Linking neo- and palaeolimnology: a case study using crater lake diatoms from central Turkey

Palaeoenvironmental interpretations from analyses of lake sediment archives are dependent on our understanding of the modern lake environment and the link between neo- and palaeolimnology. Diatom assemblages represent different lake habitats and vary in species composition and abundance on seasonal and annual timescales. These factors, along with mixing at the mud-water interface, alter the information recorded in the sedimentary archive and the resulting interpretation of environmental variables in the past. Nar Gölü (central Turkey), a crater lake with annually laminated sediments, is used to investigate the link between modern and palaeo diatom assemblages. Information about the modern diatom population has been gathered through sampling of different lake habitats (1999–2009) and installing seston sediment traps in the water column (2002–2007). The palaeo diatom assemblage has been analysed through annual-resolution percentage counts and biovolume calculations on core sediments (1927–2006), along with identification of sub-annual assemblage changes through microscopic analysis of core thin sections. Comparisons of data collected using these different methods has shown a good correspondence between sediment trap and core results, but revealed that blooms of two diatom species (Nitzschia paleacea and Synedra acus), in each case occurring on average 1 year in five, can have major impacts on percentage calculations and may result in distorted and misleading palaeoenvironmental interpretations. These two species distort the average signal and may bloom for short periods of time as a result of short duration changes in lake water chemistry, such as following fresh-water input from snow melt. Blurring of sedimentary records through mixing at the sediment–water interface can mask the full magnitude of environmental variation and low sampling resolution may fail to capture high amplitude events of short duration.