Water contained within the estuary basin is primary renewed by the action of tides, and to a lesser extent river inflow. The lower and middle reaches of the estuary are marine dominated and their ecology and structure are determined by the ebb and flow of the large tidal prism (estimated at spring tide at 19 x 106 m-3) that enters through the Knysna heads twice daily (Largier et al., 2000). Based on hydrographical characteristics, the Knysna Estuary can broadly be divided into three distinct regimes (Largier et al., 2000):
Upper estuary regime, which is strongly influenced by the inflow of freshwater from the Knysna River and demonstrates strong vertical stratification in salinity and water temperature.
• Lagoon regime, which is characterised by aged salty water with little direct influence from freshwater. The lagoon is considered as a transition zone between the estuary and the marine embayment.
• Large marine embayment regime, which if flushed by tidal flows and exhibits temperatures and salinities similar to the ocean.
The boundaries of these three regimes demonstrate a high degree of temporal variability reflecting amongst others, the magnitude of freshwater inflow from the Knysna River into the estuary and the inflow of marine waters through the heads (Largier et al 2000). Thus, during periods of high rainfall within the catchment area of the Knysna River, the upper estuary may extend as far down as the large marine embayment resulting in the disappearance of the lagoon (Largier et al., 2000). (Ref 5)
