We celebrate the Youth in June this month with a student study of Lake St Lucia salinity levels.

On the 4th of January 2021 the St Lucia estuary mouth was manually opened by the use of heavy machinery which removed sand dunes that blocked the Ocean flowing into the St Lucia Estuary. The estuary mouth initially closed in June 2002, opened again in 2014 and reclosed in October 2016, because of drought, afterwhich the salinity levels had dropped dramatically. 

Once the mouth was opened to the sea, it was expected that the salinity levels would increase, however this was not the case in the recent testing which was conducted in Makakatana Bay, approximately 20km up the St Lucia Estuary where it opens up into the main body of the Lake.

These results measured in figure 1, were done so by the use of a handheld TDS/EC conductivity tester, which tests the conductivity of water. Salt is a particle which, when present, causes conductivity to increase. So, these results indirectly determine how much salt is present in the water samples used.

Figure 1:Graph showing the conductivity of the water in Lake St Lucia in February 2021 after the estuary mouth was artificially breached.

As can be seen in the above graph the conductivity of the water was tested over about a month in February 2021 which would have been a month after the estuary mouth was reopened. This graph was expected to show an increase in conductivity, as salt particles in the water cause conductivity to increase. However, the graph shows a decrease in salinity. This was most likely due to the rainfall over the previous two months after the mouth was opened which would have caused the concentration of salt to decrease as the volume of water increased.

In figure 2 below, it is evident that the rainfall over January and February has been significant which caused the water level in Lake St Lucia to rise, and brought life back to the wetlands. Due to the increased volume of water in the lake, more water would have been pushed out of the estuary mouth causing less sea water to come into the Lake which means there would not be as much of an increase in salinity as there has been in past years when the mouth was open and the water levels were lower. This explains why the water in the lake has such high salt content during a drought, because the volume of the water in the lake is low so the sea is pumping more salt water into the lake to keep the water level constant. This can have many negative effects on the ecosystems in the lake as their habitat normally consists of a balance between salt and fresh water. This shows how necessary the rainfall experienced over the past few months really is, as it allows for that balance of salt and fresh water to be maintained as well as promoting the flow of the rivers and movement within the lake to keep the water relatively clean and healthy for its many ecosystems.

by Mia Poulter - Roedean Matric student 2021

Left: Mia Poulter.

Left :  Water samples taken from Lake St Lucia taken by Mia for her study.

Below: Table showing the rainfall data measured in millimeters over January and February 2021

* An interesting note - January 2021 rainfall of 358.5mm was the highest monthly rainfall recorded in over a decade.

*All rainfall statistics are from measurements recorded at Makakatana Bay Lodge*

Stay tuned for our next blog on rainfall.