Significance of macroscale peat flux for carbon export in upland fluvial systems

Lead Research Organisation: Durham University
Department Name: Geography

Abstract

Peatlands contain up to 30% of global terrestrial carbon. However, many of these sensitive environments are seriously threatened by changing climate and human-induced degradation. Upland peatlands form a significant part of this resource in the UK and scientists have developed carbon budget models to describe these systems. Although such models are becoming increasingly sophisticated, the export of large (c.100-1000 mm length scale) peat blocks and debris has never been included. However, peat block transport is an important component of the total organic sediment flux in upland fluvial systems and must be included in carbon budget estimates. Understanding the nature of this unique process and the potential magnitude of this flux is essential because fluvial components are the second most important component in the upland terrestrial carbon budget and carbon budget studies are now being used by upland managers to implement strategies on carbon stewardship. Our aim is to determine, for the first time, the significance of large peat block transport (> 100 mm) in the carbon budget of an upland catchment. We will test three main hypotheses: 1. The initiation of peat block transport (by flotation) is a function of block dimensions and the local hydraulics of the flowing river. Such a process has NEVER been fully documented. 2. The total peat block flux is a significant component of the total fluvial carbon flux and, in some high flow events may exceed the particulate organic flux. There is currently NO quantitative data to test this. 3. Where active in-channel peat block transport occurs, the river reaches downstream will show elevated values of particulate organic carbon due to rapid abrasion and breakdown of peat blocks. These three hypotheses will be tested at a range of scales: micro (individual peat block scale); meso (peat blocks store at the river bar scale), macro (peat block river reach sediment budget) and catchment scale. These experiments will be carried out at the Moor House and Upper Teesdale National Nature Reserve in the North Pennines. Monitoring is nested at these four scales and involves a novel, integrated system of data loggers, environmental sensors and three remote digital cameras.
 
Description With reference to the original project objectives:

1.Establish a relationship between block size, local flow conditions and entrainment.

A full range of block sizes ranging from 0.1 to 0.75m diameter were tested in the experimental approach. The largest block size was determined by limitations relating to the safe manual handling of blocks in the field. Block entrainment showed a strong relationship with local flow conditions although some variability was inherent due to local river bed roughness.



2.Quantify the total BOC storage and changes in storage through a range of hydrological events.

Peat block storage was monitored over a 15 month period and included a detailed inventory of the spatial distribution and movements of 242 individual peat blocks. Measurements of block dimensions and detailed mapping of the changing distribution of the blocks allowed the volume of stored peat blocks to be evaluated between significant hydrological events. The period of record includes an extreme flood which occurred on 31 August 2009. Results clearly demonstrate the importance of channel storage of macroscale peat blocks with distinct areas of deposition within the channel system reflecting the active, semi-active and inactive status of the peat blocks.



3. Apply a sediment budget approach to assess the importance of BOC breakdown in the generation of POC in the channel. This entails monitoring inputs and outputs of POC from the control reach.

Although the full sediment budget is still to be calculated we have established there is clear breakdown of blocks due to active transport in the study reach. Excellent time series of POC inputs and outputs from the study reach have been collected. However, the errors in the measurements data together with the uncertainty over the relative importance of splitting vs. abrasion mechanisms may make the precise quantification of these processes difficult. Furthermore, the degradation of peat blocks by oxidation, even over this relatively short period may also be processes that will need to be factored in to the analysis.



4. Assess the relative contribution of BOC and POC flux and relate this to established DOC fluxes (monitored by the NERC-sponsored Environmental Change Network).

Using the BOC inventory (Ob. 2) and the excellent POC time series (Ob. 3) the relative contribution of these components to the down stream organic sediment flux can readily be calculated. We are still waiting receipt of the Doc time series from ECN data collection team. Preliminary analysis suggests that the BOC flux represent only about 10% of the POC load.



5. Evaluate the significance of these revised fluvial components within the framework of the overall carbon budget for the study catchment.

On completion of the final calculations under Ob.3 and Ob.4 this information will used to calculate an improved estimate of the catchment carbon budget. This will build on two previous publications of the PI and provide an important refinement to our evolving upland carbon budget models.
Exploitation Route The results are of interest to land managers who wish to mainatian upland carbon stores in peatlands. For the first time we have established a relationship between peat block size, local flow conditions and entrainment resulting in a predctive equation of peat block transport. This has enabled the significance of Block Organic Carbon (BOC) to be defined and we have quantified the total BOC storage and changes in storage through a range of hydrological events. Assessment of relative importance of BOC, POC and DOC in carbon budgets is now possible. This is of potential use to land managers who wish to reduce C lossess from upland areas via fluvial systems.
Sectors Environment
 
Description The main use of these findings has still to be realised through further publication which will demonstrate: (1) The significance of macroscale peat flux in the terrestrial carbon budget - with implications for carbon accounting. (2) The transport mechanisc of large peat blocks which can impact of stream/river crossing structures such as bridges and culverts.
First Year Of Impact 2014
Sector Environment
Impact Types Economic,Policy & public services
 
Description Leverhulme Artist-in-Residence grant
Amount £18,000 (GBP)
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2013 
End 02/2015
 
Description European GeoSciences Union General Assembly 2014 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk generated comments and discussion.

Invited to submit a ppaer to a journal
Year(s) Of Engagement Activity 2014