Re-evaluating vegetation community dynamics and well-being of the lower uThukela and uMvoti rivers in KwaZulu-Natal, South Africa.

Abstract

Aquatic ecosystems are deteriorating due to the numerous anthropogenic stressors affecting them. Riparian vegetation and macrophytes can all be established as ecological indicators of the well-being of rivers and contribute to the sustainable management of water resources. In this study riparian vegetation and macrophyte communities were assessed to determine their condition and structures at the lower uThukela and uMvoti Rivers using historical data from 2013 and data collected from the same study sites in 2023.To determine the ecological state of the macrophytes and riparian vegetation communities, data was collected to represent the vegetation communities of the riparian ecosystem and macrophytes ecosystem using phytosociological methods. For this study, cross-sections for the sampled rivers, river discharge data (m³/s ), water quality variables, substrate types, water levels (m) and vegetation communities data were determined. The plant community data was superimposed into cross-sections of the rivers and used to provide information pertaining to the relative elevation and hydraulic niche requirements of each individual plant species. Substrate at each site associated with plant communities were estimated and categorized into either silt, mud, sand, gravel, cobble, boulders and bedrock following the grain-size analysis and sieving technique. The biological community structures and their associated abiotic variables were assessed using multivariate statistical techniques in the form of Canonical Correspondence Analysis (CCA) and Redundancy Analysis (RDA) multivariate approaches to determine the well-being of uThukela and uMvoti Rivers across the sampled sites. The significant threshold selected for this study was restricted to p = 0.05 where p< 0.05 is significant and p> 0.05 is regarded as not significant. Change detection results indicated a decline in both the macrophytes (4914 Ha to 4002 Ha) and riparian vegetation (23292 Ha to 19055 Ha) communities of the uThukela River in the 10-year period (2013-2023).Most of the plant communities were categorized under sub-adult and adult age groups except for few juveniles. The multivariate analysis indicated that water quality is a significant driver to the observed vegetation communities of the lower uThukela River. Macrophytes and riparian vegetation communities data from the sampled sites of the lower uMvoti River were also evaluated using similar multivariate analysis methods. Change detection method indicated an increase in the riparian vegetation communities and a decrease in macrophytes communities. The riparian vegetation communities of uMvoti River increased from 8620 Ha to 10952 Ha between 2013 and 2023, respectively. A decline in macrophytes communities was observed in the period between 2013 and 2023 with 4488 Ha and 2106 Ha, respectively. Most of the sampled plant communities in the lower uMvoti River were observed as adults. The population structure of the river’s vegetation communities is concerning as it is lacking juvenile individuals. The multivariate analysis indicated that water quality and substrates are not significant drivers of the observed plant communities in the lower uMvoti River. The plant communities superimposed to the cross sections for both uMvoti and uThukela Rivers did not show any clear association with elevation and water requirements.