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

Title

Ecological significance of surface and subsurface exchange in lowland catchments.

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.

The river bed is dominated by a positive vertical hydraulic gradient, i.e. potential upwelling subsurface water. However, the large positive vertical hydraulic gradient does not translate to rapid upwelling because hydraulic conductivity generally is low in the compacted river bed sediments. 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. Nitrate reduction is seasonal, being more pronounced in the warmer spring/summer months, and is often restricted to a thin layer in the surface sediments (top 4-8cm). There is removal of nitrogen via denitrification and patches of the substratum contain high concentrations of ammonium. Ammonium concentration was highest in the top 10-15cm of the river bed, particularly in coarse gravel, silt/sand patches and beneath macrophytes (plants visible with the naked eye), where organic matter accumulates. Nitrous oxide concentration is consistently very high in the river bed, with background concentration about 1500% saturation relative to air equilibration. In hypoxic areas, there is wide variation both above and below the background concentration, reflecting micro-scale production and reduction of nitrous oxide. In addition, the river water was supersaturated with methane but only the organic-rich silt/sand deposits were significant sources of methane, whereas both of the coarser substrata (gravels/sands) were in fact sinks for methane ie. areas of methane oxidation.

Having identified substratum patches with different biogeochemical characteristics, the second phase of the project sought to trace differences in nutrient processing characteristics into both primary and secondary production (objectives 3 and 4). Stable isotope analysis revealed wide variability in d15N and d13C signatures within the invertebrate community. Variability was due to dietary differences among taxa, with many grazers having particularly ‘light’ d13C signatures (< about 40‰). Most species exhibited a strong seasonal shift with lightest d13C in spring (reflecting the spring algal bloom) and heaviest d15N in winter, possibly reflecting instream recycling of nitrogen over the year. Some taxa showed subtle differences among patch types, but these were overwhelmed by the strong seasonal patterns. Furthermore, similar carbon:nitrogen ratios for all taxa across different patches and seasons show that nitrogen was not a limiting resource and differences in nitrogen cycling among patches were not strong enough to translate to differences in the d15N at the base of the food web.

Hyporheic invertebrate densities of both meiofauna and macrofauna declined rapidly with sediment depth. The annual mean contribution at each depth was 80% at 10cm, 15% at 20cm, and 5% at 30cm. Although invertebrate densities were comparable with other studies, the depth distribution was not consistent. The lack of a significant hyporheic invertebrate community within the Lambourn could be due to a combination of factors such as stable flows and infilling with fine sediment. Meiofaunal abundance in the surface sediments showed a clear seasonal pattern with peak densities occurring in the sediments beneath macrophytes in late spring/summer and reduced densities following sediment washout after macrophyte cutting or die back later in the year.

Therefore, the Lambourn has very different surface-subsurface exchange dynamics to those described in the 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.

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 river health/quality and the impacts of changes in land use and nutrient enrichment on river functioning, which is 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:

Other researchers and staff:

  • Dr James Pretty
  • J Petersen
  • A Voak

Publications

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

Refereed journal papers

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.

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