Detention ponds

Our research within fields through the SOWAP and MOPS projects reveals that soil erosion associated with surface runoff can result in the mobilisation of high levels of soil and associated phosphorus. 

Under some circumstances, soil and phosphorus can enter watercourses and our research within the PARIS project reveals their ecological impact.  Grass buffer strips can reduce this risk.  However, soil and nutrients are also transported to watercourses via field drains, and as these flow more consistently than surface runoff, their nutrient contribution to ditches and streams can be considerable.

A decade ago, we took steps to address this issue at the base of our longest arable slope.  An area of ground that flooded most winters and was consequently unproductive was taken out of the cropped area before the project started and we diverted a ditch and broke field drains into this to create a series of small pools.  Our recent monitoring of nutrient concentrations reveals that the phosphorus concentration is 40 – 50% lower at the tail end of the pools series than it is in the supplying ditch or drain.  Nitrogen concentration is also reduced in the longer pool series.

Although these pools are inevitably eutrophic, they support around 80 species of aquatic invertebrates, including six nationally scarce water beetle species.  Despite often being very small, the pools supported 24 – 52 species.  Fifteen dragonfly species have been recorded and the diverse habitat associated with the pools supports five species of grasshoppers and crickets. 

In terms of birds, whitethroat and reed bunting numbers increased in the buffer strip and the rank vegetation associated with the pools supported a grasshopper warbler breeding territory in 2007.  The same habitat provides nest sites for wild pheasants and winter habitat for ducks and snipe.

Although our buffer strip pools approach has wider application, the buffer strip area is larger than is acceptable in most fields and we are currently investigating the potential of condensed versions of these pool sequences for adoption at the field corner scale.  Some of this work has been carried out within the recently completed project, Wetting up Farmland for Biodiversity.