The Gamtoos River estuary has closed for the first time in nearly 70 years in the latest sign of the fearsome drought gripping the Eastern Cape.
The last time the Gamtoos closed was in 1949 following a similar bone-dry period, Southern African Environmental Observation Network regional manager Dr Tommy Bornman said on Tuesday.
Despite the dwindling flow down the river, the level of the estuary has been rising since it closed at the end of June and the prediction is it should breach again within days.
But the situation highlighted the grim pressures facing estuaries around the country sandwiched as they were between a changing climate and increasing human demand, which together were eroding key natural benefits, NMU estuarine ecologist Prof Tris Wooldridge said this week.
The one aspect to consider with the Gamtoos closing was the estuary’s role as an important nursery for threatened spotted grunter, white steenbras and dusky kob, which were also popular recreational angling fish, he said.
The life cycle of these three fish species included a marine phase in which the juveniles needed to exit the estuary into the sea before returning later to mature and spawn.
“So if the estuary stays closed it could have a negative effect on their populations.”
The larvae of mudprawn, an important bait species, similarly needed to exit into the sea in spring before returning, guided by the chemical queue of outflowing river water, to mature and spawn in the estuary.
“Mud prawns live three years so if this cycle is repeatedly disrupted by the closure of the Gamtoos they could become locally extinct.”
Wooldridge said artificially opening the estuary would lead to more problems as without sufficiently strong river flow, the berm of blocking sand which had to be scoured out would just repeatedly reform.
On the other hand, if the estuary remained closed, trapped seawater, evaporation and reduced freshwater inflow could result in excessive salinity and a possible mass die-off of estuarine species.
There were 300 estuaries along the South African coast and more than 70% of these were now permanently or periodically closed, he said.
“We should not see this situation with the Gamtoos on its own. Our estuaries are in dire straits.”
Part of the pressure on the Gamtoos came from agriculture which was allocated water from the Kouga Dam at the top of the river, he said, and this extraction issue was an increasingly fraught one nationally.
“Estuaries in general across the country are affected by over-abstraction, drought and climate change.”
St Lucia in KwaZulu-Natal, a conservation and tourism flagship, the largest estuary in SA and one of the largest in Africa, is now closed for years at a time, with consequent hypersalinity and ecosystem crashes.
Healthy estuaries were crucial for biodiversity conservation, food security, recreation, tourism and the value of properties developed alongside them, Wooldridge said.
“We need to take cognisance of this and put pressure on the authorities to put maximum effort into properly balancing river flow quotas allocated for ecological functions, on the one hand, and direct human requirements on the other.”
Bornman and his team, who have a monitoring station at the Gamtoos, have been checking a range of pointers to estuary health, including salinity, oxygen, light penetration and the density of organisms.
The natural flow of the Gamtoos was supplemented by the return flow of farm irrigation water, heavy with fertiliser phosphates and ammonia from cattle manure.
With this nutrient load no longer being flushed out the mouth, one concern was that the estuary could become eutrophic and noxious algae could develop, Bornman said.
“But we think that things will sort themselves out and that the sand berm should be breached in the next few days.”