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Accueil du site → Doctorat → Afrique du Sud → < 2000 → The regulation of phytoplankton productivity in a shallow, turbid, oligotrophic lake.

University of KwaZulu-Natal (1988)

The regulation of phytoplankton productivity in a shallow, turbid, oligotrophic lake.

Akhurst, Edward Gordon John

Titre : The regulation of phytoplankton productivity in a shallow, turbid, oligotrophic lake.

Auteur : Akhurst, Edward Gordon John.

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy 1988

Résumé partiel
Aspects of the physical and chemical environment likely to influence phytoplankton productivity were investigated in Lake Midmar, a shallow (mean depth 11.4m), oligotrophic impoundment, over a three year period to evaluate the hypothesis that "the fundamental process regulating the functioning of a shallow lake is vertical mixing." Energy exchange at the lake surface was similar to that reported for other lakes. The principal components of the energy balance (net incoming radiation and latent heat loss) and heat content of the lake varied seasonally. However, day-to-day variation in local weather, particularly the incidence of cloud cover, masked these seasonal trends and contributed to the unusual pattern of almost constant sensible heat loss throughout the year. Consequently, heat storage was highly variable, particularly in early summer (September-December). The combination of day-to-day variation in heat storage, the pattern of heat loss, wind stress and the synergistic interaction between these components together contributed to the observed pattern of weak summer stratification (absence of vertical partitioning into an epi-, meta- and hypolimnion) and efficient heat transfer within the water column. The occurence of low Wedderburn numbers ( W < 1) on several occasions during summer (September - February) resulted in Midmar being classified as a regime 2 lake (sensu Spigel and Imberger,1980) and the thermal structure as discontinuous polymictic (sensu Lewis,1983 a). Under conditions of ’normal’ river flow (1980-81) phosphorus associated with river suspended solid loads, even at times of peak loading in late summer, had little immediate influence on the total phosphorus content of the water column or phytoplankton productivity. Internal phosphorus loading, resulting from sediment resuspension and transfer of phosphorus from sediment pore water to the overlying water, was identified as the principal source of bioavailable phosphorus. The wind-induced circulation - sediment interaction also exerted a strong influence on the underwater light climate. Inorganic suspended particulate material was the principal factor regulating the attenuation of photosynthetically active radiation (PAR). Red light being the most penetrating component of the PAR spectral range throughout the study period . The hypothesis that ’internal", autochthonous processes and not external, allochthonous processes were responsible for regulating phosphorus dynamics and PAR attenuation" was confirmed during the drought (1982-83). As lake levels decreased the increased interaction between wind- induced circulation and the deeper sediments led to increased PAR attenuation and total phosphorus concentrations in the water column

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