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Ecological monitoring leading to decision making: the use of stable isotopes technique for freshwater biological monitoring of two river systems in the Eastern Cape Province

By Samuel Motitsoe*, Biological Control Research Unit, Department of Zoology and Entomology, Rhodes University

In recent decades, the exponential increase in human populations across the globe has led to rapid developmental changes taking place within our landscapes, particularly in developing countries like South Africa.

In trying to meet the challenge of supporting these drastic increases in human populations, nations are faced with numerous social, economic and, most importantly, ecological challenges.

South Africa, like most African countries, depends directly on its natural resources to generate economic wealth. However, economic research cautions the rapid use of these natural resources so as not to surpass their rate of replenishment. With large populations to look after, governments are striving to create a better quality of life for all through food security and agricultural practices; job creation and/or economic wealth, particularly the sustainable mining of mineral resources; and the construction of dams for hydropower and to provide clean water and sanitation, with the ultimate goal of raising life expectancy.

Unfortunately, high rates of land degradation and transformation, coupled with limited enforcement of responsible agricultural and mining practices, have contributed immensely to the poor state of our landscape and catchment areas. The lack of basic services, such as proper sanitation, further worsens the situation.

Ecological impacts

The resulting ecological damage includes excessive soil erosion, physical and chemical pollution, high rates of eutrophication, alien plant invasion, loss of native biodiversity (both terrestrial and aquatic) and cascading ecosystem effects, which often result in catastrophic and irreparable degradation of aquatic environments and the extirpation of native organisms. Additionally, the degradation of these aquatic ecosystems can also lead to the introduction and proliferation of water-borne and domestic disease.

Freshwater resources have been listed among the most threatened ecosystems globally. In South Africa it is unlikely that any effective management plan will allow us to reverse the damage already done to our freshwater systems. However, understanding and protecting what is left of these systems is essential to the country’s economic and biological health viability.

Over the past decade, researchers have being trying to raise awareness about the importance of freshwater resources and there have been some positive and promising assessment protocols which are aiding in ecological monitoring and decision making for many vulnerable ecosystems.

Tracing nitrogen loading in two local river systems

As a Master’s student at Rhodes University, the primary aim of my study is to field test the applicability of d13 C and d15 N isotope values of the duckweed Spirodela spp., to trace nitrogen (N) loading in two local river systems previously reported to be impacted by anthropogenic nutrient inputs - the New Year’s-Bushman’s and Bloukrans-Kowie river systems in the Eastern Cape. Prior to this study, baseline investigation of Spirodela spp. confirmed that in the laboratory, the isotope values of these plants reflect the nitrogen regime under which they were grown, showing both nutrient species and concentration level effects.

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Samuel Mititsoe demonstrates how Spirodela spp. were incubated in all study sites (Picture: Tara O’Neill – Bushman’s-New Year’s river system)

An experimental design showing greenhouse cages housing duckweed Spirodela (Picture: Tara O’Neill – Bushman’s-New Year’s river system)

In 2013, Spirodela plants with known isotopic values were transplanted from the laboratory to the field at 10 sites (n=5) in each river system. Each replicate of transplanted plants was housed in small greenhouse cages, tethered using designed PVC weights. The study ran for a period of 13 months (August 2013 - August 2014). Isotope samples were collected monthly from each greenhouse cage in both systems, together with on-site physical and chemical variables. Ecological health assessments (water quality, using SASS5) were also conducted quarterly.

Pollutant pathways

Preliminary investigations of nitrogen tracing using d15N ratios (August 2013 - March 2014) have yielded impressive results, with incubated and translocated Spirodela plants showing clear dynamic patterns in N-loading over time. The majority of sites in the Bushman’s-New Year’s and Bloukrans-Kowie river systems proved to be heavily impacted, with enriched d15N ratios describing strong inputs of sewage and/or manure nitrogen (d15N > 10 ‰).

Stable isotope techniques not only identify anthropogenic nitrogen pollution, but also broadly support the current existing ecosystem health assessments determined by SASS5 applications, yet provide a better time-scale resolution on the dynamics of anthropogenic nutrient loading over time (sites indicating impairment via SASS Score of <90, 90 - 115, 115 - 140 showed enriched d15N of > 10). Further data mining, graphical interpretation and statistical analysis will be completed once all data has been analysed.

In the longer term, the technique should also allow us to identify the eutrophication status of freshwater systems, to provide an early warning system for deteriorating watershed quality, to determine the origin of nitrogen sources, to investigate pollutant pathways through our environment in order to identify sites of management interest.


I would like to thank my supervisors, Dr Jaclyn Hill and Prof. Martins Hill of Rhodes University, for their guidance and support during this study. The Water Research Commission (WRC) and National Research Foundation (NRF) are acknowledged for financial assistance.

* Samuel Motitsoe is a member of SAEON's Graduate Student Network (GSN). He scooped top honours for his presentation entitled “Mapping nitrogen loading in freshwater systems: using aquatic biota to trace nutrient” at the 2014 GSN Indibano. This article is based on his research. For further information, contact Samuel on samy4045(at)

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