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A strategic programme for NERC Lowland catchment research
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Ecological significance of surface & subsurface exchanges: summary

A fuller description of this project is also available.

Overview of project

One goal of the LOCAR programme was to improve understanding of surface-subsurface exchange in permeable catchments from a biological perspective. This project combined measurements of nutrient pools (nitrogen & phosphorus) and a characterisation of isotopic signatures of the available nitrogen and seasonal biomass (primary and secondary biomass production) with an assessment of biogeochemical function through gas profiles (carbon dioxide, methane and nitrous oxide), porewater nutrients and stable isotope tracers (15NO3) within exchange patches.This project has been successful in characterising the River Lambourn in terms of its surface-subsurface hydrological exchange, dominant biogeochemical processes and the role of the hyporheic zone as a nutrient source or sink. Despite having a diverse and productive invertebrate fauna indicative of a 'healthy' river, there is strong evidence that the rivers are awash with inorganic N, the substrata are clogged with fine material and supersaturation with greenhouse gases like nitrous oxide and methane would indicate that the river may, in fact, be heavily impacted and far from healthy.

Aims

The primary aim of the project was to assess how patch configuration affects retention, hyporheic biogeochemical processes and, in turn, the bio-availability of nutrients for production; and whether seasonal variations in abundance of production correlates with biogeochemical function. In addition, the project aimed to provide data for a hydrogeochemical model to improve prediction of hydrochemical and nutrient function in permeable lowland catchments.

Main findings

This project has been successful in characterising the River Lambourn in terms of its surface-subsurface hydrological exchange, dominant biogeochemical processes and the role of the hyporheic zone as a nutrient source or sink.

Vertical hydraulic gradient data shows that only shallow river bed sediments (down to about 10-15 cm) are well connected with the surface water, and this is supported by steep temperature profiles within the bed and the limited vertical distribution of hyporheic invertebrates.

Biogeochemical processing is modified by the mosaic of different types of substratum patches in the river channel. Nitrate reduction is closely associated with hypoxic areas in the organic-rich silt/sand deposits and can occur in coarser substrata (fine gravel/sand) if the organic content is high.  More about the main findings of this project

Stable isotope analysis revealed wide variability in d15N and d13C signatures within the invertebrate community. This variability was related to dietary differences among taxa and showed strong seasonal patterns.  More about the main findings of this project

Hyporheic invertebrate densities of both meiofauna and macrofauna declined rapidly with sediment depth. Meiofaunal abundance in the surface sediments showed a clear seasonal pattern.  More about the main findings of this project

The Lambourn has very different surface-subsurface exchange dynamics to other rivers described in the peer-reviewed literature. The lack of clearly defined downwelling areas and the shallowness of the hyporheic zone meant that some of our original ideas had to be modified, but nevertheless, this alters our perception of chalk streams. The limited extent of the hyporheic zone has implications for its functional role, in terms of being a sink for nutrients and other contaminants within the catchment. Despite having a diverse and productive invertebrate fauna indicative of a 'healthy' river, there is strong evidence that the rivers are awash with inorganic nitrogen, the substrata are clogged with fine material and supersaturation with greenhouse gases like nitrous oxide and methane would indicate that the river may, in fact, be heavily impacted and far from healthy.  More about the main findings of this project

Description of activities

The project characterised part of the River Lambourn catchment in terms of its surface-subsurface hydrological exchange, dominant biogeochemical processes and the role of the hyporheic zone as a nutrient source or sink. Methods used included stable isotope analysis and determinations of invertebrate density.

Areas of application

The Water Framework Directive calls for an understanding of river health/quality and the impacts of changes in land use and nutrient enrichment on river functioning. This is currently lacking in permeable catchments. The data generated by this project is directly relevant to these issues and will help the Environment Agency and Defra to address their Water Framework Directive and EU Nitrates Directive obligations.

Researchers' details

Principal Investigator:

Co-investigators:

More researchers who worked on this project More researchers who worked on this project

Publications

All publications from this and other LOCAR projects are listed in the publications database.

Selected publications from this project are listed below. The full list of publications and recommended reading can be viewed in the full description page for this project Full publication list for this project

Selected publications

Pretty J., Hildrew A.G. and Trimmer M. (2006) Nutrient dynamics in relation to surface–subsurface hydrological exchange in a groundwater fed chalk stream. Journal of Hydrology. 330 (1-2): 84-100.

Full list of publications resulting from this project Full publication list for this project

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