14C as a tool to trace terrestrial carbon in a complex lake: implications for food-web structure and carbon cycling

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Many people are familiar with life in freshwater either from direct experience with angling or from nature documentaries. Most are probably aware that food chains in aquatic habitats differ from those on the ground. However not all are aware of the details of the complex ecosystems found in lakes, or indeed of the links between the lake and its terrestrial catchment. The problems of pollution in lakes are well known as nutrients from fertilisers can enter the water from agricultural land causing plant life to take over the lake (eutrophication) - this issue is regularly highlighted in the media. However the influx of terrestrial carbon into the lake and subsequent utilisation of this resource in lakes is unexpected. Fish are known to eat aquatic insects and plant life - not many people would name peat bog or soil amongst the food groups of the brown trout! We have shown that such terrestrial material does in fact make it's way into the foodchain and therefore fish diet using a technique known as stable isotope analysis. We have also used radiocarbon - more familiar as a dating method - to clarify the importance of terrestrial material in the diet of fish in Irish lakes. Using radiocarbon, or 14C, we can show that a fish is consuming carbon produced by aquatic plants. This 'within-lake' carbon is partly sourced from weathered limestone and is dissolved in the water. This rock weathered carbon does not contain the 14C radio-isotope and as a result artificially appears to be thousands of years old. Most terrestrial carbon on the other hand is in equilibrium with the earth's atmosphere and contains higher levels of radiocarbon - this carbon is 'modern' and can be distinguished from 'within-lake' carbon. Other carbon stored in peat can be 'old'; this can also be found in lakes and we don't yet know what proportions of 'dead', 'modern' and 'old' carbon are used by plants and animals in lakes. We can separate carbon components dissolved in the water which are used by plants, animals and bacteria in the lake. We can measure the stable isotopes in these carbon components as well as their 14C levels and find out where the terrestrial carbon entering the lake goes. We can also measure the 'radiocarbon age' and stable isotope values of the animals and plants living in the lake can show whether they are consuming 'within-lake' carbon or terrestrial ('modern' or 'old') carbon entering from surrounding land. This research is important as the amount of terrestrial material entering a lake can be affected by climate change and land management practices. The consumption of terrestrial carbon by species in the lake can also be affected by invasive species such as the zebra mussel which voraciously consumes 'within-lake carbon' and is rapidly spreading through Irish and U.K. lakes, causing fish to rely more on terrestrial material. Our proposal to combine the use of stable isotopes with radiocarbon in Queen's University Belfast will investigate this important new field of research to shed light on the complicated food webs in freshwater lakes.