Invasive alien aquatic plants in estuaries: From science to management

Global change, under the influence of increased anthropogenic disruption, presents one of the greatest challenges of the modern age. Estuaries are situated at the confluence of terrestrial and marine environments and are especially vulnerable to these pressures due to their proximity to land-based activities. These activities typically enhance anthropogenic nutrient enrichment of these ecosystems, culminating in accelerated rates of eutrophication. The heightened proliferation of invasive alien aquatic plants (IAAPs) is a common eutrophic response in the freshwater reaches of estuaries that highlights the escalating impact of human activities. This study investigates the significant threats IAAPs pose to the ecological integrity of estuaries by disrupting native biodiversity and altering ecosystem function. This therefore has implications for local fauna and flora as well as the socio-economic implications for communities reliant on these ecosystems. Focusing on the National Biodiversity Act as a framework for managing estuaries in South Africa, the study explores the mechanisms of invasion in various estuarine environments, emphasising the vulnerability of highly polluted estuaries and those that periodically close to the sea (temporarily closed estuaries). Estuaries known to support IAAPs in South Africa were investigated to assess the impacts that IAAPs pose to their ecological integrity. The distribution and density of IAAPs (i.e., Azolla filiculoides, Ceratophyllum demersum, Lemna gibba, Myriophyllum aquaticum, Pistia stratiotes, Pontederia crassipes and Salvinia molesta) in selected South African estuaries were quantified to understand their prevalence and spread. Moreover, the study explored the pathways and factors contributing to IAAP invasion in estuaries, including nutrient input from land-based activities and habitat alterations. This was achieved through field surveys, water quality analysis, and stakeholder engagement, allowing for a comprehensive assessment of the mechanisms of invasion and their potential ecological impacts. The study found that IAAPs are present in almost all estuary types in South Africa and are common along the subtropical east coast due to substantial wastewater inputs and the predominance of freshwater conditions which favour their growth. By synthesising the current scientific findings with actionable management strategies, this research aims to bridge the gap between science and policy, providing stakeholders with a roadmap to address the challenges posed by IAAPs. The findings will contribute to management efforts geared towards preventing and/or reversing the continued deterioration of water quality and spread of IAAPs in South African estuaries, as highlighted in the latest National Biodiversity Assessment. Investigations such as these are critical given that the persistence of invasive species is an indication of ecosystem deterioration and a loss of natural resilience. Thus, early detection of ecosystem health degradation enables more cost-effective and successful management interventions.