Environ. Sci. Technol. 2009, 43, 4320–4326
Increased Dissolved Organic Carbon (DOC) in Central European Streams is Driven by Reductions in Ionic Strength Rather than Climate Change or Decreasing Acidity ´ M,† J A K U B H R U Sˇ K A , * , † P A V E L K R A WILLIAM H. MCDOWELL,‡ AND FILIP OULEHLE† Czech Geological Survey, Kla´rov 3, 118 21, Prague 1, Czech Republic, and Department of Natural Resources and the Environment, University of New Hampshire, Durham, New Hampshire 03824
Received December 25, 2008. Revised manuscript received April 21, 2009. Accepted April 27, 2009.
Temporal trends in DOC concentration and flux were investigated at two geochemically distinct forested catchments in western Czech Republic. Mean discharge-weighted DOC concentrations averaged 18.8 mg L-1 at the acidic Lysina catchment, and 20.2 mg L-1 at base-rich and well-buffered Pluhuv Bor. Between 1993 and 2007 DOC in streamwater increased significantly in both catchments: the mean annual increase was 0.42 mg L-1yr-1 (p < 0.001) at Lysina and 0.43 mg L-1yr-1 (p < 0.001) at Pluhuv Bor, resulting in cumulative increases of 64 and 65%, respectively. These long-term increases in streamwater DOC were correlated with only modest increases in stream pH in both catchments, but large declines in ionic strength (IS), that resulted from declining atmospheric deposition. Neither catchment has undergone changes in soil-water pH, yetDOCconcentrationstripledinthesoil-waterofbothcatchments. We conclude that changes in ionic strength of soil-water and streamwater, rather than acidity, are the primary drivers of changes in streamwater DOC in this region. Temperature, precipitation and discharge show no statistically significant trends during the study period, suggesting that climate change has played no role in the changes in DOC that we have observed.
Introduction Dissolved organic carbon (DOC) concentrations have increased over the past decade or more in streams throughout the Northern Hemisphere, with most reported increases occurring in catchments that are forested or have organicrich soils (1-6). These increases have important implications for drinking water quality (7) as well as long-term carbon balance at the catchment scale (8), but it has proven difficult to ascribe changes in DOC concentrations to a specific environmental driver. Several hypotheses have been invoked to explain the observed increases in DOC concentrations. Increasing temperature leading to higher production of humic and fulvic acids in soils and peatlands was initially proposed as the * Corresponding author phone: +420 251085433; fax: +420 251818748; e-mail:
[email protected]. † Czech Geological Survey. ‡ University of New Hampshire. 4320
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cause of observed DOC increases (3, 9). Other hypotheses based on climatic changes were connected with rising CO2 in the atmosphere and increasing net primary production (10) or increasing amounts of precipitation and runoff (11, 12). Evans et al. (13), however, concluded that hydrological drivers are unlikely to explain the widespread monotonic rise of DOC. More aerobic conditions in drier and warmer peats have also been proposed as the driver of DOC increases in peaty soils (13). Recent studies (1, 2) conclude that DOC has increased in streams as a result of decreases in the ionic strength or acidity of soil solution and streamwaterresultingfromchangesinatmosphericdeposition. In this paper we present data from a 15-year study of DOC concentrations in soil-waters and streams draining two catchments in the Czech Republic with contrasting lithology and stream pH. The Lysina catchment is underlain by granite and its poorly buffered soils were highly acidified due to acidic deposition during the 1980s. Nearby Pluhuv Bor catchment is underlain by serpentinite but was never acidified due to its well-buffered soils (14). Both catchments have recovered during the last 15 years from exceedingly high levels of acidic deposition (14), and thus provide a unique opportunity to disentangle the impacts of ionic strength, acidity, and climate on stream DOC concentrations because other recent studies (1, 2, 12, 13) have focused mainly on acid-sensitive and acidified streams. We also examine the influence of large DOC increases on streamwater recovery from acidification. Site Description. The Lysina and Pluhuv Bor catchments (Table 1) are situated in the western part of the Czech Republic in the mountainous region of the Slavkov Forest (Slavkovsky´ les). The site selection criteria included proximity to each other; similar climate, atmospheric deposition, topography, vegetation cover, size; and chemically differing bedrock (15).
Materials and Methods Stream flow from the catchments was monitored continuously using a V-notch weir and a mechanical water level recorder (LG 503, Geospol Uhrˇ´ınov, Czech Republic and Thalimedes, OTT, Germany). Stream water samples were collected weekly, soil-water was collected monthly using zerotension lysimeters (placed under the forest floor at Lysina and in mineral topsoil at Pluhuv Bor). Bulk precipitation and throughfall were collected monthly. All annual mean concentrations are volume-weighted and are based on a November-October water year. Concentrations of Cl-, SO42-, and NO3- in water samples were determined by HPLC and concentrations of Ca2+, Mg2+, Na+, K+, and Al were determined by AAS in unfiltered samples (15). Solution pH was determined using a combination glass electrode (15). DOC was determined using platinumcatalyzed, high-temperature oxidation on unfiltered samples using the nonpurgeable organic carbon method. Several instruments were used during the study period of 1993-2007. A Dohrmann Carbon Analyzer (Santa Clara, CA) was used between 1993 and 1997, a TOC 5000 (Shimadzu Corporation, Kyoto, Japan) was used between 1998 and 2004, and a Tekmar-Dohrman Apollo 9000 (Teledyne Tekmar, OH) was used from 2005 to 2007. Since there is very little particulate carbon in these waters (99%) European beech (Fagus sylvatica) (
