An ecological integrity assessment of the lower Amatikulu, Thukela and Umvoti rivers, KwaZulu–Natal, South Africa
Venter, Johannes Jacobus
MetadataShow full item record
The ecosystem services of the lower Amatikulu, Thukela and Umvoti Rivers are used extensively through sugarcane agricultural activities, heavy industries and rural sewage-treatment works. These activities affect the ecological integrity of the lower Amatikulu, Thukela and Umvoti Rivers. The Umvoti River is already being referred to as a „working river‟. This study aims to determine the current state of ecological integrity of the lower Amatikulu, Thukela and Umvoti Rivers and to establish trends between current and historical periods for the evaluation of changing trends in ecological integrity. Abiotic (driver) and biotic (responder) indicator components were used in order to identify and monitor any changes in the surrounding environment as well as to determine the ecological integrity of the lower Amatikulu, Thukela and Umvoti Rivers. Driver components include water quality, sediment grain size, moisture and organic content as well as habitat state, whereas responder components involve macro-invertebrates and fish assemblages. Two surveys were carried out; one during the low-flow period (5-11 August 2011) and the other during the high-flow period (20-28 March 2012). Current data and findings together with historical data from 1999 to 2010 were used to establish trends of selected driver and responder components. Water quality variables selected include general variables such as water temperature, chemical oxygen demand (COD), electrical conductivity (EC), pH and total alkalinity (TAL) as well as salts, nutrients and toxics. These variables provide indications as to the state of the water-quality component of this study. The Target Water Quality Requirements (TWQR) as developed by the Department of Water Affairs and Forestry for domestic use (Volume 1) and Aquatic Ecosystems (Volume 7) were used to evaluate the quality of the water sampled in this study. The water quality as well as quantity was also compared to historical data obtained from previous studies that have been done for the same study area. The sediment analyses were performed according to the protocol set out by the United States Environmental Protection Agency. Habitat availability, diversity and state were assessed by means of the Integrated Habitat Assessment System Version 2 (IHAS v 2) and the Index of Habitat Integrity (IHI). The water quality of the lower Amatikulu River was found to be in a slightly modified state with the majority of water quality parameters within the target values as set by the TWQG. Water quality parameters considered on the lower Thukela River such as water temperatures, oxygen levels, nutrient and salt loads occurred at elevated levels and were not within the TWQG requirements. The water quality of the Thukela River was considered to be in a modified state which may cause negative impacts on the structure and function of the river, while the water quality of the lower Umvoti River was seriously modified. Sediment analyses revealed that the organic content of the Lower Amatikulu, Thukela and Umvoti Rivers was low. Sediment grain-size distributions are dominated by well-sorted larger soil grain-sizes (>500 μm) which is not ideal for the biodiversity. This is an indication that erosion and transportation are taking place in the Amatikulu, Thukela and Umvoti Rivers. The removal of riparian vegetation by agricultural activities and water abstraction contributes to the habitat deterioration as well as erosion and transportation of sediments that occurs in lower Amatikulu, Thukela and Umvoti Rivers. The use of macro invertabrates as biological indicators in the determination of the ecological integrity, state or health of lotic ecosystems is globally well established. The South African Scoring System, version 5 (SASS 5), the Macro invertebrate Response Assessment Index (MIRAI) and multivariate statistical analyses were implemented in order to determine the ecological integrity of the lower Amatikulu, Thukela and Umvoti Rivers. Results revealed that the SASS 5 integrity classes were generally one class higher than the integrity classes of MIRAI. SASS 5 and MIRAI integrity classes of the Amatikulu River ranged from natural (Class A) to largely modified (Class D/E) while the Thukela and Umvoti rivers ranged from natural to seriously modified (Class E/F). Fish assemblages are commonly used as key indicators to describe the ecological state of aquatic ecosystems. Methods used to sample fish included electronarcosis and a 5m wide 12mm meshed seine net. The Fish Response Assessment Index (FRAI) and multivariate statistical analyses were implemented in order to determine the ecological integrity of the lower Amatikulu, Thukela and Umvoti rivers. Results revealed that the automated FRAI integrity classes were constantly lower than the adjusted FRAI integrity classes. Automated and adjusted FRAI integrity classes of the Amatikulu River ranged from largely natural (Class B) to largely modified (Class D) while the Thukela and Umvoti rivers ranged from natural (Class A) to seriously modified (Class E/F). The current ecological integrity of the lower Amatikulu River was found to be in a largely natural, with few modifications (Class B) state. The Thukela River was found to be in a moderately modified (Class C) state while the Umvoti River was found to be in a largely modified (Class D) state. The trends in ecological integrity of the selected driver components which include water quality, sediment and habitat availability fluctuated noticeably. The general trend in water quality of the lower Amatikulu, Thukela and Umvoti Rivers slightly recovered towards 2012. Sediment analyses revealed that the sediment grain-size distribution as well as the moisture and organic contents generally remained stable. There was a decline in the general state of habitat integrity towards 2012. As a result of the decline in the habitat ecological integrity the ecological integrity of macro invertebrates also slightly declined towards 2012. However, a noticeable improvement in the ecological integrity of fish assemblages was observed towards 2012. To conclude, the ecological integrity of water quality and fish assemblages improved towards 2012, while habitat and macro invertebrates deteriorated and sediment stayed the same. The general and overall state of ecological integrity of the lower Amatikulu, Thukela and Umvoti Rivers did not deteriorate nor did it improve, but rather it stayed the same. Impacts on the ecological integrity of the lower Amatikulu, Thukela and Umvoti Rivers include a multitude of different sources. To prevent the current ecological integrity of the lower Amatikulu, Thukela and Umvoti Rivers from deteriorating further, a collective effort involving all parties is essential.