Using carbon isotopes to study nutrient enrichment at Lake Baikal

The sediment cores collected in March 2013 and August 2013 are being analysed for organic carbon (δ13Corganic) at the British Geological Survey (BGS) to investigate past changes in Lake Baikal’s primary productivity.

To find out more check out this post: ‘Using carbon isotopes to study Lake Baikal’ on the BGS GeoBlogy…

 

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Our first pigment analyses

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After all the long days of filtering water samples in our lab kunk back in March, the samples have been stored within a freezer ready for pigment analyses to begin. Filtration was carried out on the collected lake-water samples, to concentrate phytoplankton cells on a Whatman GF/F filter. All these samples have been run through the pigment analyser (High-Performance-Liquid-Chromatography system), to provide us with data on the algal community composition. This analysis can be performed as all phytoplankton groups produce pigments (chlorophylls and carotenoids) to enable cell photosynthesis, and some of these pigments can be used as important biomarkers for algal species presence and absence. The detection of these pigments provides information on the entire species assemblage within the samples, and not just a single group. This is an important aspect of the research, as phytoplankton respond very quickly to any changes within their environment, and provide excellent indicators of nutrient enrichment and climate change through species assemblage changes. Pigment records from across Lake Baikal, extending over natural timescales, will then enable the impact of recent nutrient loading on the ecosystem to be assessed.

To find out more about the application of algal biomarkers within this research, click HERE.

HPLC system: Agilent 1200 series

HPLC system: Agilent 1200 series

Before the samples are analysed on this HPLC system, sample preparation is carried out under special conditions, to enable the separation and detection of individual pigments. A mixture of organic solvents is used to extract the pigments from the cells, consisting of acetone, methanol and water. This sample preparation has to be carried out under dim light conditions, to avoid unnecessary photochemical degradation of pigments, which reduces their chemical stability. They are then stored for at least 12 hours within a freezer in the dark, to ensure pigment extraction from all the algal groups. This is important as algal species have different cell walls, and species with heavily silicified walls (such as large diatoms) require more time to fully extract their pigment composition. These samples are further filtered and HPLC-grade acetone is added as part of the final extraction step. Samples are then dried within the glass vials under nitrogen gas before analysis on the HPLC Agilent 1200 series.

Samples are placed in the autosampler within HPLC

Samples are placed in the auto-sampler tray within HPLC

Once dried, injection solvent is added and the glass vials are placed within the auto-sampler tray before being injected into the HPLC. As each sample is injected into the system, a diode-array detector then analyses the pigments. This produces a pigment chromatogram, which displays the pigment composition within the samples and individual pigments can be identified from their peak retention times and spectra.

Interpretation of these pigment chromatograms is now underway, and fortunately all the manual filtering of litres and litres of water paid off…as well as provoking some arm wrestling from all the bicep exercising…

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…Shortly our first pigment analyses on the sediment cores collected from the March 2013 field trip will be carried out, so more on this to follow soon…

Analysing our first water samples

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Part of the research that we are conducting is to analyse the silicon concentration and isotopc composition of lake waters. As we are interested in productivity, it is very important to understand silicon cycling through the year (e.g. winter versus summer conditions). This allows us to constrain down core interpretations. To find out more on why silicon is so important and what organisms use it in Lake Baikal, click here.

P1050563Having collected and filtered all water samples in the field, we are now ready to analyse the silicon concentration of the samples. This is conducted at the Brtish Geological Survey, UK. Concentration of trace metals are also given at the same time which is of great interest when looking at pollution stressors on the lake.

The next step is to measure the isotopc composition of the lake waters (δ30SDSi). In order to do this there are a number of purification steps that must be conducted, with samples being passed through a pre-cleaned resin to remove all cations (e.g. Na+, Ca2+) from the waters. This process takes rather a long time as the liquid must pass through the resin at a slow rate to ensure that there is full Si recovery of the sample.

The acid cleaning stage of the resin.

The acid cleaning stage of the resin.

However, this can only be done once the resin has been acid cleaned. Results from the ICP-MS are used at this stage in order to load onto the resin an exact volume of sample to ensure that the concentrations of all samples are high enough to precisely measure their isotopc composition.

Cationic resin preparation of samples and standards for MC-ICP_MS analyses

Cationic resin cleaning step of samples and standards for MC-ICP_MS analyses

Once samples are fully prepared they are measured once more on the ICP-MS to ensure that there is fully silicon recovery and to know the final concentration of the samples. This is important so that we know the volume of sample we need to analyse on the Multi-Collector Intercoupled Mass Spectrometer (MC-ICP-MS) and match concentrations of Si in bracketing standards.

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A lot of the action happens under the bonnet of the Multi Collector! Samples are injected into a plasma which ionises it, as the sample passes through the magnet the different masses of silicon (28, 29, 30)are separated and collected in three detectors. Based on the ratio between 30/28 we are able to understand the degree of biological uptake by diatoms that has taken place.

Laboratory work begins!

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Since returning from our March winter expedition, the large amount of laboratory work is now underway. We have collated a large variety of measurements, both physical and chemical, to help us assess the environmental conditions and water quality. These measurements were obtained using a Secchi disk, probes, water sampler and nets to determine the physical and chemical properties of the water. Our overall aim is to build a picture of the algal community activity and water quality conditions in Lake Baikal, and importantly aim to find any localised areas of eutrophication and climate change impacts on this unique ecosystem.

After collating and cataloguing all the water column samples, sediment core material and sediment trap samples the exciting part of science (as well as the incredible fieldwork expedition of course!) now begins. From the three sampling sites, a total of 180 samples where collected from within the water profile alone. The water column was sampled at depths of 1m, 3m, 5m, 10m, 20m, 30m, 50m, 100m and 180m, at sites situated along a transect (from the Neutrino scientific site to a site close to one of the biggest mills in Russia). These were taken for phytoplankton and diatom, total phosphorus, dissolved organic carbon, nutrients and major ions, photosynthetic pigments (carotenoids and chlorophyll a derivatives), zooplankton, chlorophyll a, diatom silicon isotope, diatom oxygen isotope, and anion analyses.

Nutrient concentrations are very low in Lake Baikal, and therefore only small additions of nutrients (Nitrogen, phosphorus and Silicate) can stimulate algal growth and productivity. Currently, water chemistry analysis is underway within the laboratory facilities at the University of Nottingham. To begin with the filtered nutrient and major ion water samples are being analysed on the Ion Chromatography (IC) for Nitrates. In addition to this, the Total Phosphorus and Silicate analyses on the water samples are near to completion.

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From the water samples, the factors driving the modern-day algal community and silicon isotope composition of diatom shells will be investigated, in order to look back through time within the sediment core samples and help us understand past primary productivity (palaeolimnological reconstructions). Thus, the present is the key to the past, and there will be more to follow shortly on our laboratory progress…