 89, 457-463 25 Mulkey, S.S., Wright, SJ. and Smith, A.P. ( 199 I ) Am. I. Bat. 78, 579-587 26 Hubbell, S.P. and Foster, R.B. (1990) in Reproductive Ecology of Tropical Forest P/ants (Bawa, KS. and Hadley, M., eds), pp. 3 17-34 I, UNESCO and Parthenon 27 Hubbell, S.P. and Foster, R.B. (1990) in Four Neotropical Rain Forests (Gentry, A.H., ed.), pp. 522-541, Yale University Press

28 Hubbell, S.P. and Foster, R.B. Ecology (in press) 29 Wright, S.I. and Van Schaik, C.P. Am. Nat. (in press) 30 Fisher, B., Howe, H.F. and Wright, SJ. ( I99 I 1 Oecologia 86, 292-297 PI Becker, P. and Castillo, A. (1990) Biotmpica 22, 242-249 32 Wright, S.I. and Comejo, F.H. ( 1990) Ecology 71, 1165-l 175 33 Wright, S.I. and Comejo, F.H. ( 1990) in Reproductive Ecology of Tropical Forest P/ants (Bawa, K.S. and Hadley, M., eds), pp. 49-61, UNESCO and Parthenon 34 Wolda, H. ( I9881 Annu. Rev. Ecol. Syst. 19, l-18 35 Bjorkman, 0. and Ludlow, M.M. (19721 Carnegie Inst. Washington, Yearb. 71, 75-94 36 Chazdon, R.L. and Fetcher, N. (1984) in Physiological Ecology of Plants of the Wet Tropics (Medina, E., Mooney, H.A. and Vazquez-Yanes, C., eds), pp. 27-36, Dr W. lunk 37 Denslow, IS. (1987) Annu. Rev. Ecol. Syst. 18, 43 l-452

Positive Feedback in Aquatic Ecosystems Aquatic ecosystems offer striking examples

of how positive feedback can be integral ko the dynamics of complex communities. In particular, microorganisms (bacteria and protozoa) introduce a multitude of positive feedback pathways by rapidly recycling nutrients at the very base of many aquatic food webs. The relatively large magnitude of fluxes being shunted through this ‘microbial loop’ allows an accumulation of nutrients in localized areas, promotes a general build-up of biomass, and acts as a ‘life-support system’ in harsh environments. In contrast to customary notions which portray positive feedback effects as undesirable, a reassessment indicates that this ‘bootstrapping’ can often be advan- tageous for many organisms.

Positive interactions, arising by either direct or indirect means, are widespread and intrinsic to virtually all ecosystems. Organisms that confer benefits upon one another through mutualistic or symbiotic interactions are crucial components of many ecological communitiesi-5. Similarly, organisms that have the capacity to recycle resources in

Lewi Stone is at the Yigal Allon Kinneret Limnologi- cal Laboratory, Israel Oceanographicand Limnologi- cal Research, PO Box 345, Tiberias 14102, Israel; Richard Weisburd is at the National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, lbaraki 305, lapan.

@ 1992. Elsewer Science Publishers Ltd (UK)

Lewi Stone and Richard S.J. Weisburd

loops of positive feedback allow many communities to ‘continually pull themselves up by their own bootstraps’6 - a feature especially useful when nutrients are limited, or in periods of environmental stress”lO.

Historically speaking, there has been only scant interest in positive feedback relationships since they have generally been dismissed as destabilizing to Nature’s ‘har- monious balance’. Models of mutualistic or beneficial interactions between organisms routinely predicted explosive and unstable population growth4. Comprehen- sive models of nutrient cycling demonstrated that the positive feedback effects, so inherent to recycling, are destabilizing, in that they delay the time needed for a system to recover from disturb- ances7s8. It is only recently, after careful re-evaluation of the modelling process, that these results, together with their implications, are beginning to be questioned2*“J2.

By viewing positive feedback as destabilizing, ecologists supposed that it played only a minor role in natural communities; evolutionary

arguments deemed it unlikely that organisms develop strategies that depend on unstable processes. Even the presence of positive feedback pathways was often regarded as something of a paradox’*. Such fixed ways of thinking hindered any serious study of the functioning of positive relationships in ecological communities.

In contrast, here we examine some significant properties of positive feedback mechanisms and their importance in the context of aquatic ecosystems. This focus has been chosen since fundamental ideas in aquatic ecology and community structure have recently undergone a conceptual revolution.

Feedback: a question of perspective Even the simplest of ecosystems

may contain a multitude of positive feedback pathway9. Unfortunately this straightforward fact is seldom acknowledged. Instead it has been stressed that ecosystems are predominantly controlled by self- correcting negative feedback mechanisms. We are often reminded, for example, that homeostasis - a

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concept that is frequently viewed as fundamental in ecology - ‘depends upon negative feedback’13. But overemphasizing the importance of negative feedback can be misleading and tends to conceal other significant processes (see Box I).

Like many other physical processes, feedback has characteristics that are reference-frame dependent. Its direction, positive or negative, depends entirely on the relative reference frame of the observer (see Box 1 and DeAngelis et a1.2). Either because of lax terminology, or from using a poorly

chosen or misleading reference frame, many ecological processes have been considered stabilized by negative feedback, but paradoxi- cally, if viewed from another reference frame, they may equally well be situations in which positive feedback features prominently. Rather than being destabilizing, positive feedback can play an important role in many regulatory processes.

When considering the dynamics of aquatic ecosystems, perhaps the most natural choice of reference frame is that of nutrient flows. Since nutrients, once used, are literally fed back to the available nutrient pools, we define this recycling to be a loop of positive feedback - a definition that is used throughout the paper. We emphasize that one of Nature’s fundamental processes - nutrient recycling - may be viewed as a simple form of positive feedback.

Changing views of aquatic ecosystem structure, cycling and control

Conventionally, aquatic communities were regarded as examples of ascending trophic level systems. Nutrients passed up through a linear, vertical food chain (Fig. 11, the major trophic flows being from primary producers to zooplankton to fish, with some excreted products recycled along the way. Nearly all phytoplankton production was considered to be consumed by metazoan herbivores.

The two principal hypotheses of ecosystem control have been for- mulated in terms of this vertical food chain structure: ‘bottom up’ control’4 suggests that competition between primary producers for limiting nutrients determines the state of higher trophic levels; and ‘top down’ controll argues that the effects of fish predation cascade down the trophic chain and are responsible for controlling the state of the entire ecosystem. These views invoke notions of either competition or predation as the controlling factors in aquatic systems; both of these ecological interactions have been traditionally associated with negative feedback regulatory processes.

Over the last two decades, our understanding of the structure and function of aquatic ecosystems has

been revised considerably. While the activity of bacteria and protozoa was considered inconsequential in the linear food chain paradigm, we now know that these microorganisms can control major fluxes of energy and nutrients1”20. Rather than the bulk of photosynthetic production passing directly to higher trophic levels, in some cases more than 50% is diverted into a ‘microbial 100~‘~’ where nutrients are rapidly remineralized and fed back to the dissolved inorganic pools. The microbial activity effec- tively creates a huge positive feedback loop at the very base of the food web (Fig. 2).

With the realization that small organisms can control large nutrient fluxes in this manner, came the need to rework existing ecological theory22. In particular, the trophic status and functional role of organisms, both of which had been easily identifiable in the linear food chain paradigm, are now difficult to classify in light of the microbial loop. For example, when demarcating a trophic level, it became common practice to lump together large num- bers of organisms from widely different size classes. As size often determines an organism’s functional role, such practice disregards complex interactions occurring between the many different size classes within a single level. The perceived complexity of a food web rapidly increases when size class is taken into account (Fig. 3)23.

Microbes embrace a number of size categories, and can have either a single mode of nutrition (e.g. photoautotrophy, chemoauto- trophy, osmotrophy), or multiple modes (mixotrophs). The sheer complexity of the microbial community means that there is a whole maze of pathways by which microbes may interact, both among themselves and with the rest of the food web. Assigning them to any particular trophic level in a linear food chain model is impossible.

The blurring of trophic structure and the large diversion of primary production through the microbial feedback loop are hard to reconcile with the traditional linear food chain19e24. The concepts of ‘top- down’ or ‘bottom-up’ control also need careful interpretation when there is no clear vertical ecosystem

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TREE vol. 7, no, 8, August 1992

structure (Fig. 3). Further, it appears somewhat misleading to describe these systems as being dominated by negative feedback stabilizing processes, if positive feedback - in the form of nutrient cycling - can be so quantitatively important.

Life at small size scales: the microbial loop

Traditionally the ecological activity and functional importance of organisms has been viewed to be in direct proportion to their size, so that attention has mainly focused on life at the upper end of the size spectrum. Yet processes that in- volve smaller organisms are proving to be of at least equal significance. In one key study, Platt et al.** noted the presence of large num- bers of unicellular autotrophic picoplankton (organisms less than 1 Pm) in the North Atlantic, responsible for some 60% of the primary production in the area. Other studies in oligotrophic seas have estimated the photosynthetic production of these phytoplankton to be 80-90% of the total primary production’9.25.

Bacteria were once believed to be of minor importance in pelagic ecosystems20. However, active free- living bacteria are now recognized as a ubiquitous component of the water column biota in the near- surface zones of many lake and ocean settings. Bacteria commonly consume 20-60% of primary production and their biomass (carbon) has been found to be typically lO-40% of total phytoplankton standing stockl”20. Primary production becomes available to bacteria as dissolved organic matter (DOM) in three major ways: from zooplankton (faeces and sloppy feeding)*+ from algae (release of exudates)‘6,20; and from bacteria (hydrolysis of organic particles)27.

Recent exciting developments indicate that high virus abundance is not uncommon in the oceans. Viral infections have the capacity to cause serious damage to populations of phytoplankton and bacteria, thus further controlling primary production and nutrient flows to and through the microbial 100~~~.

The hitherto-neglected protozoa, with intrinsic growth rates that exceed those of many marine bacteria, are highly active bac-

terivores. Since picophytoplankton may be responsible for a large portion of the primary production in oligotrophic water, much of this production can enter the microbial loop directly via protozoan grazing. Because protozoans enjoy numerous and varied modes of feeding29, rapid rates of growth, and have an exceptional ability to mineralize nutrients efficiently, they are placed in a pivotal pos- ition for controlling the cycling of nutrients’6,i9,23.

The size of microbes facilitates nutrient recycling in many ways. Being small, microorganisms have relatively high surface area to biomass ratios, permitting a more intimate contact with the environment, a greater uptake potential for nutrients and a more rapid turnover of nutrients and organic matter than larger organisms20. Further, they have slow sinking rates and tend to remain in the upper waters of lakes and oceans for long periods before settling out to greater depths. Therefore, the nutrients contained in organic matter that enters the microbial loop have a greater like- lihood of remaining in the photic zone longer than nutrients that are incorporated directly into larger metazoans with faster sinking rates.

A study of the microbial loop must necessarily treat the microscale environment as a factor of some sig-

1 - FISH

‘L- ZOOPLA A

PHYTOPLANKTON

Fig. 1. The major trophic pathways in the traditional vertical food chain structure of aquatic ecosystems.

nificance. GoldmanY discusses how organic microaggregates (e.g. particles of marine ‘snow’) can serve as ‘life-support systems’ for consortia of phytoplankton, bacteria, flagel- lates and ciliates. These aggregates can form sites of intense nutrient cycling, or as Goldman describes them, ‘oases’ in an ‘aquatic desert’. At present the overall importance of microaggregates is still uncertainIs; however, it is plausible that microscale patchiness encourages the development of microbial consortia and enhances both the capture of nutrients and the interactions

Fig. 2. Recycling introduced by the microbial loop at the very base of the food web. Note that the pathway from phytoplankton to bacteria indicates that a large proportion of primary production becomes available for bacterial uptake via indirect pathways (e.g. algal exudates, sloppy grazing).

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CLASSIC FOOD W E B

Mmophytoplonkton

MICROBIAL FOOD W E B I I I I I

02 PICOPLANKTON 2.0 NANOPLANKTON 20 MICROPLANKTON 200 MESOPLANKTON 2000

SIZE,pm

Fig. 3. Compartmental diagram of the important groups of autotrophic and heterotrophic organisms at the base of marine pelagic food webs, Producers of biomass from either dissolved organic or inorganic carbon are above the diagonal line B, consumers of this biomass are below line B. Organisms considered to be part of the ‘microbial loop’ are below diagonal line A and those considered to be in the ‘classic’ phytoplankton-copepod-fish food web are above line A. Adapted from Ref. 23.

between organisms16*30. Delineating the characteristics and internal interactions of these consortia, as well as their relation to the surrounding m icroenvironment, is a challenge that currently faces m icrobial ecologists20.

Positive feedback: a keystone process Simply stated, enhanced nutrient

cycling builds strong positive feedback links in pelagic environments. The m icrobial loop is adapted to this activity since it facilitates the capture and localization of nutrients and controls their distribution. By a process of rapid recycling and efficient remineralization, the m icrobial loop locks the nutrients into the system, making them available for phytoplankton uptake many times over. In turn this bootstrapping promotes the build-up of m icrobial biomass, and reinforces the food web for leaner periods when nutrients are scarce.

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A striking example of this bootstrapping effect is given by Goldman when discussing the euphotic zones of oligotrophic water columns. These zones are characterized by extremely low ambient concentrations of inorganic nutrients, suggesting nutrient lim i- tation of phytoplankton growth. Remarkably, phytoplankton elemen- tal-composition ratios consistently imply growth rates near maximal. The ‘spinning wheel’ hypothesis30 posits that high growth rates can only be achieved if there is a

very tight coupling between herbivore grazing and nutrient recycling. Bacteria and protozoa are considered instrumental in rapid remineralization of much of the organically bound nutrients, thereby keeping the ‘wheel’ spinning and allowing phytoplankton to utilize scarce nutrients repeatedly. This positive feedback of nutrients in euphotic zones, introduced by the m icrobial loop, could be viewed as a keystone process, for in its absence there would be an in- adequate supply of nutrients available to maintain phytoplankton standing stocks.

The feedback of nutrients appears to be a crucial process in several other environments. Coral reef communities, for example, exhibit very high biomass standing stocks and rates of gross primary production, even when located in oligotrophic areas. High production is maintained by rapid and efficient recycling of nutrients3’. If the positive feedback mechanisms by which coral reef communities recycle nutrients were not effec- tive, the coral reef system would run down in both biomass and productivity. Instead, the bootstrapping pathways prevalent on reefs promote the development and persistence of one of the most pro- ductive (gross), diverse and complex ecosystems in nature.

Recent studies have indicated that grazing on algal turfs (which provide most of the primary produc-

tivity in coral reefs) seems to have the counterintuitive effect of increasing algal productivity. The grazing losses suffered by turf algae appear to be compensated by the positive feedback of three resources: nutrients, light for relief from self-shading) and availability of benthic substrate for growth and colonization. Biomass-specific algal productivity was found to be two to ten times greater on turfs exposed to natural levels of grazing by the sea urchin Diadema anrillarum as com- pared to turfs from which the sea urchins had been experimentally exc1uded32,33. Similar stimulation of productivity by herbivores is also well documented for streams and ponds34. These observations indicate that grazing, together with the positive feedback it gener- ates, can be viewed as a keystone mechanism. This feedback has the capacity to stimulate primary productivity and to alter the physical structure and species composition of aquatic communities.

Research in aquaculture ponds has also demonstrated the presence of vital positive feedback links. Some eutrophic aquaculture ponds exhibit relatively low concentrations of inorganic nitrogen and phos- phorus but high rates of organic production. A mass-balance evaluation implied that in the absence of rapid recycling, the ambient stock of inorganic nutrients in ponds could be completely depleted within six m inutes to one hour (R.S.J. Weisburd, PhD thesis, University of Hawaii, 1988). In a series of related experiments, where tracers of labelled nutrients were added to bottles of pond water, the label was completely removed from the dissolved phase within half an hour35. If recycling in these environments were to be interrupted, the inorganic nutrient pool in the pond water could be reduced to extremely low levels within m inutes. In fact, aquaculture ponds commonly undergo repeated bloom- crash cycles, which are at least in some cases controlled by the ‘m icrobial 100~‘~~. It is tempting to speculate that such crashes are triggered by conditions that either unbalance or decouple nutrient uptake and recycling; that is, conditions that interrupt keystone positive feedback effects.


Nutrient cycling in varying environments Ecologists are now highly div-

ided in their opinion regarding the importance of negative feedback control and the stability it is assumed to endow. Many argue that for a large number of ecosystems, stochastic environmental factors can be of decisive influence and render any assumptions of steady state or stable equilibrium meaningless. If ecosystems do not possess stable equilibria, how do organisms survive in a changing environment? By what means are they capable of recovering from hazardous disturbances? In what ways do they persist in periods of extreme resource scarcity?

For aquatic ecosystems, the above questions may not be fully answered until a more complete picture of the role of positive feedback is acquired. Theoretical models are proving to be a useful tool in addressing such questions, by making it possible to explore the complex relationship between nutrient cycling and key ecosystem parameters”12. Some of the more recent models have demonstrated recycling to be particularly ad- vantageous for stochastically dis- turbed ecosystems that receive pulsed nutrient inputs. Ebenhoh” modelled a nonequilibrium multi- species phytoplankton community and found that species coexistence was not possible without the pulsed nutrient inputs produced by recycling processes. Similar studies have found that in model ecosystems incorporating a rapidly recycling ‘microbial loop’, positive feedback with its bootstrapping effects allows the capture and full exploitation of transient nutrient inputs, which might otherwise be dispersed and exported from the system (Ref. 37; L. Stone and T. Berman, unpublished).

Modelling attempts such as these, together with recent theoretical studies and field experiments, all indicate a fresh interest in positive feedback and its ramifi- cations. This interest is not unique to ecology; it has emerged in many diverse disciplines including economics, meteorology, physics and psychology2JsJ9. in ecology, evidence is accumulating that demonstrates positive feedback to be an operative mechanism,

important - even critical - to the persistence of many communities. This is certainly true in aquatic ecosystems, where positive feedback pathways abound.

Acknowledgements We thank Tom Berman, Barry Sherr and

anonymous referees for their help and con- structive criticisms. This work was supported by Griffith University, grant no. 870006 from the US-Israel Binational Science Foundation, the US-Israel Binational Agricultural Research and Development Fund Postdoctoral Fellow- ship US-0076-87 (RW), and Science and Tech- nology Agency of Japan Fellowship 190081 UW.

References I Boucher, D.H., ed. (1985) The Biology of Mutualism, Croom Helm 2 DeAngelis, D.L., Post, W.M. and Travis, C.C. (1986) Positive Feedback in Natural Systems, Springer-Verlag 3 Stone, L. and Roberts, A. ( 1991) Ecology 72, 1964-1974 4 May, R.M. f 19821 Nature 296, 803-806 5 Bianchi, T.S., Jones, C.G. and Shachak, M. ( 1989) Trends Ecol. Evol. 4, 234-238 6 Perry, D.A., Amaranthus, M.P., Borchers, LG., Borchers, S.L. and Brainerd, R.E. (1989) BioScience 39, 230-237 7 DeAngelis, D.L. f 1980) Eco/ogy 6, 764-771 8 Jordan, CF., Kline, J.R. and Sasscer, D.S. f 1972) Am. Nat. 106, 237-253 9 DeAngelis, D.L.. Mulholland, P.J., Palumbo, A.V.. Steinman, A.D., Huston, M.A. and Elwood, I.W. (1989) Annu. Rev. Ecol. syst. 20, 71-95 IO Hutchinson, G.E. (1948) Ann. N.Y. Acad. Sci. 50, 22 l-246 I I Ebenhoh, W. ( 1988) Theor. Popul. Biol. 34, 130-144 12 Stone, L. (1990) Mar. Eco/. Prog. Ser. 64, 137-145 13 Ricklefs, R.E. f 1990) Ecology (3rd edn), W.H. Freeman 14 Tilman, D. (1982) Resource Competition and Community Structure, Princeton University Press 15 Carpenter, S.R. and Kitchell, J.F. (1988)

Bioscience 38, 764-769 16 Berman, T. (1990) in Large Lakes: Ecological Structure and Function (Tilzer, M. and Serruya, C., eds), pp. 51 l-525, Springer-Verlag 17 Ducklow, H.W. f 1983) Bioscience 33, 494-50 I 18 Fenchel, T. ( 1988) Annu. Rev. Ecol. Syst. 19, 19-38 19 Sherr, E.B. and Sherr, B.F. (1991) Trends Ecol. Evol. 6, 50-54 20 Azam, F., Cho, B.C., Smith, DC. and Simon, M. (1990) in Large Lakes: Ecological Structure and Function (Tilzer, M. and Serruya, C., eds), pp. 477-488. Springer-Verlag 21 Azam, F. et al. (1983) Mar. Ecol. Prog. Ser. 10, 257-263 22 Platt, T., Subba Rao, D.V. and Irwin, B. (1983) Nature 301, 702-704 23 Sherr, E.B., Sherr, B.F. and Paffenhoffer GA. I19861 Mar. Microb. Food Webs I, 61-80 24 Bums, T.P. f 19891 Ecology 70, 1355-l 362 25 Stockner, LG. (19881 Limnol. Oceanogr. 33, 765-775 26 fumars, P.A. et al. ( 1989) Deep-Sea Res. Oceanogr. Abstr. 36, 483-495 27 Cho, B.C. and Azam, F. (1988) Nature 332, 441-443 28 Suttle, C.A., Chan, A.M. and Cottrell, M.T. ( 1990) Nature 347, 467-469 29 Finlay, B. and Fenchel, T. ( 1989) New Sci. 123, 66-69 30 Goldman, J.C. (1984) Bull. Mar. Sci 35, 462-476 31 Smith, S.V. (1988) in Coastal-Offshore Ecosystem Interactions (fansson, B.O., ed.), pp. 209-226, Springer-Verlag 32 Carpenter, R.C. f 1986) Ecol. Monogr. 56, 345-363 33 Will iams, S.L. and Carpenter, R.C. (1988) Mar. Ecol. Prog. Ser. 47, 145-l 52 34 Sterner, R.W. ( 1986) Science 23 I, 605-607 35 Fox, D.D. (1986) in Proceedings of the Pacific Conference on Marine Technology (Saxena, N., ed.), pp. M R M 1011-1015 36 Krom. M.D. and Neori. A. II9891 Aquaculture 83, 345-358 37 Moloney, C.L., Bergh, M.O., Field, LG. and Newell. R.C. I I9861 1. Plankton Res. 8, 427-445

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38 Arthur, W.B. (1990) Sci Am. 262, 92-99 39 Cane, M.A., Munnich, M. and Zebiak, S.E. (19901 I. Atmos. Sci. 47, 1562-1577

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