Management of water hyacinth (Pontederia crassipes) on the Hartbeespoort Dam in the North West, South Africa

Abstract

Invasion of freshwater bodies in South Africa by Invasive Alien Plants (IAPs) is common. This is primarily due to the construction of impoundments to meet water demands of the water-scarce country which have altered the hydrology of rivers, and this linked to anthopogenically driven eutrophication creates the ideal habitat for these plants. The Hartbeespoort Dam, a hypertrophic urban freshwater system in a temperate region of South Africa, is prone to aquatic weed infestations and algal blooms owing to the influx of nutrient from the urban spaces and agricultural activities upstream of the Dam. Pontederia crassipes (water hyacinth) first infested the dam in the early 1970s, leading to a 24-year chemical control programme which ended in 2016 with a moratorium on herbicide use. Biological control agents, introduced to the dam in the 1980s, had been hindered by the constant use of herbicides, and the moratorium enabled the proliferation of these agents. Although the Neochetina weevil spp. N. eichhorniae, and N. bruchi as well as the mite, Orthogalumna terebrantis, mirid, Eccritotarsus catarensis, and the moth, Niphograpta albuguttalis, were present, the weed was still not under control. The hypertrophic state of the dam helped the plants compensate for herbivory by the agents while the cool winters with frequent frosting events experienced in the highveld had a deleterious impact on the biological control. The main aim of this thesis was to evaluate the impact of Megamelus scutellaris, a newly introduced water hyacinth biological control agent, on water hyacinth coverage on Hartbeespoort Dam. The second aim was to compare the impact biological control and mechanical control (manual removal) had on the aquatic ecosystem and if monocultural stands could still provide ecosystem services. The final aim was to establish the perception residents and visitors had about water hyacinth and to determine the most suitable methods for funding biological control activities. The hypothesis that augmentative releases of the newly introduced plant hopper, Megamelus scutellaris would reduce the water hyacinth coverage on the dam was not rejected, as, in January 2020 the insect density for M. scutellaris exceeded 6000 insects/m2 which coincided with the reduction of water hyacinth coverage to below 5%. Monthly plant parameters and insect densities were assessed from October 2020- October 2022 to establish impact the introduction of M. scutellaris had on plant parameters and overall coverage of the dam by water hyacinth as confirmed through satellite imagery. Annually, in the spring there was a reinfestation of the dam through water hyacinth seedling recruitment necessitating the annual augmentative releases of M. scutellaris. The cold winters and frost adversely impacted insect numbers resulting in the spring recruitment of seedlings being largely free of insect damage. To reduce the lag phase between the spring growth of water hyacinth coverage and the increase in agent populations augmentative releases of the insects are essential. The involvement of community-based satellite mass-rearing stations assisted with the release of high numbers of healthy insects into the system, even during the colder months. This is the first study where biological control alone has managed to reduce water hyacinth coverage on a high elevation, cool, temperate hypertrophic system. This approach is contrary to previous literature which indicated that an integrated management method that integrates elements of mechanical, chemical and biological control would be the only way to achieve control of water hyacinth. Prior to 2020, the residents around the dam had used manual removal and mechanical control to manage the weed. The impact of mechanical and biological control on biodiversity of a previously colonized aquatic ecosystem was assessed. This was used to acquire empirical evidence that would inform future management strategies for water hyacinth. Mechanical removal leads to a drastic increase of nutrients in the water column which led to a proliferation of cyanobacterial blooms and therefore macrophyte removal has a positive impact on phytoplankton which is in direct competition with water hyacinth for nutrients and light. For other biodiversity indices macroinvertebrates decreased with macrophyte removal while removal had no impact on zooplankton. Comparatively, biological control allows the slow recovery of native macrophytes which led to a more diverse macrophyte population, responsible for keeping cyanobacteria numbers low. However, the biological control did lead to an increase in sedimentation. This can be reduced by integrating physical removal as the plants display severe leaf necrosis and browning. An undesired impact of the temporal absence of water hyacinth due to biological control facilitated the rapid increase in the invasive alien floating fern, Salvinia minima on the Dam. Hartbeespoort Dam’s revenue is generated through tourism and residential estate developments; therefore, the pristine state of the dam is essential for business, residents and visitors. A socio-economic survey was used to determine respondents’ view of water hyacinth as a nuisance and their willingness to pay for its management. Additionally, the socio-economic factors that might influence willingness to pay were assessed. 299 electronic and printed surveys were completed but only 281 surveys were used for data analysis. Willingness to pay was not significantly influenced by the negative perception that respondents have of the aquatic weed, instead the respondent’s gross income and residential status were the most influential factors for willingness to pay. Their willingness to pay was not influenced by the clearance level for the weed. In conclusion the biological control is a better method for the management of water hyacinth and the dam’s biodiversity, however if the high nutrient influx continues unabated then the Hartbeespoort Dam will continue to have invasiv mass development through what is referred to as the invasive cascade, where one weed species is replaced by another after control. Aside from willingness to pay for its management an option of contributing labour should be given for residents to get involved in the management of water hyacinth by maintaining insect stocks at satellite mass rearing sites over winter and throughout the summer. A secondary biological control programme is needed for S. minima. Lastly, the management of water hyacinth in cool temperate regions with high levels of eutrophication requires strategic adaptive management.