Microplastic accumulation in endorheic river basins - The example of the Okavango Panhandle (Botswana).

The Okavango Panhandle is the main influent watercourse of the Okavango Delta, an inland sink of the entire sediment load of the Cubango-Okavango River Basin (CORB). The sources of pollution in the CORB, and other endorheic basins, are largely understudied when compared to exorheic systems and the world's oceans. We present the first study of the distribution of microplastic (MP) pollution in surface sediments of the Okavango Panhandle in Northern Botswana. MP concentrations (64 µm-5 mm size range) in sediment samples from the Panhandle range between 56.7 and 399.5 particles kg-1 (dry weight) when analysed with fluorescence microscopy. The concentrations of MP in the 20 µm to 5 mm grain size range (analysed with Raman spectroscopy) range between 1075.7 and 1756.3 particles kg-1. One shallow core (15 cm long) from an oxbow lake suggests that MP size decreases with depth while MP concentration increases downcore. Raman Spectroscopy revealed that the compositions of the MP are dominated by polyethene terephthalate (PET), polypropylene (PP), polyethene (PE), polystyrene (PS), and polyvinyl chloride (PVC). From this novel data set it was possible to estimate that 10.9-336.2 billion particles could be transported into the Okavango Delta annually, indicating that the region represents a significant sink for MP, raising concerns for the unique wetland ecosystem.

Low oxygen: A (tough) way of life for Okavango fishes.

Botswana's Okavango Delta is a World Heritage Site and biodiverse wilderness. In 2016-2018, following arrival of the annual flood of rainwater from Angola's highlands, and using continuous oxygen logging, we documented profound aquatic hypoxia that persisted for 3.5 to 5 months in the river channel. Within these periods, dissolved oxygen rarely exceeded 3 mg/L and dropped below 0.5 mg/L for up to two weeks at a time. Although these dissolved oxygen levels are low enough to qualify parts of the Delta as a dead zone, the region is a biodiversity hotspot, raising the question of how fish survive. In association with the hypoxia, histological samples, collected from native Oreochromis andersonii (threespot tilapia), Coptodon rendalli (redbreast tilapia), and Oreochromis macrochir (greenhead tilapia), exhibited widespread hepatic and splenic inflammation with marked granulocyte infiltration, melanomacrophage aggregates, and ceroid and hemosiderin accumulations. It is likely that direct tissue hypoxia and polycythemia-related iron deposition caused this pathology. We propose that Okavango cichlids respond to extended natural hypoxia by increasing erythrocyte production, but with significant health costs. Our findings highlight seasonal hypoxia as an important recurring stressor, which may limit fishery resilience in the Okavango as concurrent human impacts rise. Moreover, they illustrate how fish might respond to hypoxia elsewhere in the world, where dead zones are becoming more common.

Sap flow variation in selected riparian woodland species in the Okavango Delta, Botswana.

In the tropical Okavango Delta, transpiration by trees is an important process partly responsible for maintaining the basin as a freshwater environment. Quantification of evapotranspiration from terrestrial landforms of the delta, fringed by riparian woodlands, is one of the main contributors to uncertainty in current hydrological modelling. We investigated sap flow of common trees in the distal, mid- and upper delta in July-August 2012, November-December 2012 and February-April 2013 using the compensation heat pulse velocity method. In the distal delta, four Diospyros mespiliformis individuals of different sizes were studied. Four trees of different species were studied in the mid- and upper delta. Sap flow density (SFD; flow per unit cross-sectional area) was used as a common unit to facilitate comparison. Sap flow varied with tree size, species, season and location. It was positively correlated with tree size (r 2 = 0.67). Sap flow variation between seasons and across locations in all the species studied indicated two distinct groups. Group 1 transpired the least during the hottest season, November-December, and Group 2 the most. In Group 1, the highest average SFD was 1.17 l cm-2 day-1 during July-August; in Group 2, it was 1.07 l cm-2 day-1 during November-December. Changes in the hydrology of the delta would negatively affect the riparian woodland.

Dans le delta de l'Okavango, tropical, la transpiration des arbres est un processus important partiellement responsable du fait que ce bassin reste un environnement d'eau douce. La quantification de l'évapotranspiration de la topographie terrestre du delta, bordée de forêts riveraines, est un des principaux contributeurs de l'incertitude dans la modélisation hydrologique actuelle. Nous avons étudié la coulée de sève d'arbres communs dans le delta distal, moyen et supérieur en juillet­août 2012, novembre­décembre 2012 et février­avril 2013 par la méthode de compensation heat pulse velocity. Dans le delta distal, nous avons étudié quatre individus de Diospyros mespiliformis de taille différente. Quatre arbres d'espèces différentes ont été étudiés dans les deltas moyens et supérieur. La densité du flux de sève (DFS), flux par unité de surface de section, a servi d'unité commune pour faciliter la comparaison. Le flux de sève variait avec la taille et l'espèce des arbres, la saison et l'emplacement. Il était positivement lié à la taille des arbres (r² = 0,67). La variation du flux de sève avec les saisons et l'emplacement indiquait deux groupes distincts parmi toutes les espèces étudiées. Le Groupe 1 transpirait le moins pendant la saison la plus chaude, novembre­décembre, et le Groupe 2 transpirait le plus. Dans le Groupe 1, la DSF moyenne la plus haute était de 1,17L/cm²/jour en juillet­août; dans le Groupe 2, elle était de 1,07L/cm²/jour en novembre­décembre. Des changements dans l'hydrologie du delta affecteraient négativement la forêt riveraine.

Input, behaviour and distribution of multiple elements in abiotic matrices along a transect within the Okavango Delta, northern Botswana.

Wetlands fed by rivers can be a sink for elements depending on elemental concentrations, wetland hydrology, geochemistry, vegetation and climate. In the case of the Okavango Delta, northern Botswana, the outflow discharge is a small fraction (2-5%) of the inflow. This has strong potential consequences for the Delta, as it strongly affects element cycling and storage within the Delta. We estimated the inputs, behaviour and distribution of multiple elements along a longitudinal transect within the Okavango Delta, to show potential effects of retention mechanisms of different elements. High annual element input is rather attributed to discharge than to the concentration within the water, which is generally extremely low. We observed minimal enrichment of the elements within the water pathway along the transect from inflow to outlets, implying that element output is negligible. For most elements, we observed a high correlation between storage and sediment organic matter content. The organic matter content within the sediments was higher in the vegetated sediments than in non-vegetated sediments (factor âˆ¼ 10), and a similar trend was found for most elements. In conclusion, organic matter dominated in sediments from vegetated plots and thus plays an important role in retaining the elements within the sediments of the Delta. This finding has major implications for e.g. planning constructed wetlands for water purification or element retention especially in areas with high evapotranspiration.