The littoral of Lake Dunstan, Central Otago, New Zealand, is dominated by the introduced submersed macrophyte Lagarosiphon major (hereafter Lagarosiphon) which effectively excludes native macrophytes from most of the littoral. Lake Dunstan is a large hydro lake that was - after reaching its operational water level in 1993 - rapidly colonized by Lagarosiphon from upstream sources. Because of its extensive biomass and growth, it is believed the macrophyte plays an important part in the lake ecosystem but also that it requires frequent management to allow for recreational activities, to restore aesthetic values of beaches and especially to prevent further spread into unaffected waterways. To assess the integration of Lagarosiphon into the lake ecosystem and possible impacts of extensive macrophyte removal a 3 year study was initiated focusing on invertebrate and fish distribution and diet in relation to Lagarosiphon. A stable isotope analysis was used to reconstruct carbon flows and trophic structure within the littoral of Lake Dunstan. Additionally, a small scale weed management experiment was carried out to assess possible impacts of macrophyte removal on invertebrates and fish. Lagarosiphon harboured a high density and diversity of macroinvertebrates. The invertebrate community was dominated by mollusc species in terms of biomass and abundance. There was little temporal and spatial variation in invertebrate distribution and communities on a large scale (lake-wide variation), but there were differences in invertebrate biomass on a smaller scale in relation to the location of samples within the macrophyte belt. Invertebrate biomass was lowest at the bottom of the dense macrophyte beds. The positive association of common bully (Gobiomorphus cotidianus: Eleotridae) with the dense Lagarosiphon stands and a diet - mainly consisting of invertebrates common on Lagarosiphon - suggested the important role of this macrophyte as a provider of food and shelter for small fish species. Brown and rainbow trout (Salmo trutta, Oncorhynchus mykiss: Salmonidae) diet was dominated by invertebrate species common on this macrophyte as well. Results of the stable isotope analysis suggested a secondary role of Lagarosiphon in the lake food web as a surface provider for the growth of epiphytic algae that appeared to be a major primary producer in the littoral of Lake Dunstan. Detritus (FPOM), assumed to consist partly of decaying Lagarosiphon, played a less important role as a base for the food chain. The main carbon flow pathway was from the primary producers (epiphyton - FPOM) through primary consumers (Mollusca, Chironomidae, Trichoptera) through to the top predators (tertiary consumers: trout and bullies) in the system. A small scale experiment on the influence of macrophyte harvest, i.e. the cutting of channels into dense macrophyte beds, showed that a partial removal of macrophyte biomass could help achieve several management goals: it not only reduced macrophyte biomass to enable recreational activities but also had no negative impacts on invertebrate biomass and diversity. The channels even increased invertebrate abundance and biomass in the treated compared to the untreated control plots along the cut edges. Regrowth of Lagarosiphon was rapid, with the cut channels being completely overgrown within 4 months. All these results taken together suggested a major role of Lagarosiphon in the littoral of Lake Dunstan. Lagarosiphon supported a species rich invertebrate community and was the main provider of food and shelter for fish in this lake. A partial removal of macrophyte biomass might be a more suitable management option than large scale eradication to avoid negative impacts on the lake ecosystem. The most important aspect in management of this weed remains the prevention of further spread into pristine waters.

Potentially damaging submerged invasive freshwater macrophytes have been identified in South African freshwater systems, but have received less attention than their floating counterparts. To ascertain the changes and effects that these species may have on macrophyte ecology, an understanding of the drivers of macrophyte assemblages is essential. The aims of this thesis were to investigate select abiotic and biotic factors driving introduction, establishment and spread of submerged macrophytes in South Africa. Surveys on the status of submerged plant species in South Africa were conducted to find out the distribution and diversity of the species present, imported to, and traded in South Africa. Numerous submerged indigenous and invasive macrophyte locality records were collected during field surveys, of which many were first time records. Pet stores and aquarist trading activities were identified as potential vectors for the spread of submerged macrophytes through online surveys and personal interviews. These results highlighted the potential these species have for continuing to enter, and spread within South African water bodies. Maximum Entropy (MAXENT) is a general-purpose method used to predict or infer distributions from incomplete information, and was used here to predict areas suitable for the establishment of five of these invasive macrophytes. Many systems throughout South Africa, particularly those in the subtropical coastal regions, were found to be climatically suitable for the establishment of Elodea canadensis Michx., Egeria densa Planch., Hydrilla verticillata (L.f.) Royle (all Hydrocharitaceae), Myriophyllum spicatum L. (Haloragaceae), and Cabomba caroliniana Gray (Cabombaceae). Despite the high probability of invasion, facilitated by vectors and suitable climate, South Africa’s rich indigenous submerged aquatic flora may be preventing the establishment of these submerged invasive species. Studies on the competitive interactions between a common indigenous submerged macrophytes, Lagarosiphon major (Roxb.) (Hydrocharitaceae) and M.spicatum, an invasive native to Eurasia, were conducted to ascertain which conditions influence competitive superiority. High sediment nutrient conditions significantly increased the growth rate and competitive ability of both species, while clay sediments significantly increased the competitive ability of L. major over M. spicatum, but sandy sediments improved the competitive ability of M. spicatum. These results highlighted the dynamic changes in competition between submerged species driven by abiotic factors, but did not take into consideration the effect that herbivory, a biotic factor, could have on competition between the two species. The effect of herbivory by phytophagous insects of submerged plant species has been regarded as negligible. To find out what this effect is, multiple field surveys were undertaken throughout South Africa to find natural enemies of indigenous Lagarosiphon species with the aim of identifying such species, and quantifying their influence on plant growth dynamics. Several new phytophagous species were recorded for the first time. An ephydrid fly, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae) was ascertained to be the most ubiquitous and abundant species associated with L. major in South Africa. The influence of herbivory by this fly on the competitive ability of L. major in the presence of M. spicatum was investigated using an inverse linear model, which showed that herbivory by H. lagarosiphon reduced the competitive ability of L. major by approximately five times in favour of M. spicatum. This study served to highlight the importance of herbivory as a driver of submerged aquatic plant dynamics. Current ecological theory emphasises the importance of investigating beyond plant-herbivore interactions, by including multitrophic interactions in community dynamics. Therefore, the potential of parasitism by a parasitoid wasp, Chaenusa luteostigma sp. n. Achterberg (Hymenoptera: Braconidae: Alysiinae) on H. lagarosiphon to shift the competitive interactions between the two plant species was also examined. The addition of the parasitoid reduced the effect of herbivory by the fly on L. major by half, thereby shifting the competitive balance in favour of L. major over M. spicatum. This study provides valuable insight into a selection of drivers of submerged macrophyte assemblages of South Africa. It highlights the precarious position of South African freshwater systems with regard to the potential invasion by damaging submerged invasive species. It also provides interesting insights into the effect of competition, herbivory and parasitism on the establishment and spread of species within submerged freshwater systems. Understanding the different influences could assist managers and policy makers to make validated decisions ensuring the integrity of South African freshwater systems.