Nutrients in a Changing Environment: Implications on the Sustainability of the Okavango Delta

Abstract

The sustainability and integrity of riverine ecosystems depend partly on the balance between environmental requirements and the socio-economic developmental needs of a country. Scientific data can provide a critical foresight into the possible impacts on environment, brought about by natural geologic processes, land-use activities and climate change. Developmental changes in the shared Cubango/Okavango River basin are increasing, posing risk to the long term integrity of one of Africa’s Ramsar site housing rich biological diversity and Botswana’s tourism pride. Scientists have shown that the Okavango delta is an oligotrophic system, reflecting low levels of nutrients in surface waters and this state is critical for the sustainability of the delta. This study assessed the spatial and temporal variations of inorganic nitrates and phosphates species within the Okavango Delta to evaluate possible impact of reported land use changes on surface water quality and sustainability of the Okavango delta. The study was conducted over 3 years, from 2014-2017, covering a total of seven sites; four in the upper delta and three in the lower delta. Concentrations of NO3-N and PO4-P were measured monthly at Mohembo, Sepopa, Etsatsa, Xakanaka, Khwai, Maun and Lake Ngami in the Okavango delta. The results show nitrate concentrations ranging from below detection limit to 12.72 mg/L and 9.72 mg/L at high and low flows respectively. Phosphates on the other hand, ranged from below detection limit to 1.26 and 0.50 mg/L at high and low floods respectively. The mean concentrations of these inorganic nutrients were relatively low in the upper delta (Mohembo to Xakanaka) and increased downstream at Khwai, Maun and Lake Ngami, probably due to evapo-concentration and nutrient loadings from livestock because animals have direct access to the river. Only two sites, Khwai and Lake Ngami, had nitrate concentrations which were above the US EPA and the South African guideline for aquatic ecosystems value of 0.5 mg/L. Phosphate concentrations at all study sites were well below the South African guideline of <5 mg/L for aquatic ecosystems and may actually become a limiting nutrient for productivity in the upper delta. The nutrient concentration ranges at the upper delta (Mohembo, Sepopa, Etsatsa and Xakanaka) when compared with international trophic classes confirm past studies that the upper Okavango delta is an oligotrophic system, with low nutrient concentrations. Nitrate concentrations varied significantly between sites, with some downstream values 20-40 times higher than upstream values. However there were no significant concentration trends over the 3 years of study for both nutrients, implying a stable ecosystem. This finding may be attributed to the effective management of the river basin and if continued and periodically reviewed with more scientific data, may provide protection and sustainable use of the resource in line with National water policy and UN SDG #15 both advocating for the protection, restoration and promotion of sustainable use of ecosystems, sustainable management of forests, combating desertification, halting land degradation and biodiversity loss.