Assessing chlorophyll-a and water quality dynamics in arid-zone temporary pan systems along a disturbance gradient

Temporary pans are susceptible to various anthropogenic effects such as pollution, resource extraction, and land use intensification. However, given their small endorheic nature, they are almost entirely influenced by activities close to their internally drained catchments. Human-mediated nutrient enrichment within the pans can lead to eutrophica-tion, resulting in increased primary productivity and decreased associated alpha diversity. The Khakhea-Bray Transboundary Aquifer region and the pan systems that characterise the area are understudied area with no records available of the biodiversity therein. Additionally, the pans are a major water source for the people in these areas. This study assessed differences in nutrients (i.e., ammonium, phosphates) and their effect on chlorophyll-a (chl-a) con-centrations in pans along a disturbance gradient in the Khakhea-Bray Transboundary Aquifer region, South Africa. Physicochemical variables, nutrients, and chl-a were measured from 33 pans representing variable anthropogenic ex-posure during the cool-dry season in May 2022. Five environmental variables (i.e., temperature, pH, dissolved oxygen, ammonium, and phosphates) showed significant differences between the undisturbed and disturbed pans. The dis-turbed pans generally had elevated pH, ammonium, phosphates and dissolved oxygen compared to the undisturbed pans. A strong positive relationship was observed between chl-a and temperature, pH, dissolved oxygen, phosphates and ammonium. Chlorophyll-a concentration increased as surface area, and the distance from kraals, buildings and latrines decreased. Anthropogenic activities were found to have an overall effect on the pan water quality within the Khakhea-Bray Transboundary Aquifer region. Therefore, continuous monitoring strategies should be established to better understand the nutrient dynamics through time and the effect that this may have on productivity and diver-sity in these small endorheic systems.

Automated summary

Temporary pans are vulnerable to various human activities and can be impacted by pollution, resource extraction, and land use intensification. Human-mediated nutrient enrichment can cause eutrophication and reduce alpha diversity. This study in the Khakhea-Bray Transboundary Aquifer region, South Africa, examined the differences in nutrients and their effect on chlorophyll-a concentrations in pans along a disturbance gradient. The findings suggest that anthropogenic activities have a significant impact on pan water quality and continuous monitoring is needed to understand the nutrient dynamics and its effect on productivity and diversity in these systems.