'Novel ecosystem' is a concept which was introduced in the 21st Century to describe ecosystems heavily modified by humans, about 15 years after 'ecohydrology' had been introduced as concept within UNESCO IHP, to facilitate the sustainable management of aquatic ecosystems by humans and about 5 years after the concept of IHP 'Demonstration Sites' had been introduced to promote ecohydrological principles globally. The tropical African Lake Naivasha became a DS initially to demonstrate the importance of papyrus-dominated wetland edges for nutrient and climate control in an aquatic ecosystem driven by regional hydrological instability, but it already represented a ‘novel’ ecosystem. This paper critically examines the consequences of the aquatic food web restructuring by the major alien species, common carp (Cyprinus carpio L.) – whose arrival is directly an ecological consequence of hydrological instability. Carp were first recorded in Lake Naivasha in 2001 and reached dominance in the commercial fishery by 2003. The costs and benefits of its dominance are shown by comparing aspects of the ecosystem state before and after arrival. These were hypothesized and tested by comparing data in 2012-4 with data gathered in the 1990s using the same methodologies. Carp have filled a previously vacant benthivorous niche. The species achieved moderate density but has not caused ecological disruption. Overall, carp has been a positive contributor to the local community. More intensive management strategies, better post-harvest processing and new marketing techniques need to be developed to enhance financial gain.