The transfer of terrestrial organic matter by terrestrial wild and domestic animals when they urinate and defecate directly in savanna rivers has already been studied. However, the eulittoral zone around ponds receives organic matter during the dry season, which is diluted in the ecosystem when the water body returns to its wet season level. In our study, we evaluated this pathway of subsidies in Hwange National Park, Zimbabwe, by estimating dung density in the eulittoral zone at the peak of the dry season. We also collected dung from different herbivore species during the dry and wet seasons to measure nutrient content and estimate nitrogen and phosphorus leaching rates. Our results show a decrease in carbon:phosphorus and nitrogen:phosphorus ratios in the dry season compared to the wet season. During the dry season, the deposition of total carbon, nitrogen, and phosphorus, mostly due to elephants, is estimated to be 8.65, 0.25, and 0.06 g/m²/day, respectively, while the leaching rates of dissolved inorganic nitrogen and phosphorus are 1.52 mg and 6.59 mg m²/day, respectively. No specific coloured dissolved organic matter signature for dung was identified.We discuss the temporal dynamics of the subsidies as a distinctive feature of the system.

No abstract

Small aquatic ecosystems in semi-arid environments are characterised by strong seasonal water level fluctuations. In addition, land use as well as artificial pumping of groundwater to maintain water resources throughout the dry season may affect the functioning of aquatic ecosystems. In this study, we investigated pans situated in and around Hwange National Park, Zimbabwe, where certain waterholes are artificially maintained during the dry season for conservation purposes. We monitored 30 temporary and permanent waterholes for 7 months across the wet and dry seasons in 2013, and analysed them for standard parameters to investigate seasonal variations, assess the effects of land use and pumping on lake functioning, and determine the driving factors of these aquatic systems. Results show an increase in conductivity, hardness, and turbidity when temporary pans dry up and permanent ones are filled with groundwater. Prominent parameters explaining the diversity of aquatic ecosystems are water hardness, conductivity, turbidity, and the presence of vegetation. Seasonality differences in certain parameters suggest the influence of water level fluctuations associated with rainfall, evaporation, and pumping activities. Further, the distinction between turbid pans and those with clear water and vegetation suggests the alternative functioning of pans. Land use had no significant effects, while the effects of pumping are discussed. In times of water scarcity, animals gather around artificially maintained waterholes and foul water with faeces and urine, thus inducing water eutrophication.

Semi-arid climate aquatic ecosystems may be highly seasonal due to large-amplitude waterlevel fluctuations. These resulting habitat changes impact aquatic communities and food web structures as a result of species replacement or loss. To study the impact of water-level fluctuations on planktonic food web structure, we compared food web descriptors in contrasting seasons. We examined the planktonic food webs associated with semi-arid natural and artificial waterholes sampled in Hwange National Pak, Zimbabwe, during the hot dry (HDS), wet (WS), and cold dry seasons (CDS). Natural waterholes are filled by rainwater, while water is pumped into artificial waterholes to provide drinking water to wildlife during the dry season. We computed the descriptors of 66 food webs. WS food webs tend to be more complex than those of HDS and CDS; the percentage of basal species was lower, whereas the food web connectance, percentage of intermediate species, omnivory index, mean trophic level, and maximum trophic height of species were higher. Pumping water to maintain pans in the dry season had no effect on food web properties. The drying up of waterholes in semi-arid environments simplifies planktonic food webs. The dominance of small rotifers may explain the observed low omnivory index and food web connectance in HDS compared to those observed in other studies.

Freshwater communities can vary greatly across space and time. Studying these variations, 2 that is spatial and temporal beta diversity, provides fundamental information on the processes 3 that maintain diversity and on the consequences of environmental changes on communities. 4Recently, drying events have been shown to strongly affect the spatial and temporal beta 5 diversity of temperate freshwater ecosystems, but the effects of such events are mostly 6 unknown for freshwater communities in semi-arid climates that are frequently submitted to 7 drying up. In addition, studies have so far focused on variations in species composition that is 8 species beta diversity, while variations in species trophic interactions that is food-web beta 9 diversity can give additional insights on how community functioning vary in space and time. 10 Here, we combine species and food-web perspectives to explore the spatiotemporal beta 11 diversity of plankton species and their trophic interactions in waterholes undergoing different 12 water-level regimes: either an alternation between dry and water-full states, or a permanent 13 water-full state due to water pumping. Our results show that waterholes with artificial water 14 pumping do not differ from natural waterholes in their contribution to spatiotemporal beta 15 diversity. Instead, beta diversity is strongly driven by temporal variations of species 16 composition and food-web structure during the dry season, which is characterized by 17 degraded planktonic communities with a low richness, diversity and connectance. Species-18 and interaction-based approaches give complementary information on the spatiotemporal beta 19 diversity, as they highlight different planktonic communities with contrasted functioning. 20 21 Keywords 22 Food-web beta diversity, food web connectance, trophic level, Hwange National Park, planktonic food 23 web, temporary ponds 24 communities and their dynamics have been widely studied in isolated freshwater ecosystems. 5 However these ecosystems are connected by biological processes such as individual active 6