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New SU study focuses on how black wattle impacts water quality
Author: Engela Duvenage
Published: 29/01/2016

​​​​​​Stellenbosch University (SU) conservation ecologists are taking a close look at how the quality of river water is improved once invasive black wattle is removed. Clearing projects around rivers have been ongoing nationwide for many years, in an effort to ensure increased water availability to South Africans.

"The current drought conditions are a stark reminder that water is the most limiting natural resource to development and a sustainable future for South Africa, and that we should take special care of it," says Dr John Simaika of the SU Department of Conservation Ecology.

He received a grant from South Africa's Water Research Commission (WRC) to study how the removal of black wattle (Acacia mearnsii) trees impact the surface water quality of rivers. The project is run in collaboration with colleague Dr Shayne Jacobs. Black wattle is ranked as one of the invasive species that have over the years caused the most damage to the distribution of riparian plants and animal life that are found naturally around the rivers of the Cape Floristic Region.

It is well known that the removal of these fast-growing and water-thirsty invasive plants around rivers helps to increase water flow and availability. In the process, natural vegetation such as fynbos gets a chance to regrow, while local insect species such as dragonflies often return to an area. This has been seen in many cases where tracts of invasive plants have been cleared by, for instance, teams from the national Working for Water program, after which specific management efforts have been put in place to replace the natural vegetation.

"While we know that clearing efforts improve water volume, little is currently known about the effect that these initiatives have on the actual quality of the surface water that passes through these cleared areas" says Dr Simaika. Good quality water is needed to ensure that the various lifeforms that rely on a river system are able to persist or return once a site has been cleared.

"The project will close a major research gap on the quality of water after clearing, and also about the dynamics behind stream ecosystem recovery," he believes.

Black wattle takes up nitrogen from the atmosphere and then enriches the soils in which these plants grow. This process of nitrogen fixing may boosts the invasive plants' ability to outcompete the natural plants around it. Researchers do not yet know what happens to the nitrogen in the soil once the invasive plants are removed.

"We want to find out if it washes into the streams during rain events, if it enters groundwater or does it have a negative effect on stream health," Dr Simaika explains some of the research questions that he hopes to answer. "Cape streams are typically nutrient poor, and therefore an increased nitrogen load could potentially have negative knock-on effects on algal growth and even the diversity of bigger insects."

The main aim of the project is to focus on the nitrogen cycling in soils within stands of black wattle and whether clearing efforts have a positive spin-off for the aquatic plants, insects and animals that naturally occur around South African rivers.

Three MSc students are part of the project. Zaid Railoun is investigated the rate of nitrogen fixation by Acacia plants, and how digestible their leaves are to bigger insects that forage on instream leaf litter.

Kenwinn Wiener is studying the amount of sediment trapped by the extensive root systems of black wattle and then settles at the river bottom and banks "It is likely that detritus trapped from blackwattle will significantly increase the amount of nutrient in the sediment," says Dr Simaika.

Jay-Dee Don is having a close look at the water plants and animals that are found around rivers, and what the effect of clearing invasive plants are on the various organisms that serve specific purposes within the water system. This includes algae, filter feeders and predators. "Measuring such functional diversity can be very informative and helps to tell you whether a specific area is healthy or impaired," Dr Simaika explains.

 For more information, contact Dr John Simaika of the Department of Conservation Ecology and Entomology at simaikaj@sun.ac.za