Lim r

Pia ag s

1G2 msv-1

Figure 5.10

Morphological ordination of some species of freshwater phytoplanton, against axes invoking maximal linear dimension (m), surface area (s) and volume (v) of the vegetative units, with the C-, S- and R-strategic tendencies. The algae are: Ana, Anabaena flos-aquae; Aphan, Aphanizomenon flos-aquae; Ast, Asterionella formosa; Aul, Aulacoseira subarctica; Cer, Ceratium hirundinella; Chla, Chlamydomonas; Chlo, Chlorella sp.; Cry, Cryptomonas ovata; Din, Dinobryon divergens; Eud, Eudorina unicocca; Fra, Fragilaria crotonensis; Lim r, Limnothrix redekei; Mic, Microcystis aeruginosa; Monod, Monodus sp.; Monor, Monoraphidium contortum; Per, Peridinium cinctum; Pla ag, Planktothrix agardhii; Plg, Plagioselmis nannoplanctica; Scq, Scenedesmus quadricauda; Sth, Stephanodiscus hantzschii; Syn, Synechococcus sp.; Tab, Tabellaria flocculosa var. asterionelloides; Vol, Volvox aureus. Redrawn with permission from Reynolds (1997a).

to which he gave the label 'S-strategists'; or (c) tolerance of disturbance, through making good opportunity of transient habitats and interrupted opportunities to process resources into biomass ('^-strategists').

The three primary strategies of Grime's CSR model form the apices of a triangular ordination (Fig. 5.9), which representation readily allows the accommodation of numerous intermediates and trait-combinations. Reynolds (1988a, 1995a) found only minor difficulties in analogising the r-, K- and w-selected groups to exemplifying, respectively, C, Sor Rstrategies, on the satisfying basis of agreement among the morphological properties, growth rates and life-history traits. The distribution of phytoplankton species according to their individual morphologies plotted against axes of sv-1 and msv-1 (Fig. 5.10). Just as with Grime's (1979) scheme, species are not exclusively C or S or R in their strategic adaptations. Many species of phytoplankton show intermediate characters. Interestingly, intermediacy in morphological and physiological characters matches well the intermediacy of their ecologies. The C-S gap is spanned by genera such as Dinobryon, Dictyosphaerium, Coenochloris, Pseudosphaerocystis, Eudorina and, arguably, Volvox (Reynolds, 1983b), and by Aphanocapsa and Aphanothece. The series spans diminishing sv-1 ratios, maximum growth rates and low-temperature tolerance but increasing ability to exploit and conserve nutrient resources. Algae in the C-R axis include predominantly centric diatoms of varying tolerance of turbidity and the Scenedesmus-Pediastrum element of enriched shallow ponds and rivers). The R-S possibility is represented by the slow-growing, long-surviving, acquisitive but highly acclimated species of density gradients, like Planktothrix rubescens and Lyngbya limnetica. Certain (not all) members of the genus Cryptomonas show a blend of the characteristics of all three primary strategies in being unicellular, having cells of moderate size (1-4 x103 |m3) and of intermediate sv-1 (0.3-0.5 |m-1), and being capable of intermediate replication rates (r20~10 x 10-6 s-1; r0~0.9 x 10-6 s-1).

It is right to point out that Grime's CSR concept of plant stategies is not universally accepted and it has been subject of vehement and challenging debate (see Tilman, 1977, 1987, 1988; Loehle, 1988, a.o.). Although there is much common ground shared by the adversaries and, in truth, the differences are more of perspective and emphasis (Grace, 1991), the differences have never been entirely resolved. The application to plankton has not been so criticised and some (Huszar and Caraco, 1998; Fabbro and Duiven-vorden, 2000; Gosselain and Descy, 2000; Kruk et al., 2002; Padisak, 2003) but by no means all (Morabito et al., 2002), have found the arguments convincing and helpful to interpretation. The applicability of a scheme devised for plant species is not a barrier: it is now quite evident that the idea has a long pedigree among other ecological schools (Ramenskii, 1938) and has been applied successfully to the 'violent', 'patient' and 'explerent' strategies of zooplankton (Romanovsky, 1985). The CSR model has been applied to fungi (Pugh, 1980) and periphyton (Biggs et al., 1998).

An updated application to phytoplankton is set out in Box 5.1. A notable modifica tion recognises that motility and large size are not necessary adaptations to function in chronically very resource-depleted pelagic environments. Indeed, resource gathering in spatially continuous, rarefied environments is favoured by small size, whereas the low levels of diffuse biomass is an unattractive resource for direct grazing by mesozooplankters (see Chapter 6). The adaptive strategies for surviving the 'resource desert' of the ultraoligotrophy of the oceanic pelagic are accorded the additional stress-tolerant category SS.

The original ascriptions of C, S and R categories to phytoplankton (Reynolds, 1988a) separate quite satisfactorily on the plot of the areas projected by various species of phytoplankton and the product of maximum dimension and surface-to-volume ratio (msv-1) Fig. 3.12). Near-spherical forms align close to msv-1 [d x 4n (d/2)2 ^ 4n(d/2)3/3] = 6 but separate broadly in to C and S species according to size, because the carbon and chlorophyll contents vary with v = 4n (d/2)3/3 but the light interception increases as a function of the disk area, a = n(d/2)2. The morphological attenuation of the R species pulls out the plot to much higher msv-1 values. Thus, we distinguish species that are capable of rapid growth in benign, resource-replete environments, those that are able to go on squeezing out increased biomass from diminishing light income and those who are physiologically or behaviourally adapted to function in spite of developing nutrient stress. The model appears in various guises later in the book, demonstrating the power and flexibility of the strategy-process-ecosystem interactions. It even provides the bridge to the light : nutrient hypothesis (Sterner et al., 1997) in so far as the species best adapted to cope with low doses of I* are most able to cope with high particulate content in the water and the C : P ratio of the seston available to secondary consumers.

5.4.6 Resource exhaustion and survival

It is reasonable to assume that the growth of phytoplankters distinguished by efficient, high-affinity resource-gathering capabilities may continue until they deplete their growth-limiting resource to near exhaustion. It was often and

Box 5.1

Summary of behavioural, morphometric and

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