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Abstract
The fall and recovery of dissolved oxygen (DO) is documented throughout the Gamtoos Estuary, South Africa during dry conditions and following light and heavy rainfall over a 13-month period from November 1992 to November 1993. Hypoxic conditions generally occurred in the near-bottom waters of the upper estuary. Localized fluctuations in DO levels are related to the natural diurnal fluctuation associated with photosynthesis of aquatic flora. The drop in DO levels following light rainfall is associated with the volume of oxygen-consuming compounds entering the estuary via runoff from adjacent agricultural fields. This hypoxia was short-lived. Following an extreme rainfall event, however, almost immediate hypoxia was recorded throughout the estuary, and DO levels deteriorated for some time thereafter as a result of the substantial input of organic matter into the estuary. The area of hypoxia and recovery was governed by the freshwater input at the tidal head, estuarine hydrometry and hydrodynamics. Tidal processes were identified as a source of replenishment of oxygen when, during high tide, seawater with a higher DO content penetrated the estuary.