0 'Phase' refers to part of the year: V, vernal, or spring bloom period, temperatures 5 ± 2 °C; ES, early-stratification phase (temperature 11 ± 3 °C); MS, mid-stratified (temperature 17 ± 3 °C); LS, late-stratified period (temperature generally 12 ± 2 °C).

divided off at this rate yields maximum rates of population increase equivalent to 0.26-0.34 d-1. The typical net rate of population increase of Peri-dinium in Kinneret over a sequence of 20 consecutive years was found to be 0.22 ± 0.03 d-1 (Berman et al, 1992).

Heller (1977) and Frempong (1984) estimated FDC of Ceratium in Esthwaite Water to be between 2% and 10%, occasionally 15%, sufficient to explain in-situ seasonal increase rates of 0.09-0.14 d-1. Alvarez Cobelas et al. (1988) estimated growth rates from afternoon FDC peaks in the population of Staurastrum longiradiatum in a eutrophic reservoir near Madrid to be between 0.13 and 0.16 d-1. More recently, Tsujimura (2003) has estimated in-situ growth rates from FDC among cell suspensions of Microcystis aeruginosa and M. wesenbergii from Biwa-Ko (prepared by ultrasoni-cation of field-sampled colonies). In both species, the frequency of diving cells varied between 10% and 15% in offshore stations and between 15% and 40% at inshore stations, with the average duration of cytokinesis varying from 25 h to 3-6 h. Growth rates of 0.34 d-1 thus appear sustainable in the near-shore harbour areas of Biwa-Ko, whence they are liable to become more widely distributed in the circulation of the lake (Ishikawa et al., 2002).

Frequency of nuclear division

For phytoplankton species that are less amenable to the tracking of cell division, the principle may be extended to the monitoring the frequency of karyokinesis (Braunwarth and Sommer, 1985). The success of this method relies on the good fixation of field samples followed by careful staining with the DNA-specific fluorchrome, 4,6-diamidino-2-phenylindole (DAPI) (Coleman, 1980; Porter and Feig, 1980). This precise and sensitive method has been applied to a natural Cryp-tomonas population (Ojala and Jones, 1993), the results being broadly predictable on the basis of growth rates under culture conditions. Like any methods based upon the events in the cell cycle, its prospects for measuring replication accurately are high (cf. Chang and Carpenter, 1994).

There is also keen interest in sensing the DNA replication itself. Since the groundbreaking study of Dortch et al. (1983), microbial ecolologists have been debating the validity of DNA to cell carbon as an index of the rate of DNA replication. As an indicator of the capacity for protein synthesis, the RNA : DNA ratio is already in use as a barometer of the cell growth cycle in marine flagellates (Carpenter and Chang, 1988; Chang and Carpenter, 1990) and bacteria (Kemp et al, 1993; Kerkhof and Ward, 1993).

Growth from the depletion of resource In contrast to monitoring growth-cycle indicators, methods for reconstructing growth rates from resource consumption are unsophisticated in approach and notoriously imprecise. However, methods invoking uptake of resources deployed in specific structures, such as silicon for diatom frustules (Reynolds, 1986a) or sulphur for protein synthesis (Cuhel and Lean, 1987a, b), offer more promise. Reynolds and Wiseman (1982) were able to combine the advantages of the spatial constraints of enclosure, offered by the Blel-ham tube, with frequent serial sampling of the plankton and careful accounting of the amounts of replenishing sodium silicate, in order to measure the true growth rate of a diatom population. Several datasets were collected and presented (partly also in Reynolds et al., 1982a; Reynolds,

1996b); just the case of Asterionella formosa in Blel-ham Enclosure B in 1978 is highlighted here.

The inflatable collar of the enclosure was lifted on 2 March of that year, isolating part of the lake population comprised almost wholly of Asterionella, then itself already actively increasing, at a concentration of ~630 cells mL-1). Over the next 19 days, the population increased exponentially at an average rate of 0.147 d-1, then more slowly to its eventual maximum (a total of 24,780 cells mL-1, though by then including 1950 cells mL-1 judged to be dead or moribund) on 4 April. The decline in the standing population was slow at first but accelerated enormously as warmer and sunnier weather mediated the thermal stratification of the Tarn and the enclosure. Nutrients were added to the enclosure each week, by dispersal into solution across the enclosure surface, in measured doses respectively designed to restore the levels of available resource to 300 |g N, 20 |g P, 100 |g TFe and 1000 |g SiO2 (466.7 |g Si) L-1. None of these fell to growth-limiting thresholds. However, there was no artificial relief for either high pH or probable carbon limitation. The consumption of silicon was calculated as the sum of the observed decline in the initial concentration on 2 March, aggregated with the Si added, and averaged out across the whole volume. The conversion to Asterionella cells between additions was calculated using contemporaneous routine measurements of the Si content of cells sampled from the growing population (consistently within the range 51.8-61.1 pg Si cell-1). As there was no other significant diatom 'sink' at the time, the consumption was assumed to be equal to its deposition in new Asterionella frustules. The rate of growth, r (Si), was approximated from the numbers of new cells that the observed silicon depletion could have sponsored. Estimates were comparable with the observed rates of increase (rn) and with the rates of growth reconstructed by correcting for the simultaneous loss processes (discussed fully in Chapter 6). Simultaneous sinking losses were 'monitored' in two ways: using the flux of settling cells into sediment traps placed near the bottom of the enclosure; and using a technique of coring and subsampling the semi-liquid superficial deposit (see Reynolds, 1979a). The possible losses to grazers were estimated from contemporaneous measurements of

Table 5.5 Comparison of the observed rates of increase in a natural population of Asterionella in a large limnetic enclosure with the growth rate as estimated by silicon uptake. The difference is equated with the rate of loss of live cells (rL) and which may, itelfbe compared with simultaneous observations of settling rates (rs) and estimates of the rates of loss to grazers (rg) and to death (rd)

2 Mar-21 Mar 2IMar-4Apr 4 Apr-25 Apr 25Apr-l5

calc rs 0.007 0.007 0.044 0.242

obs rs 0.009-0.027 0.016-0.021 0.032-0.126 0.150-0.189

calc r' 0.157-0.184 0.081-0.099 0.001-0.160 0-0.051

Table 5.6 Comparison of the observed biomass (as cells per unit area) of Asterionella in Enblosure B, Spring 1978, with reconstructed production (from silicon uptake), the total eventually sedimented, the intercepted flux and the estimated loss to grazers at the time

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