Growth of phytoplankton in natural environments

The rates of cell replication and population growth that are achieved in natural habitats have long been regarded as being difficult to determine. This is primarily due to the fact that what is observable is, at best, a changing density of population, expressed as species rate of increase (rn, in Eq. 5.2) which falls short of the rate of cell replication because of unquantified dynamic losses of whole cells sustained simultaneously. The net rate of change can be negative (-rn) without necessarily signifying that true growth has failed, merely that the magnitude of rL, the rate of loss noted in Eq. (5.3), exceeds that of replication, r'. The problem of patchi-ness and advection (Section 2.7.2) provides the further complication of compounded sampling errors, in which even the observed rate of population change (±rn) may prove an inadequate base. From the other direction, the true replication rate cannot be estimated from measurable photo-synthetic or nutrient-uptake capacities, unless it can be assumed with confidence that the actual rate of growth is constrained by the capacity factor concerned.

There are ways around these problems and there are now several quite reliable, if somewhat cumbersome, methods for estimating growth rates in situ. Some of these approaches are highlighted below, through the development of an overview of dynamic trait selection in natural habitats.

5.5.1 Estimating growth from observations of natural populations

On the same basis that replication rates cannot be sustained at a faster rate than cell division can be resourced, it is clear that the observable rates of population increase cannot exceed the rates of recruitment through cell replication. The corol lary of this is that attestably rapid phases of population increase, independent of recruitment by importation from horizontally adjacent patches or from germinating resting stages, are indicative of yet higher simultaneous rates of cell replication.

Growth rates from episodic events Generically, these accumulative phases fall into two categories. One of these is the annually recurrent and broadly reproducible event, such as the spring increase of phytoplankton in temperate waters, in response to strong seasonally varying conditions of insolation (see Section 5.5.2). The second is the stochastic event, when, perhaps, a sharp change in the weather, resulting in the fortuitous stagnation of a eutrophic water column, or the relaxation from coastal upwelling, or the deepening of a nutrient-depleted mixed layer with the entrainment of nutrient-rich met-alimnetic water, or some abrupt consumer failure through herbivore mortality, leads to the realisation of potential respondent growth. In this second category, the phases of increase may be brief and sensing them, accurately and with reasonable precision, requires the close-interval sampling of well-delimited populations. The study of in-situ increase rates of phytoplank-ton in Bodensee (Lake of Constance), assembled by Sommer (1981), was one that satisfied these conditions. The research based on the large (1630 m2), limnetic enclosures in Blelham Tarn, English Lake District (variously also referred to as 'Blel-ham Tubes', 'Lund Tubes' (Fig. 5.11), being isolated water columns of —12-13.5 m in depth and including the bottom sediment from the lake; for more details, see Lund and Reynolds (1982), carried out in the period 1970-84, has similarly provided many insights into phytoplankton population dynamics. Examples of specific increase rates noted from either location are included in Table 5.4.

The evident interspecific differences are partly attributable to the time period of observation, and the seasonal changes in water temperature and in the insolation attributable to seasonally shifting day length and vertical mixing. In some instances, these environmental variations are reflected in intraspecific variability in

I The Blelham Enclosures: the positions of the three butylite cylinders (A, B and C), each measuring about 1630 m2 of water surface and a similar area of bottom sediment, are shown in relation to the bathymetry of the lake. The line X—Y was set up as a permanent transect with shallow ('S') and ('D') sampling stations. For further details, see Lund and Reynolds (1982).

increase rate. Where species are common to both locations, maximal performances are similar; species are either fast-growing or slow-growing in either location.

The observed rates of increase are also plausible in terms of the dynamic behaviours of the algae in culture. Allowing for winter temperatures and short days, only half of which might be passed in the photic zone, a vernal growth rate of 0.15 d-1 for Asterionella is perfectly explicable. For small, unicellular species such as Ankyra and Plagioselmis to be able to double the population at least once per day in summer (when they can manage it twice in grazer-free, continuously illuminated culture) also seems to be a reasonable observation. The growth-rate performances of the bloom-forming Cyanobacteria and the dinoflagellate Ceratium are about half those noted in culture at 20 °C (cf. Table 5.1).

Frequency of cell division

Relatively rapid growth rates, sustained over the equivalent of several cell divisions, lead assuredly to the establishment of populations making up a significant part of the biomass, if not actually coming to dominate it. It is equally probable that the same species may be relatively inactive for the quite long periods of their scarcity. Is their increase prevented by lack of light, or lack of resources, or losses to grazers, parasites or to the consignment to the depths? Obviously, more precise means of investigating the in-situ physiological activity of numerically scarce phy-toplankters are needed to answer this question. One of the best-known and most precise techniques for estimating the species-specific growth of sub-dominant populations is to estimate the frequency of dividing cells. This works best for algae whose division is phased (i.e. it occurs at certain times of day or night) and it may need close-interval sampling (every 1-2 h) of the field population. It works especially well with algae (e.g. desmids, dinoflagellates, coccolithophorids) that have complex external architecture which has to be reproduced at each division and often requires several hours to complete. Then the numbers of cells before and after the division phase is increased by a number that should agree with, or be within, the increment deduced from the frequency of dividing cells. Pollingher and Serruya (1976) gave details of the application of this method to the seasonal increase of the dominant dinoflagellate in Lake Kinneret, now called Peridinium gatunense. During the period of its increase (usually February to May), the number of cells in division on any one occasion was found to be variable between 1% and 40%. They showed that the variability was closely related to wind speed. While daily average wind velocities exceeded 8 m s-1, the frequency of dividing cells (FDC) was always <10%. This accelerated to 30-40% during the spring period of weak winds (and, hence, weak vertical advection) averaging <3 m s-1. Successful recruitment of new cells

Table 5.4 I Some maximal in-situ rates of increase (rn d-1) of some species of freshwater phytoplankton reported from Bodensee (Sommer, 1981) and from large limnetic enclosures in Blelham Tarn (Reynolds et al., 1982a; Reynolds, 1986b, 1998b), together with some reconstructed rates of replication (r') where available (see text)

Blelham Enclosure Bodensee -

Table 5.4 I Some maximal in-situ rates of increase (rn d-1) of some species of freshwater phytoplankton reported from Bodensee (Sommer, 1981) and from large limnetic enclosures in Blelham Tarn (Reynolds et al., 1982a; Reynolds, 1986b, 1998b), together with some reconstructed rates of replication (r') where available (see text)

Blelham Enclosure Bodensee -

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