RESUMEN
Weekly monitoring of surface water chemistry took place over a one-year period in a small boggy sub-catchment of the River Thurso, northern Scotland. Monitoring started 6 months after the felling to waste of plantation conifers. The chemistry of ground surface waters was monitored at four bog sites situated in former forestry plots as well as one control site situated in an intact bog. The chemistry of the receiving stream (Sleach Water) was monitored at seven points along a 2 km stretch. Dissolved organic carbon and metals were very significantly affected by seasonal factors. On land, seasonal variations accounted for between 35% (Al) and 80% (Fe) of the total variance in the data at the intact bog site, with similar seasonal effects observed at the impacted sites. The amplitude of the seasonal signal was generally much higher at the impacted sites than at the control site. Except for dissolved Al and Mn, the chemical composition of the stream was only marginally influenced by surface runoff from the felled plantation despite evidence of intense seasonal mobilisation of e.g. DOC, K or Fe at or near the ground surface within the felled plots. This was attributed to the presence of a buffer zone between the plantation and the stream. On the other hand, surface inputs from former forestry plots caused measurable increases in stream water [Al] and [Mn]. The likely sources of Al and Mn were the disturbance of the mineral soil that had taken place some 20 years previously as a result of forestry ground preparation and the leaching from the recently felled conifer material, respectively. Such inputs occurred in late autumn or winter for Al and in summer for Mn, thus intensifying their natural seasonal patterns in this peat draining stream.