Turbulent embedding of phytoplankton

These considerations are directly comparable with the dimensions of phytoplankton (Box 1.2, Tables 1.2, 1.7). Microplanktic algae are smaller, by one or more orders of magnitude, than the smallest eddy sizes in what are arguably among the most aggressively mixed, fastest dissipating turbulence fields that they might inhabit. There are some observations and the evidence of some experiments (Bykovskiy, 1978) that together suggest larger species of phytoplankton do not tolerate eddy diminution and intensified shear implicit in enhanced, fine-grained turbulence fields but are, instead, readily fragmented. It is an unverified hypothesis to argue that phyto-plankton have evolved along lines that exploited the viscous range of the aquatic eddy spectrum, rather than to have invested in the mechanical tissue necessary to resist the collapse and fragmentation of larger structures (Reynolds, 1997a).

If the dominant vegetation of pelagic environments is truly selected by its ability to escape the smallest scales of turbulence, then the corollary is that the individual organisms are, in effect, embedded deep within the turbulence structures to which the water has frequently to accommodate. This is worth emphasising: planktic algae live most of their lives in an immediate environment that is wholly viscous but, at a slightly larger scale, one that is simultaneously liable to be transported far and rapidly through the turbulence field, and with varying intensity and frequency. The pelagic world of phytoplankton might be analogised to one of little viscous packets being moved rapidly in any of three dimensions. In reality, the packets have no enduring integrity but it is the behaviour of phytoplankton relative to the immediate water and to the transport of the water within the mixed layer that determines the suspension and settling characteristics of the whole population.

The consequences of living in a viscous medium have been graphically recounted in a much-celebrated paper by Purcell (1977). For instance, it is not possible for a planktic alga or bacterium to 'swim' through the medium as does (say) a water beetle (3-20 mm), by means of a reciprocating, rowing movement of paddle-like limbs, any more than can a man floundering in a vat of treacle. The alternative options for forward progression that are exploited by microplank-ters and smaller organisms include the serial deformation of the protoplast (amoeboid movement), the spiral rotation of the body (as do many ciliates and euglenoids) and the rotating of a flagellum like a corkscrew (as in the bacterium Escherichia). The speed of self-propulsion relative to the medium (us) of (say) a Chlamy-domonas cell, 10 /j.m in diameter (d), is, at about 10 /j.m s-1, trivial in absolute terms though nevertheless impressive in body-lengths covered per second. The Reynolds number of its motion per second, solved by analogy to Eq. (2.3), confirms that the alga moves smoothly through the water, its motion creating no turbulence:

to the power generation of its saturated rate of photosynthesis (~1.4 kW kg-1). If it stops operating its flagella, however, the alga comes to a complete rest in ~1 /s, having travelled no more than another 10 nm (10-8 m) in relation to the adjacent medium (Purcell, 1977).

Embedding is directly relevant to the issues of entrainment and distribution of phytoplankton, insofar as the behaviour of the plankter relative to a body of water in motion is strongly influenced by the behaviour of the plankter within its immediate viscous environment. To progress this exploration requires us to account for buoyancy and gravitation behaviour in relation to suspension.

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