A 2000-year sediment record reveals rapidly changing sedimentation and land use since the 1960s in the Upper Mara-Serengeti Ecosystem.

Abstract

The Mara River basin is a trans-boundary basin of international importance. It forms the headwaters of the Nile River and serves as the primary dry season water source for an estimated 1.1 million rural people and the largest remaining overland migration of 1.4 million wildebeest in the Serengeti-Mara Ecosystem. Changes throughout the basin are impacting the quantity and quality of the Mara River, yet the historical context of environmental conditions in the basin is not well known. We collected sediment cores throughout the wetland at the mouth of the Mara River, and we used isotopic dating methods and a suite of analyses to examine historical patterns of sediment quantity and source, mercury contamination, and carbon and nutrient loading. Our results show that ecological conditions in the Mara River basin were fairly stable over paleoecological time scales (2000-1000\u202fyears before present), but there has been a period of rapid change in the basin over the last 250\u202fyears, particularly since the 1960s. A shift in the source and quantity of sediments in the river began in the late 1700s and became much more pronounced in the 1950s and 1960s, coincident with increasing mercury concentrations. The quantity of sediment from the Upper Mara increased, particularly since 1960, but the proportion of total sediment from this region decreased as the Talek and Middle Mara portions of the basin began producing more sediment. The decadal oscillation in sediment accumulation was congruent with known periods of extreme precipitation events. Carbon and nitrogen loading also increased since the 1960s, and the shift in the isotopic ratio of nitrogen provides evidence for increased anthropogenic loading. Altogether, these data likely reflect patterns of change also experienced in other basins throughout East Africa.