Phosphorus in the Eye Brook catchment

Farming is coming under increasing pressure to reduce its impact on water quality, especially through the requirements of the EU Water Framework Directive, and we intend our research at Loddington to assess this impact and help us prepare for future change. Loddington sits in the central section of the 67-square kilometre Eye Brook catchment and much of our recent research has been at the catchment and sub-catchment scale. One major Defra-funded project that has recently been completed is the Phosphorus from Agriculture: Riverine Impacts Study (PARIS). This has investigated the relationship between farming and stream nutrients, especially phosphorus.

Whereas other projects have investigated processes, associated with soil erosion, PARIS explored the physical and chemical processes in streams and their impact on wildlife. The project concentrated mainly on two sub-catchments, one pasture in the Eye Brook headwaters, and the other mainly arable around Loddington. The ecological part of the project looked at the effect of phosphorus on aquatic invertebrates and diatoms. At 80-100 µg/l (micrograms per litre) of phosphorus, the number of insect and diatom (silicaceous algae) species in the stream started to
decline and the diatom biomass started to increase. This ecological tipping point is analogous to the development of blanket weed on the surface of farm ponds and has a profound impact on freshwater wildlife.

Phosphorus is strongly associated with eroded soil particles. Weekly sampling revealed that average concentrations of phosphorus were four times as high in the arable catchment (118 µg/l) as in the grass one (28 µg/l). Inevitably, soil erosion is more likely during storm events when surface run-off occurs and field drain flow is at its peak. Automated samplers set up in the grass and arable sub-catchments enabled samples to be taken in response to rainfall. The results illustrate the increase in sediment and phosphorus following rain (see Figure1). Grassland buffers the stream up
to a point, but even here, phosphorus concentrations increase during extreme rainfall. Such intense storm events are predicted to increase under future climate change so understanding this relationship between land use and water quality is important.

Figure 1. Phosphorus concentrations in the grass and arable sub-catchments in response to rainfall

	Arable
	Grass
	Rainfall

We sampled water upstream and downstream of houses (which are potential sources of domestic phosphorus from septic tanks) to define the domestic and agricultural sources separately. The phosphorus concentration was three to four times higher downstream of houses than upstream, where farming was the only source (see Figure 2). Further sampling revealed that phosphorus concentrations from roads and tracks were intermediate between concentrations associated with field drains and surface run-off from fields, and point sources such as farm yards and septic tanks. In terms of annual phosphorus yield, diffuse agricultural sources contributed more than 95%, but the small contribution from domestic sources was a major contributor at base flow when this continuous source was not diluted by water from the surrounding land.

These results suggest that agriculture is having an impact on stream ecology by delivering phosphorus to watercourses via a number of pathways. Through additional monitoring of Eye Brook tributaries, we identified which contribute most sediment to the stream. They are predominantly arable, but there is not a simple correlation between arable area within a tributary’s catchment and the sediment produced by it. A more detailed understanding of individual pathways at the field scale is required  to address diffuse pollution. Such field-scale knowledge is held largely by farmers
themselves. The project also identifies domestic septic tanks as a source of phosphorus that can have even greater impact on stream wildlife, at least locally.

PARIS is one of several projects in the Eye Brook. Improving the Eye Brook as a habitat and a resource can only be achieved at the community level, and we work closely with other members of the catchment community, not least through the Eye Brook Community Heritage Project, which brings together research results (such as those from PARIS) with the local knowledge of farmers and others.

Figure 1. Phosphorus concentrations in the grass and arable sub-catchments in response to rainfall

	Upstream
	Downstream