Water holes typology as a function of human and ungulate Activities in Hwange National Park, Zimbabwe

Abstract

Small aquatic ecosystems in semi-arid environments are characterised by strong seasonal water level fluctuations. Artificial pumping of ground water to maintain water resources throughout the dry season may affect the limnochemical characteristics of the aquatic ecosystems. The limnology of water pans (pumped and unpumped) in Hwange National Park (Zimbabwe) was studied by determining the seasonal, spatial variation of selected physico-chemical parameters, investigating environmental variables affecting plankton communities in the study sites and assessing of organic matter inputs from terrestrial herbivores. Nineteen temporary and permanent water pans were sampled for seven months in 2013 across the wet (February, March, April, and December) and dry seasons (July, September and November) and analysed for standard physicochemical parameters. Results showed an increase in conductivity, hardness, magnesium and calcium when temporary pans dry up, and when permanent ones are filled with ground water. The concentrations of ions and hardness of water was higher in pumped pans due to groundwater. Higher turbidity was observed during the rainfall season in natural pans, while in artificial pans it occurred during the dry season. The water in all pans showed alkaline conditions. The pumped pans also recorded the highest concentrations of nutrients (total nitrogen and ammonia) during the dry season. In particular, in times of water scarcity, animals gather around artificially maintained waterholes and foul water with faeces and urine, thus inducing water eutrophication. Seasonal variation in conductivity, hardness, turbidity, calcium, magnesium and turbidity suggests the influence of water level fluctuations associated with rainfall, evaporation and pumping activities. Seasonal and spatial distribution of plankton communities and the effect of environmental variables on plankton community composition were investigated in the same temporal, natural and permanent, artificial pans for the same period. In total, forty-three phytoplankton species belonging to thirty-seven genera and fifty-six zooplankton species belonging to twenty-three genera were recorded. Species richness and diversity decreased in both phytoplankton and zooplankton communities during the dry season, although this was more pronounced for zooplankton. During both the dry and wet months, diatoms and rotifers dominated the phytoplankton and zooplankton communities, respectively, in both permanent and temporal pans. Canonical correspondence analyses showed salinity, vegetation, calcium, nutrients, dissolved oxygen and turbidity as the different environmental factors that influenced the plankton community structure in both natural and artificial water pans. An assessment of direct nutrient inputs of terrestrial wildlife into the studied shallow water bodies was also carried out for six months in 2014. Dung from different herbivore species was collected during the dry (June, August and October) and wet (February, March and May) seasons to measure nutrient content and estimate nitrogen and phosphorus leaching rates. Results obtained showed changes in dung nutrient composition between the seasons. The deposition during the dry season of total carbon, nitrogen, and phosphorus, mostly due to elephants, was estimated to be 8.65, 0.25, and 0.06 g/m²/day, respectively, while the leaching rates of dissolved inorganic nitrogen and phosphorus were 0.11 mg and 0.17 mg m²/day, respectively. Water bodies which do not dry up during the dry season attracted wild animals, which caused significant nutrient enrichment. During the wet season there were strong surface water inflows which coincided with reduction of nutrients, conductivity and water hardness levels leading to increased aquatic production (zooplankton and phytoplankton biomass).During the dry season there was contraction of the studied freshwater ecosystems coupled with loss of flow, reduction of clarity due to increase of nutrients from allochtonous organic matter and establishment of cyanobacterial blooms. Due to the small size of the studied waterholes conditions deteriorate within the ecosystems during the dry season with noticeable change in plankton assemblages. The wet season flushing resets the system. In conclusion spatial and temporal variation was observed between the study sites (natural and artificial water pans) with characteristics of both types of pans being influenced by organic matter from terrestrial herbivores.