Development of a connectivity index to assess aquatic macroinvertebrate species vulnerability to thermal change : a case study in KwaZulu-Natal Province.

Abstract

Connectivity of river systems is a critical ecological component affecting not only the integrity of aquatic systems but also freshwater species’ habitat, community structure, migration and other life activities such as feeding rate and reproduction. Connectivity is a product of longitudinal and lateral conditions of rivers that in turn impact on flow and water temperature time series patterns (temporal connectivity). No comprehensive connectivity index of rivers has previously been developed for South Africa’s KwaZulu-Natal Province. The aims of this study were to develop a connectivity index for selected main rivers in KwaZulu-Natal, where free-flowing rivers represent full connectivity, and to assess quaternary catchments which would most be vulnerable to the impacts of increased water temperatures in response to climate change in terms of aquatic biota. A connectivity index at quaternary catchment scale was developed for main rivers in KZN based on in-stream barriers, land cover fragmentation and small dams density. The temporal dimension of connectivity involved an examination of stream flow and water temperature changes at selected stations based on comparisons of data from before and after construction of in-stream barriers. Temporal connectivity was incorporated into the longitudinal connectivity index score using the concept of reset distance. Reset distance was calculated based on mean daily flow volumes and the distance between impoundments at each of the selected quaternary catchments. Based on these assessments, catchments likely to be most vulnerable to species community changes were detected as a function of connectivity of rivers and slope gradient of streams. The assumption was that aquatic macroinvertebrate species are likely to be most impacted by rapid thermal change (based on temperature lapse rates) in the high altitude catchments particularly where such rivers have reduced connectivity. The uMngeni catchment emerged with the most disconnected river in the province due to its closely located in-stream barriers and long distance recovery potential of flow. The quaternary catchment with lowest lateral connectivity also occurred within the uMngeni catchment. It is concluded that catchment management authorities should consider both connectivity and vulnerability (climate change) assessments of river systems as a tool negotiating for sustainable conservation plan of aquatic species and ecological integrity of rivers.