Coast KZN

23 Nov 2015

Lake St Lucia drying up in the drought

Tony Carnie (The Mercury)


Salinity levels threaten species

With the drought spotlight fixed on human suffering, starving livestock and economic impacts, far less attention has focused on impacts to the natural environment.

Just one example is Lake St Lucia, Africa’s largest estuarine lake.

Water in the northern part of Lake St Lucia is now almost three times saltier than seawater and the lake has been separated into at least three shallow compartments – all below normal sea level.

In short, Lake St Lucia is critically starved of fresh water, partly by the drought and partly because of a decades-long historical dispute with local sugar farmers.

But as the impacts of the drought intensify, no immediate relief is in sight following a court case mounted by the Umfolozi Sugar Planters co-operative.

Way back in the 1920s, cane farmers began to cultivate the fertile alluvial soils along the Imfolozi River flood plain.

While they enjoyed access to this fertile land, they found that their cane fields were flooded when the river flowed strongly. So a system of canals and levees was built.

The natural course of this river – which historically supplied nearly 60% of the lake’s fresh water inflow – was also altered in the early 1950s when the mouth of the Imfolozi was diverted to flow directly into the sea, instead of the Lake St Lucia estuary mouth.

This reduced flow of water into the lake also caused complications for cane farmers when the mouth closed during dry periods, as trapped river water flooded back on to some of the cane land.

To resolve this problem for farmers, the sand bar blocking the mouth of the Imfolozi has been breached repeatedly over the past six decades.

More recently, the iSimangaliso Wetland Park Authority signed an agreement with the Global Environment Facility in an attempt to find a lasting ecological solution to decades of human interference. This plan involves allowing the Imfolozi to re-establish a natural link to the St Lucia mouth, increasing the volume of fresh water feeding the lake.

But over the past few decades large volumes of sand have built up near the estuary mouth – effectively blocking the river from returning to its former course.

Nearly two years ago, a small canal was dug to re-establish a passage to the estuary. But in the absence of heavy rains the Imfolozi mouth has closed again and cane farmers went to court last month to force iSimangaliso to reopen the mouth. They said they were entitled to protect their land and argued that opening the mouth would have minimal impacts on the lake, since very little Imfolozi water was reaching the estuary mouth during the drought.

While iSimangaliso has been compelled to allow the mouth to be breached for now, both parties will go back to court next year to argue their positions in more detail.

In the meantime, however, iSimangaliso ecologists say salinity levels in the northern section of the lake have risen to almost 100 parts of salt per thousand parts of water.

By comparison, seawater has a salinity level of about 35 parts per thousand.

Wetland Park chief executive Andrew Zaloumis says the lake level has also dropped so low that it is now possible to walk from Catalina Bay to Charters Creek without getting your feet wet.

“Conditions will become extremely dire if we do not have good rains and get Imfolozi River water into Lake St Lucia this summer,” he said this week. Zaloumis said breaching the St Lucia estuary mouth to allow seawater into the system would simply raise salinity levels further.

He noted that crocodiles and hippos were hard hit by high salinity levels during a similar major drought in the early 1970s.

Eventually, several hippos and crocodiles had to be moved southwards away from the hyper-salty northern sections.

iSimangaliso scientists say several species – including crabs, prawns and worms – are unable to handle salinities above 60ppt. Invertebrates such as the white prawn and several fish species will move to areas of lower salinity or get trapped and killed by the higher salinities.


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