Effects of elevated atmospheric carbon dioxide concentration and nutrient enrichment on freshwater plant species

Despite being extremely species-rich and highly threatened, freshwater ecosystems have not attracted as much conservation and research efforts as terrestrial or marine ecosystems, specifically in relation to global environmental change. Therefore, the overarching aim of this thesis was to examine the effects of rising atmospheric CO2 concentration ([CO2]) and altered nutrient conditions on a select number of invasive exotic freshwater plant species relative to their native and naturalised counterparts in Australia. Firstly, a systematic description to establish the current status of exotic freshwater plant species with reproducing wild populations in Australia was conducted. It was found that there are 63 exotic freshwater species with wild populations including nationally invasive species and species that are invasive in some states (declared weeds). Europe, South America and North America were the main sources of these species and the most important introduction pathway was the ornamental plant trade. The systematic description was then followed by experiments testing the growth, reproduction, functional trait and competitive responses of a range of species with contrasting invasive status to elevated [CO2] and varied nutrient conditions. In most cases, the results showed that elevated [CO2] and nutrient enrichment promoted plant growth either independently or in interaction with each other, and did not mediate competition between the tested species. Furthermore, the growth effects were species-specific and not strongly linked to the invasive status of the species. It can therefore be concluded that exotic freshwater plant species are an important component of Australian freshwater systems and their response to global environmental change is largely species-specific. In addition, based on the number of species that showed increased growth under nutrient enrichment, it is likely that it will have a more profound direct effect on the freshwater systems than elevated [CO2].