Persistent surface water acidification in an organic soil-dominated upland region subject to high atmospheric deposition: the North York Moors, UK

The North York Moors National Park, in Northeast England, is one of the few upland areas of the United Kingdom located immediately downwind of major sulphur and nitrogen emission sources. Despite this, few studies of air pollution impacts have been undertaken, and there is no formal long-term upland water quality monitoring site. We examined the condition of surface waters in the National Park based on 1) a unique 20 year stream pH record from three locations; and 2) a snapshot survey of 51 surface waters draining moorland and conifer plantations. Interpretation was supported by sulphur isotope analysis of a subset of water samples, and a diatom survey of one of the monitoring streams. Long-term pH data for a stream draining the peat plateau demonstrate extreme, year-round acidification, with recovery only evident in the last few years. Lower-elevation sites are less acidic, but show similar temporal trends, and are characterised by frequent and severe acid episodes. The snapshot survey confirmed that acidification of the moorland area is widespread, to a degree observed in few other areas globally; out of 37 moorland streams sampled, 32 had an acid neutralising capacity (ANC) below -50 eq l-1. Sulphate was found to be (by far) the dominant cause of acidification, and sulphur isotope analysis confirmed that this derives primarily from atmospheric deposition. Nitrate concentrations remain low, indicating that the organic moorland soils continue to retain most incoming nitrogen. It appears that conifer planting has exacerbated acidification, leading to five-fold higher nitrate and threefold higher aluminium concentrations compared to the moorland sites. Available biological data suggest that waters in the region have been impoverished by acidification. We speculate that the slow recovery of surface waters in the North York Moors is due to the release of a legacy of stored sulphur from the peats, released during droughts. We conclude that: 1) acidification is far from being a solved problem in this sensitive near-source upland region, despite reductions in sulphur deposition; 2) plantation forestry has exacerbated the effects of atmospheric pollution, and triggered nitrogen saturation; 3) the lack of any formal long-term monitoring in the North York Moors represents a major gap in the current evidence base for the effects of long-range air pollutants on UK upland ecosystems.