SEDGEFIELD NEWS - The Garden Route Region is expected to get hotter, windier and drier conditions over the next few months. Frugal and responsible use of domestic and ground water resources is thus important.
SA National Parks marine ranger for Wilderness, Jonathan Britton, explains how SANParks is doing its bit to manage water resources in the park, mainly through conserving nature and ensuring better quality of water.
There are various monitoring systems in place and these include the following:
• Passive hydrogen sulphide monitors.
• Dissolved oxygen, pH, salinity, and temperature loggers.
• Vegetation density sampling.
• Annual fish surveys.
• Roving kreel surveys.
• Sand prawn monitoring project.
• Increased community and municipal communication and cooperation.
• Baited remote underwater video stations (Bruvs). These have been used to visually observe and record the fish species densities and diversity in the shallow parts of the estuary (about 1m) that would normally be exposed during the open phase of the estuary.
Swartvlei is classified as a temporary open/closed estuary.
During the open and closed phase there are unique ecological processes that collectively work together to maintain the delicate food web.
The river mouth will generally close naturally during a low rainfall season as there is a reduction of the inflow of freshwater. This process of closing will also be supported by the deposition of sediment from tidal inflow.
Artificial breaching is often associated with a limited head/pressure of water behind the sand berm at the mouth. Without enough pressure there will be limited scouring of sediment from the estuary, not creating a deep enough channel. If breaching occurs when the water levels are low, this can lead to gradual "shallowing" of the estuary from the accumulation of sediment.
In addition, artificial breaching when there is not enough and simultaneous river inflow often results in the introduction of large volumes of salt water that will change the ecological composition of the estuary. This can significantly impact the dynamics of the food chain.
The closed phase in the estuary is a time of "rest and recovery" when some interesting and important ecological processes come into play.
When an estuary closes, benthic micro-algal* populations begin to recover and build up. This forms an important part of the food web providing nutrition for invertebrates that are consumed by fish and birds.
The stable conditions of the closed phase also provide suitable conditions for the growth of submerged macrophytic plants. Dissolved oxygen is also higher during closed phases as a result of photosynthesis.
Photosynthesis contributes more to increased dissolved oxygen levels than from physical movement/disturbance of the water body. The plants also provide foraging and refuge habitat for juvenile fish.
*Plant-like creatures that cover the floor of the estuary that are a source of food and energy for many organisms.
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