Diatom of the Month August 2018 - Diatoms and large scale conservation ecology in the Okavango River, Botswana
On this month´s Diatom of the Month series of posts, Mia Otto explains how diatoms may inform conservation actions in one of the most ecologically important South African water system, the Okavango Delta and its river.
By Mia Otto*
Large scale ecosystem conservation and management are quickly becoming the focal points of sustainable development and associated policy and decision-making processes world-wide. Especially in Africa, where freshwater resources often cut across political boundaries, the management and conservation responsibilities are often not well defined or easy to manage. One such complex ecosystem is the Okavango Delta in Botswana, the largest of only three inland deltas on the continent and the only one emptying into a semi-desert region, the Kalahari.
Figure 1.The panhandle section of the Okavango River remains vulnerable to unsustainable upstream development , anthropogenic disturbances and unplanned rural sprawl. The panhandle is characterised by palm tree islands (Moonlight Island pictured here) and papyrus-lined channels.
The Okavango Delta is a globally important wetland that is protected by the Ramsar Convention and was recently declared a UNESCO World Heritage site. The upstream panhandle section of the Okavango River in Botswana is often neglected in ecosystem assessments and conservation debates. It is merely seen as the thread which connects the upstream Angolan highlands and Namibian Kavango sections of the river with its downstream showstopper, the delta (Sammy & Opio 2005; West 2010). So much so that the Permanent Cubango-Okavango River Basin Commission (OKACOM)  agreement between Botswana, Namibia and Angola, makes no mention of the panhandle.
In Africa, diatoms are rarely used to make the case for large scale ecosystem conservation and so, with serious ambition, we set out to work towards doing just that in July 2014. The team included my head supervisor, the then Head of Department, Prof Jo Van As, his wife and co-supervisor Prof Liesl Van As, myself and a few other aquatic science students.
We sampled diatoms in the main river channel and in some associated tributaries such as the Nxamaseri Floodplain . We identified a total of 4,486 diatom valves of 69 different species across 10 sites. It was found that the panhandle diatom community composition is characterised by Eunotia minor (Kützing) Grunow, E. formica Ehrenberg, 1843, Asterionella formosa Hassall (1850) and Fragilaria biceps Ehrenberg, 1843. Typically,E. minoris known to occur in circumneutral waters (pH 6.5 – 7.5) while E. formica is found in slow flowing dystrophic to oligotrophic waters (water with low nutrient levels and low productivity) (Taylor et al. 2007). This makes sense since we know that the undisturbed areas of the panhandle have a lower nutrient content than that of bottled water (West 2010).
The Okavango Delta is a biodiversity hotspot, but the lower Okavango River’s water is comparable to that of an aquatic desert due to the low levels of dissolved nutrients and minerals . The plants are well adapted to this and the low nutrient content has been found to contribute to keeping the delta in its mostly pristine condition. However, A. formosa and F. biceps, which more commonly occur in meso-eutrophic waters (with a moderate level of nutrients and biological activity), were also found in high abundances in the panhandle (Taylor et al. 2007). The high numbers of these diatom species may be explained by the floating papyrus beds to which the introduced substrate5 was attached. Papyrus traps nutrients forming highly productive micro-environments for algae in this otherwise nutrient poor environment. Small scale differences in physico-chemical conditions lead to significant differences in community composition, thus highlighting the sensitivity of the primary producers within this ecosystem. The annual flood and the nutrient trapping papyrus beds play a critical role in making nutrients available to animals and plants within this ecosystem (Mendelsohn et al. 2010).
Figure 3: We used boats to explore different channels within the panhandle. The drifting papyrus beds are kept open by boats, flooding/flow patterns and hippo (a keystone species in the Okavango River). Left is Prof Jo Van As driving the boat down a side channel lined by papyrus and on the right we are in the main channel, which is much wider and open. We often spotted crocodiles and hippos in the main channel.
The delta and its abundance of wildlife would suffer greatly if the panhandle were to be degraded. West (2010) already showed how local anthropogenic impacts can lead to large changes in water quality. This has potentially devastating consequences for the local communities in terms of health, potable water avalability and sustainable goods and services (Darkoh and Mbaiwa, 2014). More than a million people in Botswana rely directly on the Okavango River for their livelihoods. A change in primary productivity in the panhandle could threaten the resilience and functionality of the delta, which would not only impact the tourism and wildlife industries (the country’s largest economic contributors after diamond extraction), but also the very survival of many local people.
Figure 4:The Okavango River is the main source of water and associated ecosystem services in Botswana. Top left: Women collect reeds from the river for construction and roofing purposes. Top right: Children play at a communal water collection point in a rural village. Water in the river is so clean that the tap water in these villages is taken directly from the river. Bottom: Local fishermen in a mokoro (wooden canoe) next to a wall of floating papyrus. Not only does papyrus beds trap nutrients, but they also serve as a hiding and breeding spot for many animals, such as fish and birds.
In a time when we are losing biodiversity at an unprecedented rate, the ecosystems and people in adverse environments are most at risk. Never before has there been a more important time to document the species living on our planet, but also to conserve, value and utilise them, and the information they provide (biomonitoring), to ensure ecosystems are managed in an equitable and sustainable manner for all (Sustainable Development Goals, 2030 Agenda and the Convention on Biological Diversity). We can hope that the Okavango Delta’s world heritage site status will aid upstream biodiversity conservation efforts to ensure the Okavango River remains in its current mostly pristine condition. Diatoms greatly support water quality and ecosystem health monitoring and management around the world and will also continue to contribute to scientific information, which supports policy and managerial decisions regarding large scale ecosystem conservation in Southern Africa.
*Department Zoology and Entomology, Faculty Agricultural and Natural Sciences,University of the Free State, Bloemfontein, South Africa.
Darkoh MBK, Mbaiwa JE (2014) Okavango Delta – A Kalahari Oasis Under Environmental Threats. J Biodivers Endanger Species 2:138. doi: 10.4172/2332-2543.1000138
Junk WJ. Brown M. Cambell IC Finlayson M. Gopal B. Ramberg L. Warner B.G. 2006. The Comparative Biodiversity of Seven Globally Important Wetlands: a Synthesis. Aquatic Sciences. Vol 68: 278-309.
Mendelsohn JM. van der Post C. Ramberg L. Murray-Hudson M. Wolski P. Mosepele K. 2010. Okavango Delta: Floods of Life. RAISON, Windhoek, Namibia. 144pp.
Sammy J. Opio C. 2005. Problems and prospects for conservation and indigenous community development in rural Botswana. Development Southern Africa. Vol 22 (1): 67-85. Published online at: https://doi.org/10.1080/03768350500044644 April 2011.
Taylor JC. Harding WR. Archibald CGM. 2007. An Illustrated Guide to Some Common Diatom Species from South Africa. Water Research Commission Report. WRC Report No. TT282/07.
United Nations Educational, Scientific and Cultural Organisation (UNESCO). 2014. Okavango Delta. Available at: http://whc.unesco.org/en/list/1432.html.
West DT. 2010 The Conservation condition of the unprotected Okavango Delta, Botswana.Magister Scientiae in the Faculty of Natural and Agricultural Sciences, Department of Zoology and Entomology, University of the Free State, South Africa.
Namibia has been looking for ways to bring Okavango River water down to Windhoek via a pipeline for years. Also a proposed hydro-dam development at Popa Falls has been under constant deliberation by NamPower Ltd. Both these and possible new anticipated developments in Angola, now that the civil war has ended, threatens the Okavango River in Botswana.
In an attempt to manage the Okavango River Basin as a single entity, Angola, Botswana and Namibia agreed to sign the OKACOM Agreement in 1994, in Windhoek, Namibia.
The Nxamaseri River used to be a major tributary of the Okavango River. Today, only a fossil riverbed remains which is annually inundated by floodwaters from the Okavango River.
5. An artificial substrate was used to sample diatoms in papyrus beds to make the samples comparable across sites.