Which river catchments in Somerset require nutrient neutrality compliance?

There are four catchments, including the River Tone, River Parrett, River Brue, and River Axe. Each covers a defined geographic area. The Parrett catchment is the largest, covering much of central and south Somerset (including the former South Somerset and Sedgemoor districts). The Tone covers the Taunton area and Wellington. The Brue runs through the Mendip Hills area. The River Axe extends into East Devon and Dorset. We hold phosphate credits in all four.