Critically testing the role of delta-30 Si[diatom] as a novel productivity signal in temperate lakes

Lead Research Organisation: Loughborough University
Department Name: Geography

Abstract

Despite occupying only ~3% of the earth's land surface, lakes are productivity hotspots and play an important role in the biological fixing, mineralisation and burial (collectively cycling) of carbon, at both a landscape and global scale. Lakes are sensitive to environmental change, yet the ways that lake ecosystems respond (in terms of their overall structure, and processes such as lake productivity) are poorly understood. For this reason, we are also uncertain as to the fate of carbon within the lake system (in terms of its cycling), and this is especially problematic given the uncertainty with future climate change (which may change the way that lakes cycle carbon in the future). Given the complexity of the carbon cycle and instability of organic compounds in lake waters and sediments, finding out past lake productivity from stratigraphic profiles of carbon stored in lake sediments is problematic. Diatoms are unicellular algae, and an important group of primary producers of biomass in lakes, and may provide a possible solution in the chemical signature of their shells (or 'valves'), which are made of silica (natural glass) and preserve well in many freshwater sediments. Analysis of the ratio of stable isotopes of silicon (Si) in diatom valves (delta-30Sidiatom) offers a potential alternative means to trace past primary productivity in lakes, particularly alongside independent evidence for climate and hydrological change derived from stable isotopes of oxygen also found in diatom silica (delta-18O). Recent advances in the analytical methods now allows both silicon and oxygen isotope signals to be measured from the same sample. Preferential uptake of the lighter silicon isotope (28Si) during diatom silicification (valve building), leads to an enrichment of the heavier isotope 30Si within residual water proportional to diatom population growth, reflected by increasing delta-30Sidiatom. While the use of delta-30Si is rapidly expanding in marine studies, the potential for lake studies has yet to be assessed. In particular, the relative influence of biogenic uptake of silicon and the recycling of silicon within the lake (as it does slowly dissolve in water and from sediment on the lake floor) on the eventual delta-30Si signal in lakes is critical, yet poorly understood. Diatom oxygen isotope analysis (delta-18O) has received greater attention and the fractionation of both isotopes during silicification by diatoms in laboratory culture experiments is the aim of an ongoing NERC Fellowship by one of the co-investigators. Yet, field studies are scarce and considering the complex nature of Si cycling in lakes, there remains an exciting opportunity and urgent need to develop this approach critically for freshwater systems. The first steps towards unravelling the Si cycle in lakes, and its delta-30Sidiatom fingerprint, requires detailed case studies of well-chosen and representative sites. We propose to address these issues by monthly monitoring Si cycling in conjunction with delta-30Sidiatom and delta-18O at Rostherne Mere, Cheshire over two spring (diatom bloom) periods, using an integrated approach with weekly to fortnightly automatic sediment trapping and water sampling, in combination with seasonal measurements from water column profiles, inflows and groundwater, and annual bulk sediment traps. The project addresses fundamental questions concerning delta-30Si biogeochemistry and will provide an important step towards understanding how we can use delta-30Sidiatom from lake sediments to tell us about the past condition (function and productivity) of lakes, establishing the potential for this technique as a tracer for productivity in aquatic systems, and providing insight into nutrient recycling (valuable for environmental managers). Knowledge of the way lakes functioned in the past, under different regimes of climate and human activity, can help us predict and manage change that we see today and might expect in the future.

Publications

10 25 50
 
Description We have characterised the dynamics of silicon and oxygen isotopes in a natural lake setting over a 2 year period (especially how the delta-30 Si is related to silica concentration and diatom productivity in the lake), and have data on how these signals are incorporated into diatom silica. There is a clear and strong inverse correlation between deltaSi-30 in lake waters and silica concentration, providing strong evidence for biological control (via diatoms) on silica dynamics and isotopic signals in lake waters. Biogenic silica isotope values from freshly formed and sedimented diatoms follow the pattern in surface waters with a fractionation offset, and together these data provide support for the use of deltaSi-30 in biogenic silica as a proxy of biological utilisation of Si (supply vs. demand) in lake waters. This dataset is the largest and most comprehensive yet generated for understanding silicon isotope dynamics in lake waters.
Exploitation Route The data can be used to help understand and manage lakes in terms of seasonal productivity. Our findings support the further application of the d30 Si method to fossil diatoms in sediment cores to explore patterns of past lake productivity and help answer questions about long-term natural variability and response to change in aquatic freshwater ecosystems under human impact and climate change. This can help us manage lakes for the future in the context of the Water Framework Directive (WFD), for example, by answering questions about when lakes started to become affected by human activity (e.g. agriculture, settlement, urbanisation), how fast and how large the changes were; and how they responded to climate changes of the past. The main result provides a critical test for delta-30 Si as a valid proxy for lake algal productivity, and so test whether d-30 Si can be used to infer past patterns of aquatic productivity from fossil diatom valves (both in a freshwater, and marine, context).



The results could also be used to model silicon dynamics in the lake system, and for the oxygen data, show how the lake and diatom signal is related to hydrological variability (through climate/weather).
Sectors Agriculture/ Food and Drink,Environment

 
Description Our research aims and objectives - the basic why we are doing this research at Rostherne and what we hope to find - have been written up for a recent edition (2015) of the Natural England Cheshire Newsletter and Rostherne parish newsletter, and laminates put up around the site. This has helped explain to the general public what we have been doing at Rostherne within this and related work.
First Year Of Impact 2015
Sector Environment
Impact Types Cultural

 
Description Big Data Capital Call: Depth-resolved, high-resolution data on lakes
Amount £230,000 (GBP)
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start  
 
Description NERC DTP CENTA PhD Studentship
Amount £37,000 (GBP)
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 10/2014 
End 03/2018
 
Title Organic carbon burial calculation using sediment trap and sediment core records from Rostherne Mere, Cheshire, UK (1360-2016) 
Description This dataset includes sediment trap, sediment core and loss-on-ignition to total organic carbon measurements from a PhD research project at Rostherne Mere in Cheshire. These data were collected to show the relationship between the changing nutrient loads and subsequent organic carbon burial over the last 120 years. The sediment trap data cover the period from May 2010 to August 2016, while the sediment core was taken in September 2011 and has been 210Pb dated to circa 1360AD. All data is presented for date, loss-on-ignition (LOI) and calcium carbonate (CaCO3), with sediment trap data converted into net flux measurements and sediment core data calculated for net sedimentation rate following 210Pb dating. The conversion from LOI to total organic carbon was measured using mass spectrometry and applied to the trap and core data. The work was carried out as part of a Natural Environment Research Council (NERC) funded PhD [grant number NE/L002493/1], with part of the work also funded by the NERC small grant [grant number NE/H011978/1]. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes