Lake evolution in a semi-arid montane environment: response to catchment change and hydroclimatic variation

Pollen, geochemical and sedimentological data from Sidi Ali, a montane Moro ccan lake, provide a 7000 yr record of changes in climate, catchment vegeta tion and soil erosion intensity. Diatoms, non-silicious algae, macrophyte f ossils and ostracods from the same core record the dynamics of the lake eco system. Oxygen isotope and trace-element ratios of benthic ostracods appear to be relatively insensitive to climatic variation in this open lake with low water-residence time, but diatom plankton / periphyton (P/L) ratios sho w lake-level variations that are probably climate controlled. At least two superimposed processes are recorded, but at different timescales: catchment vegetation and soils show long-term changes due to climate and human impac t, whereas P/L ratios suggest century-scale oscillations in lake depth. The timing of changes in algal and macrophyte productivity and carbon cycling within the lake broadly corresponds to changes in terrestrial vegetation, s uggesting either that lake nutrient status is linked to catchment vegetatio n and soils, or that both were influenced by climate. The lack of a sensiti ve and independent (non-biological) climate proxy makes it more difficult t o assess the lake's ecological response to short-term climate variation. Ov erall, the lake's evolution has been influenced both by catchment-mediated nutrient flux and by changes in water balance, thus having characteristics in common with both temperate and arid zone lakes.