Sediment analyses in lakes.

The composition of lake sediments is primarily determined by the background geology of the lake watershed area while the rate of accumulation is influenced by prevailing climate and the relative size of the catchment. However, biotic and abiotic processes within the lake and in its catchment, including human impacts, have significant additional influences on lake water quality and sediment composition. Lake sediments contain a record of past events and human activities in the catchment area.

AQUENS assesses previous conditions of lake water quality by varied analyses of sequential sections of sediment cores, for the most part laid down in a chronological sequence. Accurate establishment of the time of deposition, or age, of sediment core samples is essential not only to construct a chronology but also to calculate rates of change and allow meaningful interpretation of results. Sedimentation rates are determined by radionuclide analyses. Measurement of the naturally occurring isotope 210Pb, with a half-life of 22.26 years, provides age estimates on a time scale of ca. 125 years. Measurement of 137Cs resulting from military atmospheric testing of nuclear weapons (1954 - early 1960's) in the Pacific region, allows dating from a peak representing 1963. An additional 137Cs marker for some European lakes, including many in Ireland, comes from the April 1986 Chernobyl power station incident.

Sediment chemistry provides information on catchment erosion and leaching conditions, nutrient inputs and a variety of in-lake processes. Extraction, identification and analyses of the remains of pollution-tolerant and indicator organisms (algae and diatoms and their pigments, zooplankton and insect larvae) which lived in earlier times and whose "microfossils" are incorporated into the sediments, provide information for benchmarking and changes in water quality.

These applied techniques are used to evaluate rates of change and impacts of aquatic pollutants in lake ecosystems and are particularly useful if data on previous conditions is not available. Palaeolimnologists have been broadly categorised as "hindcasters" and AQUENS hindcasters have over 30 years experience of lake sediment analyses in Ireland and other countries.

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