Estuaries: a biodiversity treasure trove
By Alexis Olds, Marine and Coastal Ecologist
Estuaries are often thought of as part of a river but they are so much more than that. They are the link between the salty ocean and the fresh rivers upstream. A river becomes an estuary as soon as you can measure salt in the water. Some estuaries are very short (a few hundred metres) and others very long. The Breede Estuary in Swellendam is over 62 km long.
Species diversity is very high in estuaries, especially the big ones. At least 176 estuarine plant species are associated with estuaries. There are an astounding 150 estuarine-associated fish species that regularly occur in our estuaries. Approximately 50 of these are southern African endemics of which 20 are exclusively found in South African waters. The Botrivierklipvis (Clinus spatulatus) is found in only two estuaries in the world, the Bot/Kleinmond estuary and the Klein estuary just outside of Hermanus. The Knysna seahorse (Hippocampus capensis) is found in only three estuaries (Keurbooms, Knysna and Swartvlei) in the Garden Route.
Many of species rely on estuaries to complete their life cycles. Mud prawns (Upogebia Africana), another southern African endemic, are found in estuaries where the estuary mouth is open permanently or the majority of the time. They have an obligate marine larval phase which means that the larvae, once hatched, must go to sea until they reach sexual maturity and then they move back into an estuary to recruit the next seasons larvae. Fish do something very similar but in the opposite direction. Species like the spotted grunter (Pomadasys commersonnii), leervis (lichia amia) and dusky kob (Argyrosomus japonicus) all rely on estuaries as juveniles. The estuary offers more protection than the ocean. It is warmer, has shelter for the young fish and has a high productivity in terms of food. When the fish reach sexual maturity, they exit the estuary and congregate offshore to spawn. Their larvae swim back into an estuary to complete the cycle.
Estuaries are home to some of South Africa’s most iconic species and CapeNature works to protect and conserve the biodiversity found within them.