Drivers of water quality in Afromontane-savanna rivers

Abstract

Although several studies have investigated the relationships between water quality in rivers and the types of land use within their catchments, many aspects of these relationships remain unclear in Afromontane-savanna rivers, especially the interactions between catchment land use, seasonality and stream size. Afromontane-savanna catchments present a unique situation where headwater regions and lowlands have experienced more dramatic land cover change, but mid-elevation regions remained rather natural. We examined the influence of seasonality, catchment land use and stream size, including their interactions, on water physico-chemistry, nutrients and major ions in the Afromontane-savanna Mara River in Kenya, using data collected from 2010 to 2018 at >150 sampling sites in the Kenyan part of the river. We developed generalized linear mixed models (GLMMs) to explore the influence of seasonality (dry and wet seasons), land use (forest, mixed, agriculture and grasslands), stream size (stream orders 1–7), and their interactions on river water quality. Water quality variables included physico-chemical measures (pH, dissolved oxygen [DO] concentration, temperature, electrical conductivity, total dissolved solids [TDS], turbidity, total suspended solids [TSS] and particulate organic matter [POM]), nutrients (NH4+, NO3−, total dissolved nitrogen [TDN], total nitrogen [TN], soluble reactive phosphorus [SRP], total phosphorus [TP] and dissolved organic carbon), and major ions (Cl−, F−, SO42−, Na+, K+, Ca2+, Mg2+, Fe2+, HCO3− and Si). There were clear differences in average values of most water quality variables among land uses with sites in savanna grasslands having high levels of major ions, ammonium and P, while agricultural sites had higher dissolved fraction of N (except ammonium). Stream order was a poor predictor of water quality, and most parameters did not display any relationship (either linear or non-linear) with stream size. Our results can be used to efficiently enhance water quality by developing strategies for stream restoration and management based on the predomination type of land use in the catchments.

Bioassessment of multiple stressors in Afrotropical rivers: Evaluating the performance of a macroinvertebrate-based index of biotic integrity, diversity, and regional biotic indices

“…Seasonality played a significant role in influencing water quality variables in the river through its influence on runoff, erosion and leaching of solutes, as also shown in a previous study (Wanderi et al, 2022). Solute concentrations and electrical conductivity were much higher during the dry season, probably because of increased evaporation and water temperatures.…”

“…Sites were grouped into four site categories depending on the percentages of the forest, and agricultural land uses and levels of stressors they were experiencing. The percentages of forest, grasslands, and agricultural land use in the entire catchment upstream of the sampling sites were calculated based on the Digital Elevation Model of Kenya (90 m by 90 m), obtained from the Shuttle Radar Topography Mission Wanderi Frontiers in Environmental Science frontiersin.org et al, 2022). Sub-catchments were delineated, and the area of each major land use category upstream of each sampling site was calculated.…”

“…Extreme cases of flow alteration that lead to cessation of flows or supra-reduced flow levels during the dry season have been reported in the Mara River and other rivers in eastern Africa (Dutton et al, 2018;Stears et al, 2018). Reduced flows lead to the accumulation of organic matter and ammonia, increased concentration of solutes and an overall decline in DO concentration (Dutton and Subalusky, 2021;Wanderi et al, 2022).…”

“…The MARA sites also experienced extreme flow variation, with seasonal tributaries ceasing flowing during the dry season. Given that this is the nature of savanna rivers-seasonal flows, high levels of organic matter, turbidity, water temperatures, and electrical conductivity (dissolved solutes) and low dissolved oxygen concentration (Wanderi et al, 2022), these taxa may be adapted to live in these conditions while maintaining high diversity and abundances. Indeed, some of the most sensitive taxa to poor water quality, such as Ephemeroptera and Trichoptera, recorded high taxa richness and abundance in organically polluted sites downstream of hippo pools and livestock watering points (Masese and Raburu, 2017).…”