Publication

Journal	Global and Planetary Change
Volume (Issue)	123
Page(s)	86 - 96
Title of proceedings	Global and Planetary Change
DOI	10.1016/j.gloplacha.2014.10.008

Varved lake sediments are excellent natural archives providing quantitative insights into climatic and environmental changes at very high resolution and chronological accuracy. However, due to the multitude of responses within lake ecosystems it is often difficult to understand how climate variability interacts with other environmental pressures such as eutrophication, and to attribute observed changes to specific causes. This is particularly challenging during the past 100 years when multiple strong trends are superposed. Here we present a high-resolution multi-proxy record of sedimentary pigments and other biogeochemical data from the varved sediments of Lake Żabioskie (Masurian Lake District, northeastern Poland, 54°N-22°E, 120m a.s.l.) spanning AD 1907 to 2008. Lake Żabinskie exhibits biogeochemical varves with highly organic late summer and winter layers separated by white layers of endogenous calcite precipitated in early summer. The aim of our study is to investigate whether climate-driven changes and anthropogenic changes can be separated in a multi-proxy sediment data set, and to explore which sediment proxies are potentially suitable for long quantitative climate reconstructions. We also test if convoluted analytical techniques (e.g. HPLC) can be substituted by rapid scanning techniques (Visible Reflectance Spectroscopy VIS-RS; 380-730 nm). We used Principal Component Analysis and Cluster Analysis to show that the recent eutrophication of Lake Żabioskie can be discriminated from climate-driven changes for the period 1907-2008 AD. The eutrophication signal (PC1 = 46.4%; TOC, TN, TS, Phe-b, high TC/CD ratios Total Carotenoids/Chlorophyll-a Derivatives) is mainly expressed as increasing aquatic primary production, increasing hypolimnetic anoxia and a change in the algal community from green algae to blue-green algae. The proxies diagnostic for eutrophication show a smooth positive trend between 1907 and ca 1980 followed by a very rapid increase from ca. 1980 ± 2 onwards. We demonstrate that PC2 (24.4%, Chl-a-related pigments) is not affected by the eutrophication signal, but instead is sensitive to spring (MAM) temperature (r = 0.63, pcorr< 0.05, RMSEP = 0.56°C; 5-yr filtered). Limnological monitoring data (2011 – 2013) support this finding. We also demonstrate that scanning visible reflectance spectroscopy (VIS-RS) data can be calibrated to HPLC-measured chloropigment data and be used to infer concentrations of sedimentary Chl-a derivatives {pheophytin a + pyropheophytin a}. This offers the possibility for very highresolution (multi)millennial-long paleoenvironmental reconstructions.