Cape Town - The biodiversity of marine and coastal environments is experiencing changes as South Africa’s estuaries are facing a growing threat from pollution.
The health of estuaries, which are where rivers meet the sea and act as sponges to absorb surplus water, is said to be worsening owing to high pollution pressures from agricultural, industrial, rainfall, and wastewater discharges.
With estuaries providing many ecosystem services, from carbon sequestration and storage, nursery habitats and water filtration, UWC’s Faculty of Natural Sciences Associate Professor Anusha Rajkaran said that estuaries contribute R 4.2 billion per annum to the country's economy and are therefore highly valuable areas.
"Estuaries play a vital role in supporting, maintaining and increasing biodiversity along South Africa's coastline. Many plants and animals find shelter and refuge in these coastal features that link the rivers to the sea, as they provide a low-energy environment away from the high-energy coast. Mangrove forests, salt marsh, sea grass, and other estuarine plants provide a home for a myriad of life in the soil (crabs, worms, mud and sand prawns), the water column (zooplankton and fish) and for those that visit the estuaries from time to time, such as birds.
"The economic contribution and quality of ecosystem services are determined by the pressures that our estuaries face and the resulting changes to their ecological state. These pressures have altered many of the estuaries in South Africa and have decreased their resilience to recover from disturbances and their ability to support our everyday lives. The number of species and population sizes of many animals have decreased. Estuaries affect the health and wellbeing of those who live close to them, and in the rural parts of our country, they may no longer be able to support the coastal communities that rely on them for sustenance and economic gain," said Rajkaran.
Estuarine Ecologist Dr Daniel Lemley said that given that estuaries are naturally vulnerable to anthropogenic manipulation due to their inherent variability, approximately 75% of the 290 estuaries in South Africa are classified as either temporarily or predominantly closed to the sea.
"Recently, the health of South African estuaries was shown to be deteriorating. This downward trajectory was largely due to a 33% reduction in the volume of freshwater inflows reaching the coast, significant modification to 63% of overall estuarine habitat, and approximately 34% of the country’s estuaries are considered to be facing severe pollution pressures related to agricultural, industrial, stormwater, and wastewater discharges.
"From an environmental and human health point of view, increased nutrient loading and modified freshwater inflow patterns via the process of eutrophication are often the root causes of undesirable changes in estuaries. Eutrophication reduces the ability of estuaries to function normally and provide desired ecosystem services. Some of the most common consequences of eutrophication include the proliferation of potentially toxic harmful algal blooms (HABs), oxygen depletion, reduced biodiversity, loss of important nursery habitats, imbalanced food webs, and mass faunal mortalities.
"There are numerous examples of South African estuaries where eutrophication has been documented to cause socio-economic and ecological losses. For example, a loss of seag rass habitats and mass faunal (e.g., fish and invertebrates) mortality events have been documented in the Zandvlei, Hartenbos, and Swartkops estuaries due to HAB events.
“Additionally, many of the small, temporarily closed estuaries along the KwaZulu-Natal coastline are subject to daily nutrient-rich discharges from wastewater treatment works, resulting in a loss of ecosystem functionality and an increased abundance of invasive alien aquatic plant species. Furthermore, increased water abstraction and development of low-lying properties have led to an increased need for management authorities to intervene with regard to managing estuarine mouth conditions. This practice of artificially breaching temporarily closed estuaries is costly and interferes with natural processes such as fish recruitment," said Lemley.
Rajkaran added that the response to these pressures must begin with increasing the understanding of the importance of estuaries and maintaining equitable access for all South African citizens to enjoy the ecosystem services provided by estuaries.
"The realisation that what happens in our rivers affects the estuaries and the sea must be clearly communicated to all, meaning that dumping in rivers alters the riverine environment as well as the estuary that connects it to the ocean. Implementation of our laws that govern estuaries and the surrounding ecosystems is paramount to decreasing the pressures that estuaries face. Estuarine management plans should be prioritised for all estuaries to ensure sustainable and equitable access and use of these systems," said Rajkaran.