Biogeochemical evidences of human intervention in a shallow lake, Zeekoevlei, South Africa

Sammanfattning: This thesis describes a multi-parameter biogeochemical investigation in a shallow hyper-eutrophic freshwater coastal lake, Zeekoevlei, in South Africa. The predominance of autochthonous algal-derived organic matter in lake sediments is indicated by low C/N, high H/C ratios and ?13Corganic values. Seepage from a nearby waste water treatment plant, rapid urbanization, raw sewage input and heavily fertilized farming in the catchments have caused enhanced productivity and is reflected by the changes in TOC concentrations, ?15N values, terrestrial to aquatic (n-alkane) ratio (TAR) and low carbon preference index (CPI) values. Eutrophic conditions were initiated in the lake with the start of recreational activities in early 1900s. Construction of dams and pondweed eradication in mid-1900s caused the transformation of the lake towards a hyper-eutrophic water body. Moreover, the aquatic macrophyte n-alkane proxy (Paq) values indicate the slow takeover by floating macrophytes after the eradication of submerged pondweeds in 1951. Low ?15N values and appearance of zeaxanthin indicate initiation of cyanobacterial bloom in the lake following pondweed eradication. Furthermore, the lake experienced intense cyanobacterial bloom after 1983 dredging. Although, cyanobacterial domination has decreased in recent years, hyper-eutrophic condition persists in the lake.Chemical weathering process supplies major fraction of trace metals, whereas fertilizers, agricultural wastes, sewage effluents and road runoff constitute the anthropogenic fraction. Low industrialization in the catchments causes low metal pollution in waters. In addition, high pH and metal scavenging by planktons result low dissolved trace metal concentrations. Adsorption by CaCO3 and planktonic assimilation control trace metal and phosphorus (P) sedimentation. Lake sediments have low P retention capacity and P is released from surface sediments by wind-induced resuspension.