Box 21 I Langmuir circulations

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Langmuir circulations are elongated, wind-induced convection cells that form at the surface of lakes and of the sea, having characters first formalised by Langmuir (1938). They take the form of parallel rotations, that spiral approximately in the direction of the wind, in the general manner sketched in Fig. 2.26. Their structure is more clearly understood than is their mechanics but it is plain that the cells arise through the interaction of the horizontal drag currents and the gravitational resistance of deeper waterto entrainment. Thus, they provide the additional means of spatially confined energy dissipation at the upper end of the eddy spectrum (Leibovich, 1983). In this way, they represent a fairly aggressive mixing process at the mesoscale but the ordered structure of the convection cells does lead naturally to a surprising level of microstructural differentiation. Adjacent spirals have interfaces where both are either upwelling simultaneously or downwelling simultaneously. In the former case, there is a divergence at the surface; in the latter there is a convergence. This gives rise to the striking formation of surface windrows or streaks that comprise bubbles and such buoyant particles as seaweed fragments, leaves and plant remains, insect exuviae and animal products as they are disentrained at the convergences of downwelling water

The dynamics and dimensions of Langmuir circulation cells are now fairly well known. The circumstances of their formation never arise at all at low wind speeds (U < 3-4 m s-1 : Scott et al., 1969; Assaf et al., 1971). Spacing of streaks may be as little as 3-6 m apart at these lower wind speeds, when there is an rough correlation between the downwelling depth and the width of the cell (ratio 2.0-2.8). In the open water of large lakes and the sea, where there is little impediment to Langmuir circulation, the distance between the larger streaks (50-100 m) maintains this approximate dimensional proportionality being comparable with that of the mixed depth (Harris and Lott, 1973; Boyce, 1974). The velocity of downwelling (w > 2.5 x I0-2 ms-1) is said to be proportional to the wind speed (~0.8 x I0-2 U): Scott et al., 1969; Faller 1971), but the average velocities of the upwellings and cross-currents are typically less.

Consequences for microalgae have been considered (notably by Smayda, 1970, and George and Edwards, 1973) and are reviewed in the main text.

generation time (Reynolds, 1986b). Moreover, a spatial difference within a closed area of water only 45.7 m across is unlikely to persist, as the forcing of the gradient is hardly likely to be stable. A change in wind intensity and direction is likely to redistribute the same population within the same limited space.

We may follow this progression of thinking to the wider confines of an entire small lake, or to the relatively unconfined areas of the open sea. Before that, however, it is opportune to draw attention to a relatively better-known horizontal sorting of phytoplankton at the scale of a few metres and, curiously perhaps, is dependent upon significant wind forcing on the lake surface.

The mechanism concerns the Langmuir circulations, which are consequent upon a strong wind acting on a shallow surface layer, when accelerated dissipation from a spatially constrained volume generates ordered structures. These are manifest as stripe-like 'windrows' of foam bubbles on the water surface. Even now, the formation of Langmuir cells is imperfectly understood but their main properties are fairly well described (see Box 2.1).

Although the characteristic current velocities prevalent within Langmuir circulations (>10-20 mm s-1) would be well sufficient to entrain phytoplankton around the spiral trajectories, the cells do have identifiable relative dead

Figure 2.26

Diagrammatic section across wind-induced surface flow to show Langmuir circulations. Redrawn from Reynolds (1984a).

Figure 2.26

Diagrammatic section across wind-induced surface flow to show Langmuir circulations. Redrawn from Reynolds (1984a).

Figure 2.27

Schematic section through Langmuir rotations to show the likely distributions of non-buoyant (•), positively buoyant (A) and neutrally buoyant, fully entrained (*) organisms. Based on an original in George (1981) and redrawn from Reynolds (1984a).

spots, towards the centre of the spiral, at the base of the upwelling and, especially, at the top of the convergent downwellings, marked by the foamlines (see Fig. 2.26). Smayda (1970) predicted the distributions of planktic algae, categorised by their intrinsic settling velocities, within a cross-section adjacent to Langmuir spirals. Independent observations by George and Edwards (1973) and Harris and Lott (1973) on the distributions of real (Daphnia) and artificial (paper) markers in the field lent support for Smayda's predictions. Although mostly well-entrained, sinking particles (pc > Pw) take longer to clear the upwellings and accumulate selectively there, buoyant particles (pc < Pw) will similarly take longer to clear the downwellings and those entering the foamline will tend to be retained. A schematic, based on figures in Smayda (1970) and George (1981), is included as Fig. 2.27.

Such distributions of algae are not easy to verify by traditional sampling-counting methods, because the behaviour depends not only on the match of the necessary physical conditions - the circulating velocity, the width and penetration of the rotations are all wind-influenced - but their persistence (Evans and Taylor, 1980). Whereas it may take some minutes to organise and generate the circulation, a wind of fluctuating speed and direction will be constantly initiating new patterns and superimposing them on previous ones. This behaviour does not suppress the fact that larger, more motile plankters remain liable to crude sorting, on the basis of their individual buoyant properties, into a horizontal patchiness at the relatively small scales of a few metres to a few tens of metres.

Patchiness in small lake basins

With or without superimposed Langmuir spirals, the horizontal drift is likely, at least in lakes, to be interrupted by shallows, margins or islands, where the flow is subject to new constraints. Supposing that little of the drifting water escapes the basin, most is returned upwind in subsurface countercurrents (see Imberger and Spigel, 1987). In small basins, there is a clear horizontal circulation, which George and Edwards (1976) analogised to a conveyor belt. While this process seems destined towards the basin-scale horizontal integration of populations, the movements of

Figure 2.28

Whole-lake 'conveyor belt' model of non-buoyant (•) and positively buoyant (o) phytoplankters, proposed by George and Edwards (1976). Redrawn from Reynolds (1984a).

Figure 2.28

Whole-lake 'conveyor belt' model of non-buoyant (•) and positively buoyant (o) phytoplankters, proposed by George and Edwards (1976). Redrawn from Reynolds (1984a).

plankters in the vertical plane may well superimpose a distinct advective patchiness in the horizontal plane. The mechanism is analogous to the behavioural segregation in the Langmuir circulation, though on a larger scale. Put simply, the upward movement of buoyant organisms is enhanced in upwind upwellings but resisted in downwind downwellings; conversely, sinking organisms accelerate in downwellings but accumulate in the upcurrents. Positively buoyant organisms accumulate on downwind (lee) shores; negatively buoyant organisms are relatively more abundant to windward (Fig. 2.28).

Such distributions of zooplankton have been observed, with concentrations of downward-swimming crustaceans collecting upwind (Cole-brook, 1960; George and Edwards, 1976). Similar patterns have been described for downward-migrating dinoflagellates (Heaney, 1976; George and Heaney, 1978); on the other hand, the downwind accumulation of buoyant Microcystis has been verified graphically by George and Edwards (1976). Representative maps of these contrasting outcomes are shown in Fig. 2.29.

The representation in Fig. 2.29a is one of a number of such 'snapshots' of variable patchiness during a long period of Microcystis dominance in the Eglwys Nynydd reservoir. The field data allowed George and Edwards (1976) to calculate a crowding index of horizontal patchi-ness (x*), analogous to that solved for the vertical dimension (See Section 2.7.1), and to show that its relationship to the mean population (x) was a close correlative of the accumulated wind effect. Their data are redrawn here (Fig. 2.30) but with the horizontal axis rescaled as an equivalent steady wind speed. Patchiness is strongest

Figure 2.29

Advective horizontal patchiness of phytoplankters in relation to wind direction: (a) positively buoyant Microcystis in Eglwys Nynydd reservoir (after George and Edwards, 1976), isopleths in ^g chlorophyll a L-1; (b) surface-avoiding, motile Ceratium in Esthwaite Water (after Heaney, 1976), isopleths in cells mL-1. Redrawn from Reynolds (1984a).

Figure 2.29

Advective horizontal patchiness of phytoplankters in relation to wind direction: (a) positively buoyant Microcystis in Eglwys Nynydd reservoir (after George and Edwards, 1976), isopleths in ^g chlorophyll a L-1; (b) surface-avoiding, motile Ceratium in Esthwaite Water (after Heaney, 1976), isopleths in cells mL-1. Redrawn from Reynolds (1984a).

Figure 2.30

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

Figure 2.30

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

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