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Accueil du site → Doctorat → Afrique du Sud → < 2000 → On the ecology of hyperscum-forming Microsystis aeruginosa in a hypertrophic African lake.

University of KwaZulu-Natal (1987)

On the ecology of hyperscum-forming Microsystis aeruginosa in a hypertrophic African lake.

Zohary, Tamar.

Titre : On the ecology of hyperscum-forming Microsystis aeruginosa in a hypertrophic African lake.

Auteur : Zohary, Tamar.

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy 1987

Résumé partiel
Light is the primary source of energy in most of earth’s ecosystems . In freshwater ecosystems the major interacting factors that determine the abundance and species composition of planktonic phototrophs, the primary utilizers of light, are nutrients, temperature and light. With increasing eutrophication and declining geographical latitude, nutrient availability becomes in excess of the organisms’ requirements, water temperature is more favourable for growth, and community structure depends to a greater extent on light availability. This study focuses on the population dynamics of the bloom-forming cyanobacterium Microcystis aeruginosa Kutz. emend. Elenkin in subtropical Hartbeespoort Dam, South Africa. The objectives of the study were : to investigate the annual cycle, and the factors leading to the dominance and success of the cyanobacterium in this hypertrophic, warm monomictic lake, where light availability is the major factor limiting phytoplankton growth rates ; to study the surface blooms and ultimately hyperscums that this species forms ; and to assess the ecological significance of hyperscums. A 4. 5-years field study of phytoplankton abundance and species composition in relation to changes in the physical environment, was undertaken. The hypothesis was that M. aeruginosa dominated the phytoplankton population (> 80 % by volume) up to 10 months of every year because it maintained itself within shallow diurnal mixed layers and was thus ensured access to light. It was shown that wind speeds over Hartbeespoort Dam were strong enough to mix the epilimnion (7 - 18 m depth) through Langmuir circulations only 12 % of the time. At other times solar heating led to the formation of shallow ( < 2 m) diurnal mixed layers (Z[1]) that were usually shallower than the euphotic zone (Zeu ; x = 3.5 m), while the seasonal mixed layer (zrn) was always deeper than Zeu. From the correspondence between vertical gradients of chlorophyll a concentrations and density gradients, when M. aeruginosa was dominant, it was implied that this species maintained the bulk of its population within Z[1]. Under the same mixing conditions non-buoyant species sank into dark layers. These data point out the importance of distinguishing between Zrn and Z[1], and show the profound effect that the daily pattern of Z[1], as opposed to the seasonal pattern of Zrn can have on phytoplankton species composition Adaptation to strong light intensities at the surface was implicated from low cellular chlorophyll a content (0.132 μg per 10[6] cells) and high I[k ](up to 1230 μE m⁻² S¯¹). Ensured access to light, the postmaximum summer populations persisted throughout autumn and winter, despite suboptimal winter temperatures, by sustaining low losses.

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