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1 The relationship between vertical patchiness of Microcystis in the shallow Eglwys Nynydd reservoir and wind velocity (U). Data of George and Edwards (1976) and redrawn from Reynolds (1984a).

be less helpful than contoured depth-time plots (such as used in Fig. 2.21) over long periods of time. Neither do they necessarily convey the gen-eralism between vertical discontinuity and the physical heterogeneity. In consideration of a 2-year series of Microcystis depth profiles in a small, shallow reservoir, (Eglwys Nynydd: area 1.01 km2, mean depth 3.5 m), George and Edwards (1976) calculated a crowding statistic (x*, owing to Lloyd, 1967) and its ratio to the mean concentration over the full depth (x) to demonstrate the susceptibility of vertical distribution to the wind-forced energy. Putting x* = [x + s2/x - 1], where s2 is the variance between the individual samples in each vertical series, they showed that the relative crowding in the vertical, (x*/x), occurred only at low wind speeds (U < 4 ms-1) and in approximate proportion to U), but wind speeds over 4 m s-1 were always sufficient to randomise Microcystis through the full 3.5-m depth of the reservoir (Fig. 2.23).

Neutrally buoyant plankters (pc ~ pw) In this instance, 'neutral' implies 'approximately neutral'. As already discussed above (Section 2.5), it is not possible, nor particularly desirable, for plankters to be continuously isopycnic with the medium. Nevertheless, many species of phyto-plankton that are non-motile and are unencumbered by skeletal ballast (or the gas-filled spaces to offset it) survive through maintaining a state in which they do not travel far after disentrain-ment. This state is achieved among very small unicells of the nanoplankton and picoplankton

(where, regardless of density, small size determines a low Stokesian velocity) and among rather larger, mucilage-invested phytoplankters, which are genuinely able to 'dilute' the excess mass of cell protoplasm and structures in the relatively large volume of water that the mucilaginous sheath immobilises. In either case, the adaptations to pelagic survival take the organisms closest to the ideal condition for suspension. These are, literally, the most readily entrainable plank-ters according to the definition (Section 2.6.1, Eq. 2.19).

In Fig. 2.24, sequences in the vertical distributions of two non-motile green algae -the nanoplanktic chlorococcal Ankyra and the microplanktic, coenobial palmelloid Coenochloris (formerly ascribed to Sphaerocystis) - are depicted in relation to stratification in the large limnetic enclosures in Blelham Tarn. The original data refer to average concentrations in metre-thick layers (sampled by means of a 1-m long Friedinger trap: Irish, 1980) or in multiples thereof. The data are plotted as stacks of individual cylinders, the diameters of which correspond to the respective cube roots of the concentrations. In both cases, the algae are dispersed approximately uniformly through the epilimnion, while numbers in the hypolimnion remain low, owing to weak recruitment either by growth or by sedimentation (ws < 0.1 m d-1). The effect of grazers has not been excluded but simultaneous studies on loss rates suggest that the impact may have been small, while neither species was well-represented in simultaneous sediment-trap collections or samples from the surface deposits (Reynolds et al., 1982a). On the other hand, deepening of the mixed layer and depression of the thermocline result in the immediate randomisation of approximately neutrally buoyant algae throughout the newly expanded layer. Such species are thus regarded as being always likely to become freely distributed within water layers subject to turbulent mixing, then to settle from them only very slowly and, of course, to be unable to recover a former distribution when mixing weakens (Happey-Wood, 1988).

Flagellates and ciliates are capable of directed movements that, actually as well as potentially,

Figure 2.24

Instances in the vertical distribution of non-motile phytoplankton in a Lund Enclosure during the summer of 1978, shown as cylindrical curves. Ankyra is a unicellular nanoplankter; Coenochloris occurs as palmelloid colonies. Redrawn from Reynolds (1984a).

Figure 2.24

Instances in the vertical distribution of non-motile phytoplankton in a Lund Enclosure during the summer of 1978, shown as cylindrical curves. Ankyra is a unicellular nanoplankter; Coenochloris occurs as palmelloid colonies. Redrawn from Reynolds (1984a).

may result in discontinuous vertical distributions in lakes and seas. This ability is compounded by the capacity for self-propulsion of the alga (the word 'swim' is studiously avoided - see Section 2.3.4). In terms of body-lengths per unit time, the rates of progression may impress the micro-scopist but, in reality, rarely exceed the order of 0.1-1 mms-1.

In general, the rates of progress that are possible in natural water columns are related to size and to the attendant ability to disentrain from the scale of water movements (Sommer, 1988b). Moreover, the detectable impacts on vertical distribution also depend upon some directionality in the movements or some common set of responses being simultaneously expressed: if all movements are random, fast rates of movement scarcely lead to any predictable pattern of distribution. For instance, the impressive vertical migrations of populations of large dinoflag-ellates are powerfully and self-evidently responsive to the movements of individual cells within environmental gradients of light and nutrient availability. This applies even more impressively to the colonial volvocalean migrations (Section 2.6.1) where all the flagellar beating of all the cells in the colony have to be under simultaneous control. The point needs emphasis as the flagellar movements of, for instance, the colonial chrys-ophytes (including the large and superficially Volvox-like Uroglena), seem less well coordinated: they neither 'swim' so fast, nor do their movements produce such readily interpretable distributions as Volvox (Sandgren, 1988b).

On the other hand, there is a large number of published field studies attesting to the vertical heterogeneity of flagellate distribution in ponds, lakes and coastal embayments. Reference to no more than a few investigative studies is needed (Nauwerck, 1963; Moss, 1967; Reynolds, 1976a; Cloern, 1977; Moll and Stoer-mer, 1982; Donato-Rondon, 2001). Works detailing behaviour of particular phylogenetic groups include Ichimura et al. (1968) and Klaveness (1988) on cryptomonads; Pick et al. (1984) and Sandgren (1988b) on chrysophytes; Croome and Tyler (1984) and Hader (1986) on euglenoids. Note also that not all flagellate movements are directed towards the surface. There are many instances of conspicuous surface avoidance (Heaney and Furnass, 1980; Heaney and Talling, 1980a; Galvez et al, 1988; Kamykowski et al., 1992) and of assembling deep-water 'depth maxima' of flagellates, analogous to those of Planktothrix and photosynthetic bacteria (Vicente and Miracle, 1988; Gasol et al., 1992).

The example illustrated in Fig. 2.25 shows the contrasted distribution of Ceratium hirundinella. This freshwater dinoflagellate is known for its strong motility (up to 0.3 mm s-1) (see Section 2.6.1) and its well-studied capacity for vertical migration under suitable hydrographic conditions (Talling, 1971; Reynolds, 1976b; Harris et al, 1979; Heaney and Talling, 1980a, b; Frempong, 1984; Pollingher, 1988; James et al, 1992). These properties enable it quickly to take up advantageous distribution with respect to light gradients, when diffusivity permits (u* < 10-4 ms-1). The right-hand profile in Fig. 2.25 shows the vertical distribution of Ceratium during windy weather in a small, eutrophic temperate lake; the left-hand profile shows a distribution under

Figure 2.25

Contrasted vertical distributions of the motile dinoflagellate Ceratium hirundinella, in a small stratifying lake (Crose Mere, UK), in relation to temperature gradients (6) and percentage light penetration (pecked lines). (a) was observed under very calm conditions; (b) under strong winds. Redrawn from Reynolds (1984a).

Figure 2.25

Contrasted vertical distributions of the motile dinoflagellate Ceratium hirundinella, in a small stratifying lake (Crose Mere, UK), in relation to temperature gradients (6) and percentage light penetration (pecked lines). (a) was observed under very calm conditions; (b) under strong winds. Redrawn from Reynolds (1984a).

only weakly stratified conditions but one that is strongly allied to the light gradient.

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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