Summary

The chapter explores the nature of the relationship that phytoplankters have with the physical properties of their environment. Water is a dense, non-compressible, relatively viscous fluid, having aberrent, non-linear tendencies to expand and contract. The water masses of lakes and seas are subject to convection generated by solar heating and, more especially, by cooling heat losses from the surface. These motions are enhanced in the surface layers interfacing with the atmosphere, where frictional stresses impart mechanical energy to the water through boundary-layer wave generation and frictional drag. Diel cycles of insolation, geographical variations in heating and cooling, atmospheric pressure and the amplifying inertia caused by the rotation of the Earth represent continuous but variable drivers of motion in aquatic environments. These can rarely be regarded as still: water is continuously in motion. However, the viscous resistance of the water determines that the introduced motion is damped and dissipated through a spectrum of turbulent eddies of diminishing size, until molecular forces overwhelm the residual kinetic energy. Instrumentation confirms emerging turbulence theory about the extent of water layers subject to turbulent mixing and the sizes of the smallest eddies (generally around 1 mm), which, together, most characterise the medium in which all pelagic organisms, and phytoplankton in particular, have to function.

The most striking general conclusion is that most phytoplankters experience an immediate environment that is characteristically viscous. Yet the physical scale is such that individuals of most categories of plankter (those less than 0.2 mm in size) and their adjacent media are liable to be transported wherever the characteristic motion determines. From the standpoint of the plank-ter, the important criteria of the turbulent layer are its vertical extent (hm) and the rate at which it dissipates its turbulent kinetic energy (E). Both are related to the intensity of the turbulence (u*2) and, thus, to the turbulent velocity (u*).

The traditional supposition that the survival strategy of phytoplankton centres on an ability to minimise sinking is carefully updated in the context of pelagic motion. Extended residence in the upper water layers remains the central requirement at most times. This is attained, in many instances, by maximising the entrainabil-ity of the plankter within the motion. Viewed at a slightly larger scale, many phytoplankters optimise their 'embedding' within the surface mixed layer. Criteria for plankter entrainment are considered - for it can only be complete if the plankter has precisely the same density as (or is isopycnic with) the suspending aqueous medium. Even were this always desirable, it would be difficult to attain. Not only does the water vary in density with temperature and solute-content but the components of phytoplankton cells are rather more dense than water (typically amounting to 1020-1263 kgm-3, compared to <1000; Table 2.3). Following Humphries and Imberger (1982), relative entrainment is instead suggested to be governed by the relationship between particle buoyancy and turbulent diffusivity. Effectively, in order to achieve turbulent entrainment, an alga's sinking rate, ws (or its flotation rate, -ws) must be exceeded by u* by a factor of >15. Thus, the best descriptor of algal entrainability turns out to be its sinking rate and, the greater is the adaptive ability to minimise it, the better able is the alga to contribute to its persistence in an adequately mixed water column.

The adaptive mechanisms for lowering the sinking velocity are reviewed in the context of the Stokes equation and its various derivatives. Of the equation components, only particle size, particle density and particle form resistance are considered subject to evolutionary or behavioural adaptation. Examples of each adaptation are quantified. Adaptations to control or offset density and the beneficial effects of distortion from the spherical form are demonstrated. The consequences of chain formation and cylindrical elongation (into filaments) on sinking rate are explored and the effects of cell aggregation to form the distinctive coenobia of Asterionella and Fragilaria are evaluated. In relation to the presumed vital regulatory component in sinking rate, some possible mechanisms are discussed. Some of the explanations offered are eliminated but there remain others that await careful investigation.

Some larger, motile organisms are successful plankters by virtue of adaptations that are antithetical to increasing entrainability. Large, motile species of Microcystis, Volvox, Ceratium and Peridinium combine relatively large size, motil-ity and shape-streamlining to be able to escape moderate-to-low turbulent intensities in order to perform controlled migrations, at rates of several metres per day. Reducing sinking rate is far from being a unique or universal adaptation qualifying microorganisms for a planktic existence.

In the later sections of the chapter, various types of behaviour are illustrated through specific examples of the vertical distributions of planktic algae in relation to the increased differentiation of density structure in the water column. The impacts are extrapolated to horizontal distribution and to the instances of small-scale patchiness and advective patchiness in small lakes, resolving in terms of algal migratory speeds in relation to the velocity of advective currents. The viability and persistence of phytoplank-ton patches in expansive, large-scale systems, where return currents are extremely remote, relate to the comparative rates of recruitment within the patch and of the erosion at the patch periphery. Some case studies are presented to show some very contrasted large-scale outcomes, distinguishing enduring community similarities over 800 km in the horizontal from sharply localised patches in systems able to support ongoing or persistent rapid recuitment of organisms from point sources.

Many different distributional outcomes can be explained by the behaviour and dispersiveness of particular species in given systems, although the subjugated deployment of the same processes elsewhere may contribute to the formation of quite different patterns.

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