Diatom community composition and ecological gradients on selected rivers in the Eastern and Western Cape, South Africa

English: This study sought to assess diatom community composition across ecological gradients on selected rivers in the Eastern and Western Cape Provinces, South Africa, as well as in the lowland section of the Okavango River. For South Africa data were collected over a one-year period with three-monthly or seasonal sampling conducted between spring 2014 and winter 2015. Five to ten cobbles were scrubbed and diatoms fixed with 70% ethanol to produce an end product of >20% alcohol content. In Botswana samples were collected during July and August 2014. A phytoplankton net with a mesh size of 25 m and introduced substrate were sampled in the panhandle, Nxamaseri Floodplain and the Thamalakane River. The Hot HCl method was used to clean samples of organic material. Permanent slides were made using Pleurax as mounting agent. All information was added to the National Diatom Collection since one of the main objectives of this study was to fill the current information gap on these regions. A minimum of 400 individuals was identified per slide to produce a community composition. Multivariate statistics showed that diatom communities responded geospatially more strongly than seasonally. In South Africa the diatom communities responded at an Ecoregion Level 1. At Ecoregion Level II, catchment signatures were not strictly followed as would have been expected. Instead more localised impacts and natural fluctuations in physico-chemical changes were found to drive group formation. The influence of flow modification, such as the inter-basin transfer schemes in the Drought Corridor ecoregion, was indicated in diatom community composition. While diatoms are extremely useful as small-scale impact specific indicators and assessment tools, the results produced in this study showed that the use of diatom information for larger scale climate change impact monitoring initiatives towards sustainable freshwater resource management in future, remains untapped. Further research on the relationship between diatom community composition and more detailed environmental drivers of landscape scale ecosystem changes would greatly improve our understanding of the role diatoms play in the resilience of natural freshwater ecosystems to large-scale changes and impacts. The diatom data proved to be very robust and reliable in this study, suggesting that there exists a great resilience at the base of the food web in the highly volatile and dynamic African freshwater ecosystems. Properly functioning diatom communities could be a more important component of ecosystem resilience than currently recognised. This study also found that the exclusion of certain low abundance diatom information does not contribute to a more accurate result but instead removes valuable biodiversity information in a time when it should be promoted, protected and well documented. It is clear that in a country expected to experience severe and direct climate change induced impacts, the exclusion of diatoms when managing freshwater sustainability and continued optimal ecosystem functioning for the delivery of associated good and services, is rather reckless. This study has provided the foundation of updated diatom information for a majority of the major rivers in the Eastern Cape in particular and some of the smaller coastal rivers in the Western Cape. This method should be applied in other regions of the country to produce diatom reference conditions, which speak directly to Ecological Reserve scale approaches in order to contribute to a more holistic approach to ecosystem management in future. In Botswana a significant difference between the community compositions of the Thamalakane River, Nxamaseri Floodplain and the Okavango Panhandle was found to be present. These differences are suspected to be caused by micro-environments with differences in nutrient load and associated water quality. These micro-habitats allowed for some species, that are more tolerant to higher nutrient loads, to be found but did not interfere with the larger scale diatom community composition in a specific geographic region. The difference between diatom communities in the panhandle and other areas in the system highlights the importance of upstream conservation in the Okavango River for continued optimal ecological functioning of the downstream Okavango Delta and its associated systems. Diatoms are able to make a considerable contribution to our current understanding of freshwater resource functioning and the consequent conservation, monitoring and management of sustainable water security for all.