Why are the reeds at the mouth of Lake St Lucia still surviving after weeks of strong marine influence and limited freshwater flow?
THE SHORT ANSWER
The common reed, Phragmites australis, is not strictly a freshwater plant. It is able to survive in brackish estuarine conditions and can tolerate periods of substantial salinity.
WHY THE REEDS CAN SURVIVE
- The root zone may be less salty than the surface water.
Saltwater washing over the stems does not necessarily mean that the mud around the rhizomes has the same salinity. Groundwater, older pore-water and freshwater stored in the sediment can create a different environment below the surface. - The underground rhizomes provide resilience.
Reeds spread through underground stems and can continue producing shoots even when some above-ground leaves are stressed or damaged. - They have salt-tolerance mechanisms.
Salt-tolerant reeds can limit the movement of salt into sensitive tissues and survive fluctuating brackish conditions. - Exposure is probably variable.
Ocean water may wash over the mouth during particular tides, but the entire root zone may not be exposed continuously to full-strength seawater for several weeks.
WHAT THE REEDS DO—and DO NOT—TELL US
The surviving reeds are consistent with strong marine influence near the mouth, particularly alongside the return of mudskippers and the apparent reduction of freshwater indicators such as tilapia and visible bubbling activity.
However, reed survival does not prove that the entire lake is fully marine. It mainly shows that the reed beds, especially their underground root zones, can tolerate brackish or periodically salty conditions.
WHAT SHOULD BE MEASURED?
The clearest way to test the situation would be to measure surface and bottom salinity, conductivity, dissolved oxygen and temperature at several locations, together with pore-water salinity in the reed mud. Conditions at the mouth may differ greatly from conditions farther inside the lake and around the roots.
REFERENCES
Ecophysiological responses of Phragmites australis:
Frontiers in Plant Science
Effects of salinity on the growth of Phragmites australis:
ScienceDirect
Review of the Scientific Basis for Breaching the Mouth of Lake St Lucia Estuary:
Independent scientific review PDF
Biodiversity census of Lake St Lucia:
ZooKeys
Note: The survival of reeds is not a substitute for salinity measurements. It is one ecological clue among several.
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