Friday, 11 July 2014

Using carbon isotopes to study Lake Baikal... by Sarah Roberts

Today we're very pleased to share a guest post from Sarah Roberts, a Postgraduate Researcher at the School of Geography, University of Nottingham. Here she introduces her exciting collaborative work, to investigate changes in nutrient fluxes at Lake Baikal, Siberia, with the Baikal research team; Dr. George Swann, Prof. Anson Mackay, Dr. Suzanne McGowan and Dr. Virginia Panizzo (BGS Visiting Research Associates) and BGS staff.

Why are we researching nutrient enrichment at Lake Baikal?

Lake Baikal research expedition in March 2013 on the
ice in the South basin
Lake Baikal is the world’s oldest, deepest and most voluminous lake; forming over 25 million years ago, reaching water depths of 1,700 m and holding 20% of the world’s surface freshwater. It is listed as a UNESCO World Heritage site, as 80% of Lake Baikal’s living organisms are endemic. It is the uniqueness which gives the lake its well-known title; ‘The Galapagos of Russia.’ However, over the last half-century, this UNESCO site has undergone substantial catchment change, due to increased anthropogenic activities such as deforestation, development and industrialisation. Lake Baikal is still considered to be relatively pristine, however these threats are believed to have resulted in higher nutrient input into the lake in recent years, impacting upon Baikal’s water quality and unique ecosystem. Interlinked with this, climate change has lead to a decline in winter ice-cover thickness and duration and enhanced nutrient loading from regional permafrost thaw and fluvial input. These factors have had a direct effect upon Lake Baikal’s aquatic ecosystem. This project aims to investigate the impact of recent anthropogenic activity and natural climate variability at Lake Baikal, over the past 1000 years, through contemporary monitoring of its lake waters and analyses of its sediments from the bottom of the lake.

Lake Baikal research expedition in August 2013 on a
research vessel across the South, Central and North basins
Which nutrient enrichment proxies are we using?

To investigate changes in Lake Baikal’s primary productivity, we are analysing organic carbon (δ13Corganic) and silicon (δ30Si) isotopes at the BGS. Stable isotopes are different forms of an element, which have the same number of protons but differing number of neutrons. Carbon isotopes in lake sediments can be used to compare algal growth rates (i.e. productivity) under different nutrient dynamics while silicon isotopes tell us specifically about the cycling of silicon (a key nutrient) in the lake over time. As well as stable isotopes, algal pigments are also being analysed at the University of Nottingham. Algal pigments are produced by all photosynthetic organisms, and can be used as biomarkers to identify specific algal groups, such as diatoms, green algae and picoplankton. These varying nutrient enrichment proxies are being analysed on the sediment cores collected from across the South, Central and North basins of Lake Baikal in March 2013 and August 2013 and will be used to reconstruct past environmental conditions and anthropogenic impacts on the lake.

How do carbon isotopes infer past primary productivity?

The machines used to produce 13C/12C and C/N data by the
Stable Isotope Group led by Prof Melanie Leng at the BGS
During photosynthesis algae take up carbon, but they prefer to take up the lighter isotope (12C) rather than the heavier (13C) because the former is easier to utilise. In Lake Baikal, if there is lots of algal growth (if the algae are ‘productive’) then the algae take lots of 12C from the water. The more growth there is, the more reduced this pool becomes in 12C. Therefore, when subsequent algae grow, they have to use 13C instead. Stable isotope analyses compares the ratio (δ) between these two isotopes so that increased productivity is reflected by an increase in the δ13C of organic matter in Lake Baikal sediments. However factors other than primary productivity can influence the carbon isotopic composition of in-lake produced organic matter, such as the concentration of atmospheric CO2, lake-water pH, temperature, nutrient limitation and algal growth rates. Therefore, in addition to comparing δ13C values, the carbon to nitrogen ratio (C/N) of the organic matter can be of used to quantify/assess algal versus higher plant (e.g. terrestrial) production. In-lake organic carbon produced by algae is distinct from the terrestrial input of organic carbon from the watershed produced by vascular plants, based on their respective C/N ratios. This is as algae have a much lower C/N ratio in comparison to vascular plants, such as grasses, shrubs and trees, and aquatic macrophytes.

What will the carbon isotope records at Lake Baikal show?

location diagram
The analysed sediment cores collected from Lake Baikal (with water depths ranging from 66 m to 1,360 m) have low C/N values, suggesting the source of organic matter is predominately from an algal, rather than terrestrial, origin. Therefore δ13C values will largely provide records of aquatic productivity. As a result higher δ13C values are expected to coincide with periods of increased anthropogenic impacts over the last half-century when nutrient inputs to the lake are believed to have increased, thereby promoting more algal growth. The sediment cores collected from the South basin are expected to show a shift towards more positive δ13C values (higher values) over the last 60 years, in comparison to the sediment cores collected from the North basin. This is as the South basin sites are hypothesised to be influenced by both climate and anthropogenic effects, whereas the North basin sites are hypothesised to be influenced by climatic changes alone. Down-core reconstructions of primary productivity along the sediment cores, which are c. 50 cm long, will enable the natural variability of the system over the last c. 1000 years to be established, and will help to disentangle the recent impacts on the lake from both anthropogenic activity and natural variability. Importantly, the nutrient enrichment proxies being used (carbon and silicon isotopes and algal pigments) will enable a more holistic picture of past primary productivity response to changing environmental conditions than has been achieved in previous work on the lake.

Sarah

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