Coast KZN

18 May 2019

Sewage leak at Durban harbour threatens unique ecosystem

Duncan Guy ( Independent on Saturday) Picture: Wayne Bezuidenhout watches the dirt after sewage leaked onto the Durban harbour last week Picture Bongani Mbatha / African News Agency (ANA)

Durban – Durban Bay may give life to the country’s economy as ships come and go through its mouth, but for far longer, the estuary has been the heartbeat of marine life in its waters and in the open sea.

“It plays a really important role in terms of being nurseries for a whole host of living things, from tiny things to brindle bass and sharks,” said Jone Porter, education director at uShaka Sea World.

“It provides food for several areas of marine life.”

Being filled with sewage at a rate of 720000 litres an hour, as happened recently, has a huge impact.

“There’s a huge response from microscopic plants, which grow fast, using oxygen which they suck out of the water. Also, sunlight doesn’t get through the sewage.”

Durban Bay is unique because it is at a temperate latitude but is sub-tropical in nature. This is because the fast-flowing Agulhas Current brings warm water from the Equator that flows fast because of the coastlines of Madagascar and Africa creating a channel. After Kosi Bay and St Lucia, Durban Bay is also the largest estuary on the KZN coast.

Although the face of the bay has changed – with the Customs House standing where the Mgeni River once entered the bay, the Congella Hotel sitting on what was once a sandy beach, and several stormwater drains playing the role of former natural streams – nature remains hard at work.  Porter stressed that the drains needed to be properly managed.

“Stormwater is very important, it’s part of the freshwater inflow that makes the Durban Bay work. That’s why it’s important to manage the stormwater drains properly,” she said.

“The Bayhead mangroves are remnants of the huge number of mangroves once in the bay, but are still vital and seen as one of the critical global habitats,” Porter added.

“Because of how a mangrove ecosystem works, they pump a whole variety of nutrients into the bay’s ecosystem. The mangrove forest is considered some of the most productive land in the world. But it can’t be used for agriculture.

“If you took scoops of mud from the mangroves, or the mudflats or inter-tidal sandbars, you’d see a layer on the top covered in animal life. People think it’s just yucky mud, but there’s an amazing structure within it that houses a whole range of different species and food webs in that layer.”

Porter said estuaries such as Durban Bay acted as important nurseries for many coastal species.

“A number of fish retreat at some stage of their life cycle to an estuary.”

It also attracts many wader birds.

“Then it acts as a filter, cleaning river water before it enters the sea.”

Porter said every corner of the bay was a unique habitat, shaped by currents, winds, water depths, temperature and the available sunlight. She also said the harbour filled a huge space in the lives of people who used it for recreation and fishing.

“I hope incidents like the sewage leak put pressure on everyone to come up with a plan to fix the problems that caused it.”