Non-Linear Dynamics and Regime Shifts in the Lake Chad Aquatic Ecosystem: A Long-Term Time Series Analysis

Shallow lake ecosystems are prone to catastrophic regime shifts between alternative stable states, but the detection, characterisation, and early warning of such transitions in tropical African lakes remain theoretically underdeveloped. This research applies non-linear time series analysis and alternative stable state theory to investigate regime shift dynamics in the Lake Chad aquatic ecosystem using a 60-year multi-proxy dataset compiled from remote sensing, hydrological records, fisheries statistics, and sediment core proxies. Remote sensing data from MODIS, Landsat, and Sentinel were processed to generate monthly surface area and vegetation index time series from 1960 to 2024. Wavelet analysis and empirical orthogonal function decomposition were applied to identify dominant modes of variability. Early warning signals of regime shifts including rising variance, autocorrelation, and skewness were computed using rolling window analysis. Sediment cores from the central basin were analysed for diatom assemblages, stable isotopes, and biogenic silica as paleolimnological proxies. Two confirmed regime shifts were identified: a large-lake to small-lake transition in the 1970s and a partial recovery event in the 2000s. Early warning signals successfully detected both shifts with 3 to 5 year lead times. Diatom community reconstruction revealed two compositionally distinct alternative states corresponding to open pelagic and shallow macrophyte-dominated configurations. Bifurcation analysis of a simplified lake ecosystem model incorporating hydrology, nutrients, and turbidity identified the critical threshold for state transition at a lake surface area of approximately 12,400 square kilometres. The study establishes the first rigorous evidence-based regime shift chronology for Lake Chad. Keywords: regime shifts, Lake Chad, alternative stable states, non-linear dynamics, early warning signals.