The delicate balance in Estuaries

Estuaries are generally described as the meeting place between rivers and oceans. These can be open temporarily or permanently. The physical nature of the river mouths are changed regularly by the tides, wave action and rainfall.  The interaction between inbound salty seawater and outbound fresh river water is regulated by the tides, but extreme conditions like droughts and floods can cause dramatic changes.  High rainfall can change an estuary into a mighty flowing, brown, muddy freshwater river, while strong coastal currents can cause a slow flowing saltwater river, which can lead to the mouth being closed temporarily by sand sediments. Rapid daily fluctuations in temperature, salt content and oxygen concentrations in the estuarine environment make estuaries one of the harshest ecological systems on earth.  Furthermore, salty water is more dense than freshwater which causes the freshwater to float on top of the salty water with more or less no mixing taking place. That's why we'll find the characteristic wedge of seawater underneath the freshwater. Organisms living in estuaries have to cope with constant and big changes.


One way to survive the fluctuations of the salinity, is to migrate to a more suitable area, but this is a privilege only available to mobile creatures like crabs, fish and prawn. Less mobile creatures have to stay put and have to survive the changing salinity or regulate the salt via a physiological process like osmosis.    

In contrast with the ocean where the water temperature normally stays more or less the same, temperatures in estuaries can fluctuate dramatically; from around 0°C to 30°C or more.  High temperatures is a bigger problem to animals than lower temps, since it negatively influences the metabolic processes of animals. The solubility of oxygen in warmer water is also remarkably lower than in colder water - this can lead to oxygen deprivation. Some animals migrate to cooler water if temperatures rise too high, but digging mudprawn just stop pumping water through their systems when water reaches temps of more than 32°C in order to help them survive higher water temperatures. This will continue till water temps normalise again.

The food chain in estuaries relies heavily on the death and decay of various types of plants.  The fragmented remains of plants, called detrius, together with organic matter that is introduced from the rivers and sea, form the main source of food for the prolific estuarine animals.  Tidal action is important for estuaries.  The flood tide carries in the larvae of invertebrates and juvenile fish from the sea and the ebb tide flushes pollutants out to sea.  
Mobile species such as zooplankton and mullet (harder) are able to migrate to areas rich in detritus to feed, but mudprawns, bloodworms, mussels and other benthic invertebrates rely largely on tidal currents to make food available to them.  Surface deposit feeders, such as crabs and snails, forage on the exposed inter-tidal mudflats for material deposited during the preceding high tide.  Some fish feed on the benthic invertebrates, while others eat zooplankton, or prey on fish smaller then themselves.

More than a hundred fish species in South Africa are completely or partly dependent on estuaries, while 400 species live in estuaries at a certain time in their existence.  Most of them hatch in the ocean and swim into the estuaries as juveniles where they are safer till they reach adulthood. Known species of estuaries on the West Coast are mullet (harder), cod, white steenbras, Cape stumpnose, elf and garrick.

A lot of bird species like the migrating waders are also dependent on estuaries and come summertime one can find up to 55 000 birds on the mudflats of Langebaan. We'll post about waders at some other time.

Because of their aesthetic and relaxation value, a lot of South African estuaries are influenced badly by human activity. Holiday-goers enjoy camping, canoeing, water skiing, fishing and bird watching at the estuaries around our coast line. The physical conditions of an estuary greatly relies on the delicate balance between the fresh and salt water exchange. Any human activity which disturbs this balance, will have an enormous effect on the estuarine environment and on the fauna and flora. The biggest threats are pollution, loss of habitat, damage to habitat, withdrawal of fresh water, dams in the upper course of the river which reduce the episodic flushing, coastal development, imported species and unlimited fishing.

Pollution is a huge threat to estuaries, it comes in different forms and have their own different results in the estuaries.  Industrial pollution cause heavy metals to be absorbed in the food chain which causes the top predators to be poisoned. Sewage or the pollution by food processing plants lead to raised concentrations of organic compounds in estuaries. This in turn will lead to eutrophication - a vast growth in algae because of increased nutrients levels of the water. Eutrophication can have serious effects, e.g. algal blooms that block light from entering the water and so harming the plants and animals utilising it. If there's too much overgrowth of algae, it can also prevent oxygen from getting into the water, making it hypoxic and creating a dead zone where no organisms can survive.