Figure 5.16

The frequency of years (out of 18) in which the annual areal load of phosphorus added to a Blelham enclosure fell within the ranges stated (0-0.5, 0.5-1.0 g P m-2, etc.) and the number of years in each category during which the named phytoplankters produced a dominant or large sub-dominant population. Species abbreviations as in Fig. 5.15, plus: Cerat, Ceratium hirundinella; Coeno, Coenochloris fottii; Dinob, Dinobryon spp.; Staur, Staurastrum pingue; Urogl, Uroglena cf. lindii. Redrawn with permission from Reynolds (1986b).

subject to P loads between 0.3 and 2.5 g P m-2. Many species occurred in all sequences but in differing absolute and real proportions. Some were relatively much more frequent at high rates of P fertility and some were relatively more abundant at low rates. Using more enclosure-years (making 18 in total) but in lesser detail and in respect of a small number of representative species, Reynolds (1986b) summarised the apparent preferences of certain specific ascendencies to reveal the patterns shown in Fig. 5.16. Whereas Asterionella dominated for a time in every enclosure in every year and the incidence of dominant populations of Planktothrix, Cryptomonas, Fragilaria and Microcystis was not clearly correlated with the eightfold variations in phosphorus supplied, the growth of Eudo-rina, Ankyra and the desmid Staurastrum pingue showed preferences for phosphorus richness. On the other hand, the chrysophytes Uroglena and Dinobryon, as well as the colonial chlorophyte Coenochloris, conformed to supposition (Table 5.2) by flourishing well down the scale of phosphorus fertility. Population development of Anabaena and Ceratium also shows bias towards the less enriched conditions.

Following a similar approach, increase phases of the same group of species were blocked as a function of the pH of the water in which they grew (see Fig. 5.17). The water in Blelham Tarn has a rather low bicarbonate alkalinity (<0.4 meq L-1) and, in the enclosures, it is isolated from

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