Conserving Reservoirs

Ecological Challenges

Like many man-made water bodies, Malilangwe Dam has faced environmental challenges over the years. We are fortunate to have had the resources to study these challenges and their causes, enabling us to better understand the Malilangwe ecosystem and how to protect it moving forward.

One of the most significant issues we’ve faced was an outbreak of the parasitic crustacean Lernaea cyprinacea, commonly known as anchor worm, which severely affected fish populations. Research found that four fish species were infected, with Mozambique tilapia and black tilapia showing the highest infection rates. We believe it may have been accidentally introduced during efforts to control an earlier outbreak of Azolla, an invasive water fern. The incident highlighted how solving one ecological problem can sometimes create another, and led to stricter biosecurity measures: fish are now quarantined for two weeks and treated with potassium permanganate during the final 24 hours before being moved into any water body within the reserve.

Blue-green algal blooms have been one of the dam’s more recent ecological challenges. Malilangwe Reservoir supplies water to several of the reserve’s larger artificial water points via an abstraction pipe. In recent years, we’ve observed recurring surface algal blooms of blue-green algae in the Dam. Our research team hypothesised that sediment disturbance caused by water extraction may have been contributing to these blooms by stirring nutrients into the water column. In response, abstraction from the bottom of the dam wall to our largest pan, Banyini, was halted in 2024. To date, this intervention appears to have been effective, and water has been pumped to the Banyini pan from a surface level abstraction point instead. We plan to continue to test this hypothesis over the next 5-10 years.

Another ecological challenge that may become increasingly important over the long term is siltation resulting from catchment erosion. A large portion of the Malilangwe Reservoir catchment extends beyond the reserve’s boundaries, where land-use changes can influence the amount of water and sediment entering the reservoir. Historically, this was less of a concern, as the surrounding landscape consisted largely of lightly grazed commercial ranch land. Since 2000, however, changes in land use have increased pressure on the catchment and altered natural runoff patterns. Reduced grass cover in some areas means that heavy rainfall now generates greater surface runoff and erosion, carrying increasing quantities of sediment into the reservoir each year. Earlier studies suggested that siltation would affect the reservoir only gradually over the long term, but ongoing changes in the catchment indicate that this assessment may need to be revisited as conditions continue to evolve.