Cell replication and population growth are considered as a unified process and accorded exponential logarithmic units with the dimensions of time. The observed rates of population change in nature (±rn) are net of a series of in-situ rates of loss (rL, treated in Chapter 6). However, the true rates of replication (r ) must always be within

(and are sometimes well within) the least rate that is physiologically sustainable on the basis of the resources supplied. Cell replication is regulated internally and cannot occur without the prior mitotic division of the nucleus. Nuclear division is prevented if the cell does not have the resources to complete the division. If it does, replication can proceed at a species-specific maximum rate.

In general, division rates are strongly regulated by temperature. Species-specific division rates at 20 °C (r20) are correlated with the surface-to-volume (sv-1) ratios of the algal units, as are the slopes of the temperature sensitivity of growth. The light sensitivity of growth is subject to physiological photoadaptation but, ultimately, the shape of the alga influences its effectiveness as a light interceptor. Species which offer a high surface-to-volume ratio through distortion from the spherical form (such that the maximum linear dimension, m, is rather greater in one plane than in one or two of the others) and show high values of the product msv-1 are indicative of probable tolerance of low average insolation. Given that the potential daily photon harvest becomes severely constrained by vertical mixing through variously turbid water to depths beyond that reached by growth-saturating levels of downwelling irradiance, enhanced light interception becomes vital to maintaining growth. Below a threshold of about 1-1.5 mol photons m-2 d-1, growth-rate performances are maintained relatively better in plankters offering the combination of relevant morphological preadap-tation and physiologically mediated photoadaptative pigmentation.

Growth rates of phytoplankton species are variously sensitive to nutrient availability, though not at concentrations exceeding 10-6 M DIN or 10-7 M MRP. Above these 'critical' levels, growth rates are neither nitrogen nor phosphorus limited. Neither, it is argued, are they dependent upon the ratio of either of these resources to the other. However, differing species-specific nutrient-uptake affinities influence the potential growth performances when nutrient availabilities fall to, or are chronically below, the 'critical' levels. Species with high affinity for phosphorus are able to maintain a faster rate of growth than species having weaker affinities and may outperform them by building up larger inocula than potential competitors. Chronic or eventual deficiencies in nitrogen may have an analogous selective effect, although, subject to the fulfilment of other criteria (available phosphorus, high insolation and a supply of relevant trace metals; see Chapter 4), dinitrogen fixers may experience selective benefit.

In the frequent cases in which nutrient depletion is experienced first in the near-surface waters of the upper mixed layer and proceeds downwards, high affinity may be less helpful than high mobility. The beneficial ability to undertake controlled downward migration with respect to the relevant 'nutricline' or to perform diel or periodic forays through an increasingly structured and resource-segregated medium, is conferred through the combination of self-regulated motility and large organismic size. The greater is the isolation of the illuminated, nutrient-depleted, upper column from the dark but relatively resource-rich lower column, the greater is the value of such performance-maintaining adaptations.

In the face of silicon shortages, diatoms are unable to perform at all, whereas the performances of non-diatoms is normally considered insensitive to fluctuations in supply. The amounts of silicon required vary interspecifi-cally, as do the affinities for silicic-acid uptake. Because diatom nuclei divide before the silicic acid needed to form the daughter frustules is taken up, it is possible for otherwise resource-replete and growing populations to experience big mortalities as a consequence of sudden encounter with Si limitation. With other nutrients, planktic algae are able to 'close down' vegetative growth and to adopt a physiological (or perhaps morphological) resting condition, which improves the survival prospects for the genome.

The environments of phytoplankton may be classified, following Grime (1979), upon their ability to sustain autotrophic growth, in terms of the production that the resources will sustain and the duration of the opportunity. Most algae will grow under favourable, resource-replete conditions during long photoperiods. The species that perform best rely on an early presence, rapidity in the conversion of resources to biomass and a high frequency of cell division and recruitment of subsequent generations. By analogy with Grime's (1979) functional classification of plants, these algae are considered to be C strategists; they are typically small (<103 ^m3), usually unicellular, have high sv-1 ratios (>0.3 ^m-1) and sustain rapid growth rates (r'20 > 0.9 d-1). Algae whose growth performance is relatively tolerant of and adaptable to progressively shorter photoperiods and aggregate light doses are comparable to Grime's rud-eral (R) strategists: their sizes are varied (103-105 ^m3) but all offer favourable msv-1 ratios (range 15-1000). Algae whose growth performances are maintained in the face of diminishing nutrient availability are equipped to combat resource stress. Their conservative, self-regulating S strategies are served by the properties of large size (104-107 ^m3) and motility but at the price of low sv-1 (<0.3 ^m-1), low msv-1 (<30) and slow rates of growth (r'20 <0.7 d-1).

Neither large size nor motility offer any advantage to survival in chronically resource-stressed environments of the ultraoligotrophic oceans and the largest lakes. Moreover, the extreme resource rarification also offers a respite from direct consumption. Not only are there resource-gathering constraints against large size but the smallest picoplanktic sizes of photoau-totrophs have these provinces of the aquatic environment very nearly to themselves. It is proposed here that they be henceforth referred to as 'SS strategists'.

Many algae have adaptations and biologies that represent intermediate blends of C-, S- or R-strategist adaptations and lifestyles. The spatial and temporal distributions of particular types and species of phytoplankton, and the opportunities of replication that lead to population development, are shown to be closely correlated with the extent of their C, S or R attributes. Though best demonstrated among the freshwater phyto-plankton, the functional-strategic approach of Grime appears to hold just as well for the ecologies of the marine plankton. In both the sea and fresh waters, morphological and (presumably) physiological criteria are better predictors of ecology than are phylogenetic affinities. In a concluding section, this finding is reversed to show how functional properties of phytoplank-ton and their respnses to environmental drivers can be used to predict the structure of ascendent phytoplankton communities on the basis of their likely strategic growth responses and not the stochasticity of the processes befalling individual species.

Was this article helpful?

0 0

Post a comment