The spatial distribution of phytoplankton

The focus of this chapter, the conditions of entrainment and embedding of phytoplankton in the constant movement of natural water masses, is now extended to the conditions where water movements are either insufficiently strong or insufficiently extensive to randomise the spatial distribution of phytoplankton. This section is concerned with the circumstances of plankters becoming disentrained and the consequences of weakening entrainment for individuals, populations and communities, as augured by spatial differentiation in the vertical and horizontal distributions of natural assemblages.

Distributional variation is subject to issues of scaling which need to be clarified. It has already been made plain that aquatic environments are manifestly heterogenous, owing to spatial differences in temperature, solute content, wind stress, etc., and that each of these drivers is itself subject to almost continuous variation. However, while precise values are impossible to predict, the range of variability may be forecast with some confidence, either on the basis of averaging or experience, or both. We may not be able to predict the intensity of wind mixing in a lake some three weeks or more into the future but we may estimate from the knowledge base the probability with which a given wind intensity will prevail. The changes in temperature, insolation, hydraulic exchanges and the delivery of essential nutrients affecting a given stretch of water also occur on simultaneously differing scales of temporal oscillation - over minutes to hours, night-day alternations, with changing season, interannually and over much broader scales of climatic change. The nesting of the smaller temporal scales within the larger scales holds consequences for phytoplankters in the other direction, too, towards the probabilities of being ingested by filter-feeders, of the adequacy of light at the depths to which entrained cells may be circulated, even to the probability that the energy of the next photon hitting the photo-synthetic apparatus will be captured. The point is that the reactions of individual organelles, cells, populations and assemblages are now generally predictable, but the impacts can only be judged at the relevant temporal scales. These responses and their outcomes are considered in later chapters in the context of the relevant processes (photosynthesis, assimilation, growth and population dynamics). However, the interrelation of scales makes for fascinating study (see, for instance, Reynolds, 1999a, 2002a): in the end, the distinction is determined by the reactivity of the response. This means that critical variations alter more rapidly than the process of interest can respond (for instance, light fluctuations are more frequent than cell division) or so much less rapidly that it is perceived as a constant (such as annual temperature fluctuations having a much lower frequency than cell division) which will be no more relevant to today's populations than is the onset of the next ice age to the maintenance of present-day forests (Reynolds, 1993b). In between, where driver and response scales are more closely matched, the interactions are rather more profound, as in the frequency with which new generations are recruited to a water column mixed to a different extent on successive days.

The variability in the instantaneous distribution of phytoplankton may be considered in relation to an analogous spatial scale. Consider first a randomised suspension of unicellular flagellates, such as Chlamydomonas or Dunaliella. Viewed at the 1-10 |im scale, distribution appears highly patchy, resolvable on the basis of presence or absence. In the range 10-1000 | m, the same distribution is increasingly perceived to be near-uniform but, in the turbulence field of a wind-mixed layer, variability over the 1-10 mm scale may attest to the interaction of algal movements with water at the viscous scale (Reynolds et al., 1993a). In the range 10-100 mm and, perhaps, 10-1000 mm, the distribution may again appear uniform. Beyond that, the increasing tendency for there to be variations in the intensity of mixing leads to the separation of water masses in the vertical (at the scale of tens to hundreds of metres) and in the horizontal (hundreds of metres to hundreds of kilometres), at least to the extent that they represent quite isolated and coexisting environments, each having quite distinct conditions for the survival of the flagellates and the rate of their recruitment by growth. This is but one example of the principle that the relative uniformity or heterogeneity within an ecological system depends mainly upon the spatial and temporal scale at which it is observed (Juhasz-Nagy, 1992).

Uniformity and randomisation, on the one hand, and differentiation ('patchiness'), on the other, may thus be detected simultaneously within a single, often quite small system.

Moreover, the biological differentiation of individual patches may well increase the longer their mutual isolation persists. Thus it remains important to make clear the spatial scale that is under consideration, whether in the context of vertical or horizontal distribution.

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