title: Trends in hydrometeorological conditions and stream water organic carbon in boreal forested catchments.
abstract: Temporal trends in stream water total organic carbon (TOC) concentration and export were studied in 8 forested headwater catchments situated in eastern Finland. The Seasonal Kendall test was conducted to identify the trends and a mixed model regression analysis was used to describe how catchment characteristics and hydrometeorological variables (e.g. precipitation, air and stream water temperatures, and atmospheric deposition) related to the variation in the concentration and export of stream water TOC. The 8 catchments varied in size from 29 to 494 ha and in the proportion of peatland they contained, from 8 to 70%. Runoff and TOC concentration were monitored for 15-29 years (1979-2006). Trends and variation in TOC levels were analysed from annual and seasonal time series. Mean annual TOC concentration increased significantly in seven of the eight catchments. The trends were the strongest in spring and most apparent during the last decade of the study period. The slopes of the trends were generally smaller than the variation in TOC concentration between years and seasons and between catchments. The annual TOC export showed no clear trends and values were largely determined by the temporal variability in runoff. Annual runoff showed a decreasing trend in two of the eight catchments. Mean annual air and stream water temperatures showed increasing trends, most clearly seen in the summer and autumn series. According to our modeling results, stream water temperature, precipitation and peatland percentage were the most important variables explaining annual and most seasonal TOC concentrations. The atmospheric deposition of SO4, NH4, and NO3 decreased significantly over the study period, but no significant link with TOC concentration was found. Precipitation was the main hydrometeorological driver of the TOC export. We concluded that stream water TOC concentrations and exports are mainly driven by catchment characteristics and hydrometeorological factors rather than trends in atmospheric acid deposition.
authors.affiliation: Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, FI-01301 Vantaa, Finland. sakari.sarkkola@metla.fi
authors.forename: Sakari
authors.initials: S
authors.lastname: Sarkkola
authors.forename: Harri
authors.initials: H
authors.lastname: Koivusalo
authors.forename: Ari
authors.initials: A
authors.lastname: Laurén
authors.forename: Pirkko
authors.initials: P
authors.lastname: Kortelainen
authors.forename: Tuija
authors.initials: T
authors.lastname: Mattsson
authors.forename: Marjo
authors.initials: M
authors.lastname: Palviainen
authors.forename: Sirpa
authors.initials: S
authors.lastname: Piirainen
authors.forename: Mike
authors.initials: M
authors.lastname: Starr
authors.forename: Leena
authors.initials: L
authors.lastname: Finér
mesh_terms: D002244:Carbon / Q000032:analysis*; D017753:Ecosystem; D004784:Environmental Monitoring; D005387:Finland; D008685:Meteorological Concepts; D009566:Nitrates / Q000032:analysis; D009930:Organic Chemicals / Q000032:analysis*; D000644:Quaternary Ammonium Compounds / Q000032:analysis; D012044:Regression Analysis; D045483:Rivers / Q000737:chemistry*; D013431:Sulfates / Q000032:analysis; D013997:Time Factors; D014197:Trees; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D009566:Nitrates; D009930:Organic Chemicals; D000644:Quaternary Ammonium Compounds; D013431:Sulfates; D014874:Water Pollutants, Chemical; D002244:Carbon