Impacts Of Land Use And Land Cover On Water Quality And Benthic Macroinvertebrates In Theta River Catchment

Abstract

Landuse and landcover (LULC) changes continue to pose a threat to water quality, and aquatic biodiversity in many watersheds across the world. There is a growing scientific consensus of the close relationship between land use and land cover types and water quality. Understanding this relationship is vital for water management. This study addressed the impacts of land use/cover types on water quality and benthic macroinvertebrates in Theta River catchment in Kiambu County of Kenya. Theta River spans Lari, Gatundu South, Githunguri, Ruiru, and Juja Sub- Counties covering a total area of approximately 143 km2. The objectives of the study were to: - a) detect the spatial changes in LULC types along Theta River gradient, b) establish the influence of spatial LULC on water quality along Theta River, c) assess the influence of LULC changes on macroinvertebrates composition in Theta River; and d) analyze the relationship between water quality and macroinvertebrates composition in Theta River. Primary data included water quality parameters and benthic macroinvertebrates. The physico-chemical water quality parameters were measured in situ using Hanna H1 9829 this included turbidity, pH, dissolved oxygen (DO), salinity, temperature, and electrical conductivity. Nitrates were determined in the laboratory using the Brucine method. Macroinvertebrates were sampled using D net following 1-minute standard kicks in the river biotopes and identified to family level. The secondary data comprised of remote sensing datasets made up of digital elevation model (DEM), Sentinel 2a satellite image for 2018 sourced from the United States Geological Survey (USGS). Results of the study showed that the main land use and landcover were agriculture (61.4 %,), grassland/shrub (19.9%), urban-settlements (6.7%), forest land (4.9%), riverine vegetation (6.4%), and open water (0.7%). Forest, agricultural, and urban-settlement land uses where used to represent the human disturbance gradient of how these land use affect water quality and macroinvertebrate community structure. There were clear differences in relationships between water quality characteristics and the land uses. The correlation of determination from spearman rank showed that forest land explained 64% variation in temperature and 60.8% variation in dissolved oxygen, while Agricultural land explained 51.8% of the variation in nitrates, and urban-settlement explained 46.2% of variations in temperature, 98% of variation in turbidity, and 59.3% of variations in nitrates. Water samples from stations in forest land had high levels of dissolved oxygen, low temperature, nitrates, electrical conductivity, and turbidity compared to those from agricultural and urban-settlements. The impairment of water quality along the human disturbance gradient was colloborated by macroinvertebrates data. Forested site had high family level diversity, and high abundance of the sensitive taxa of the order EPT than agricultural and urban-settlement land uses. The differences in macroinvertebrates community structure among the land uses was showed to be statistically significant from analysis of similarity (ANOSIM) test. The study recommended water pollution preventive measures such as enhancing the protection of forest, land, improvement and protection of riparian zones, and mitigation of pollution loads from agricultural land use through sustainable farming practices.