Abstract
African tropical lakes provide vital ecosystem services including food and water to some of the fastest growing human populations, yet they are among the most understudied ecosystems in the world. The consequences of climate change and other stressors on the tropical lakes of Africa have been informed by long-term analyses, but these studies have largely focused on the massive Great Rift Valley lakes. Our objective was to evaluate how recent climate change has altered the functioning and services of smaller tropical lakes, which are far more abundant on the landscape. Based on a paired analysis of 20 years of high-resolution water column data and a paleolimnological record from a small crater lake in western Uganda, we present evidence that even a modest warming of the air (∼0.9°C increase over 20 years) and changes in the timing and intensity of rainfall can have significant consequences on the dynamics of this common tropical lake type. For example, we observed a significant nonlinear increase (R2adj = 0.23, e.d.f. = 7, p<0.0001) in thermal stability over the past 20 years. This resulted in the expansion of anoxic waters and consequent deterioration of fish habitat and appears to have abated primary production; processes that may impair ecosystem services for a vulnerable human population. This study on a system representative of small tropical crater lakes highlights the far-reaching effects of global climatic change on tropical waters. Increased research efforts into tropical aquatic ecosystem health and the development of sound management practices are necessary in order to strengthen adaptive capabilities in tropical regions.
Highlights
The paucity of reliable long-term environmental and climatic data sets for the African continent is hindering progress in building capacity to adapt to global change [1], in aquatic systems where potential impacts of climate change are not adequately documented and understood [2,3]
Given the range of uncertainty surrounding climate change scenarios and increasing pressures facing African inland waters [4] including a continent-wide shortage of potable water [5], the development of local scale data sets is critical for these systems [6]
Because the rate of change in water density is greater at higher temperatures [13], warm equatorial lakes are more sensitive than temperate lakes to small increases in temperature
Summary
The paucity of reliable long-term environmental and climatic data sets for the African continent is hindering progress in building capacity to adapt to global change [1], in aquatic systems where potential impacts of climate change are not adequately documented and understood [2,3]. Given the range of uncertainty surrounding climate change scenarios and increasing pressures facing African inland waters [4] including a continent-wide shortage of potable water [5], the development of local scale data sets is critical for these systems [6] We address this problem by presenting an analysis of a 20-year highresolution record of regular monitoring in a small but locally significant equatorial crater lake, Lake Nkuruba, located in a densely populated rural region of the East African highlands. Like Lake Nkuruba (Fig. 1), provide additional sources of income for local populations (in this case, support for an orphanage in the form of a small-scale communal ecotourism venture; http://www.enfuzicommunitycampsite.com/) In this region, crater lakes have often experienced water quality degradation driven by watershed land-use changes [9]. Cultural eutrophication threats are presently low in Lake Nkuruba, making it an ideal system for studying effects of recent climate change on small, warm tropical lakes, without the confounding effects of eutrophication
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