Assessment of metals, carbon dynamics and macroinvertebrates diversity in pan wetlands across geological areas of central Kruger National Park, South Africa.

Abstract

Wetlands are ecosystems that rely on both rainfall and groundwater supply for their composition, structure and functionality. They are highly dependent on water levels and a change in climatic conditions affect water availability and this highly influences their structure and functionality. They play an important role in climate mitigation, sustaining of ecosystems, livelihood and societies by providing essential ecological services and societal benefits. The central parts of Kruger National Park comprise of a network of ephemeral pan wetlands that lay across different geological types (sandstone, granite, basalt and rhyolite) that provides an ecosystem service to both inland and aquatic species. When compared with marine and terrestrial biomes due to their limited extension, they are widely recognised as biodiversity hotspots. However, despite their ecological services and their carbon dynamics, there is still a poor understanding of these ecosystems functioning. In a pool of knowledge about what they are and what they provide, there is still no information about their distribution, pan sizes and the amount of carbon they sequestrate. Hence, this thesis aims to investigate the complex interactions between abiotic (carbon sequestration, metal accumulation and nutrient concentration) and biotic factors (invertebrates’ diversity and abundance) in pan wetlands. A field study was conducted in September 2022 when pan wetlands were drying out to assess soil organic carbon sequestration, metal and non–metal dynamics in floodplain pan wetland system across different geological types and soil profiles. Sediment samples were collected randomly across 12 pans with different geologies using a 130 cm hand auger. The results indicated that there were significant differences in pH, SOC (soil organic carbon), and concentrations of various metals (K, Mn, Na, Fe, Cu, Ca, Zn) with sediment depth. These differences were found to be statistically significant. Additionally, there were significant 3 differences in pH, SOC, SOM (soil organic matter), resistivity, carbon sequestration, non–metal/nutrients (S, P, B), and all metals. Despite the results highlighting a significant concentration of metals in the topsoil, there are no discernible adverse observed on the aquatic systems. This study provides an evaluation of sediment metal pollution to assist resource managers in effectively implementing management strategies for floodplain pan wetlands. In addition, an assessment of spatiotemporal invertebrate diversity and abundances during high and low rainfall water period in relation to water and sediment chemistry was done. The sediment and water–chemistry variables and macroinvertebrates were sampled during high water period (December 2022) and low water period (June 2023). Sediment and water chemistry–variables were measured to assess the concentrations of metals and nutrient across geological areas and how they influence aquatic invertebrate biodiversity. The study collected a total of 5 145 macroinvertebrates from 41 species across 12 pan wetlands with different geological types during high water and low water periods. The high rainfall period had a higher number of species (n = 29) compared to the low rainfall period (n = 12). The most abundant order was Hemiptera, making up 57.0 % of the total sample, followed by Crustacea (24.6 %), Diptera (9.1 %), Mollusca (3.3 %), Trichopteran (2.0 %), Coleoptera (1.5 %), Annelida (1.4 %), Odonata (0.9 %), and Ephemeroptera (0.1 %). Statistical analysis revealed significant differences in macroinvertebrate community structure among geological types and seasons. During the high–water period, the % ETOC and %EPT were higher compared to the low–water period. The % Chironomidae + Oligochaeta was only observed in the low–water period. The Shannon–Wiener diversity index was higher during the low–water period, while species abundance was higher during the high–water period. The 4 Simpson index showed a higher number of dominant species during the low–water period, and the evenness index was higher during the high–water period. While the pan wetlands located in the central Kruger National Park play a major role in maintaining biodiversity and serving as water points for inland animals. This study provided an assessment of aquatic biodiversity in pans to better understand trophic interactions within (autochthonous) and contributions from outside (allochthonous) and provide management options for aquatic taxa in a protected area.