Ecologic Stress in a Subtropical Coastal Lagoon: Lake St. Lucia, Zululand.

摘要

Lake St. Lucia is the largest of several coastal lagoons that lie behind the massive sand barriers forming the Zululand Coast, South Africa. Following the Flandrian transgression, this lagoon covered 912 sq km, forming a body of water 112 km long and more than 40 m deep. A rich and varied lagoonal ecosystem developed in which plants and animals characteristic of tropical Africa blended with species from temperate southern Africa. Today, after 5000 years of sedimentation, segmentation, and reed-swamp encroachment, Lake St. Lucia has been reduced to a shallow lagoon averaging 312 sq km in area, 40 km in length, and less than 2 m deep. Whereas most coastal lagoons are destined to be infilled over time, the processes of change at Lake St. Lucia have been accelerated recently primarily by man. Present sedimentation rates are between two and three times the mean rate for the past 5000 years, a response in part to accelerated erosion farther inland. In addition, periodic low water levels and high salinities reflect diminished discharge from watersheds affected by erratic rainfall, irrigation agriculture, afforestation, and drainage diversions. Frequent closure of the estuary by strong littoral drift further inhibits the free exchange of water and biota between the lagoon and the sea. These physical changes in Lake St. Lucia create a serious threat to the survival of the entire ecosystem.