Moving in the right direction
Kian Barker
BSc Hons Ichthyology and
Fisheries Science
It was with great interest that I noticed that there is a positive shift in the management or the facilitation of the hydrodynamics of the St Lucia Estuary system.
This remarkable ecosystem has been jerked back and forth through a series of management policies.
However, there seems to be light at the end of the tunnel - or should one say 'fresh water at the end of the tunnel?'
Like any ecosystem when well managed, there are untold benefits.
We have to appreciate that since the 1950s this ecosystem and the factors affecting the estuary have changed.
Historical data needs to be carefully interpreted and certain of this information discarded to create a sustainable ecosystem to maximise what potential remains within and around Lake St Lucia.
However, some of the popular terminology needs to be evaluated.
A drought should no longer be considered as a management option, but rather dry and wet cycles.
Dry and wet cycles are natural global features.
If we refer to a drought, the general understanding is that this is an anomaly and it will eventually go away. Not in Africa.
Genetic ability
In fact, in all ecosystems the plants and animals that live within these ecosystems have a genetic ability to deal with these cycles.
But when there is a change in a resident human population, agriculture and industry expands naturally and certain challenges are created - there is less water for everyone.
This is especially true during dry cycles when aquatic ecosystems come under great pressure, especially St Lucia as it is at the end of several river systems.
Add climate change and the projected drying of this area, careful long-term management is required.
Climate change forecasts indicate progressively less rainfall for this area.
However these dry periods will be punctuated by heavy rain in the form of cyclones and intense low pressure systems dumping tons of rain.
Therefore, long-term management practices taking this into consideration are the only way forward.
There will be more dry periods, with less rain in summer, but generally an average year-on-year winter rainfall.
Climate change
If one splits the rainfall figure over the past few years into summer rain and winter it becomes apparent that there is less summer rain and average winter rainfall - possible results of climate change, which create hotter summers. Higher air temperature means less of an ability for the atmosphere to carry moisture to create rain.
This rainfall and temperature forecast pattern will remain in place for the foreseeable future.
Trouble is that Lake St Lucia loses most of its water through evaporation in the hot summer months when there has been little or no rain. A rather bleak scenario.
What about the past history of Lake St Lucia?
When examining an area it is critical to look at how an ecosystem formed - what factors and features resulted in the final product.
Also how this 'ecosystem' that we so affectionately refer to as Lake St Lucia, sustained itself prior to the arrival and even subsequent to the arrival of man.
The Umfolosi (Imfolosi) River delivered an estimated 60% of freshwater into Lake St Lucia. However, certain factors and decisions resulted in the canalisation and diversion of the river.
This precious load of fresh water was then dumped into the sea, along with its load of silt and sediment.
Taking this amount of fresh water out of any ecosystem will have long and sustained effects.
By looking at some basic agricultural figures to determine the effects of removing the Imfolosi River, it is possible to see why these recent changes are so important.
The estimated evaporative index for this area is an estimated 2 200mm per annum.
Annual rainfall is approximately 1200mm.
Shortfall
Therefore an estimated shortfall of water required to keep lake St Lucia at sea level is one metre of water over the whole lake system.
The in-flow of the Imfolozi River assisted in this and kept lake St Lucia 'generally' balanced.
When this system is stressed by a dry cycle, then there is a net inflow of seawater (when the mouth is open).
During this period sea sand is drawn into the estuary part of Lake St Lucia. But when we enter a wet cycle there is a net-outflow of freshwater, that is diluted with the estuarine salt.
During these net out-flows, sand is also carried out to sea.
Unfortunately when the mouth was breached in March 2007 there was a net in-flow of seawater that brought in tons of sea sand because of the low lake level due to the recent hot dry summer.
It was a mere 120 days before the mouth silted closed.
Shortly after opening, the channel linking to the sea was 2.3m deep.
Closure was for two main reasons: a dry cycle so the lake was losing millions of litres of water due to evaporation and the lack of the Imfolozi River, which would have counter-balanced the evaporative pressure placed on Lake St Lucia due to the relatively large surface area to volume.
Therefore the current move to re-introduce the freshwater into lake St Lucia is critical and should have been done years ago.
We all remember the devastating effects of the dry period of 2002/3. During this period the lake lost a huge biomass of fish and invertebrates.
Had this simple policy been introduced this lake would not have suffered the permanent ecological scar of that ecologically disastrous period.
Salinities were estimated to have reached five times that of seawater.
Since very few organisms can survive in such high salinities, it is a great pity that current policies were not implemented then.
Now we are heading in the right direction.
No comments:
Post a Comment