Acid episodes in the Allt a'Mharcaidh, Scotland: an investigation based on sub-hourly monitoring data and climatic patterns

Abstract

Stream waters in the Allt aMharcaidh catchment (Cairngorms, Scotland) have been monitored for flow, conductivity and pH at sub-hourly resolution; and for a range of chemical, biological and physical parameters, less intensively, since the mid-1980s. The Allt aMharcaidh stream is subject to acidic events (pH<5.5) triggered by both hydrology and sea-salt inputs from the atmosphere. This paper investigates the drivers of these acidic events using variables derived from sub-hourly monitored data. It also examines the influence of the North Atlantic Oscillation (NAO) on episode severity. Sub-hourly datasets are used to derive multiple regression models expressing stream H + concentration as a function of the sea-salt conductivity and the peak instantaneous flow rate amongst other explanatory variables. The relationship between sea-salt conductivity and the NAO is significant but hidden due to issues such as time lags and the influence of atmospheric patterns other than the NAO.

On the factors affecting distributions of freshwater diatom species in a remote South Atlantic archipelago

“…This inference is consistent with the large variation in conductivity values found in precipitation samples from the Mount Pleasant Weather Station (above). High sea-salt deposition and ion exchange processes in peat deposits can make water bodies prone to ecologically significant acidity pulses (Harriman et al, 1995Bonjean et al, 2007). Because the Na : Cl ratios for the Falkland Islands sites are so close to the theoretical seawater ratio (0.86), seasalt corrections were possible and these can help identify other ion sources (Wright & Henriksen, 1978).…”