Changes in the chemistry of surface waters ~
25-year results at the Hubbard Brook Experimental Forest, NH
this report, we discuss how long-term changes in elemental deposition from the atmosphere have apparently altered the chemical characteristics of headwater streams at the HBEE In particular, we examine how declining atmospheric inputs of basic cations affect trends in stream acidity and discnss the implications of these observations.
Charles T.Driscnll Syracuse Universiiy Syracuse, NY 13244 Gene E. Likens Lars 0. Hedin
John S. Eaton R e New York Botanical Garden Millbrook, NY 12545
E Herbert Eonnann k l e Universiiy New Haven, CT 06105
An important focus of the current U S . assessment of effects of acidic deposition on surface waters is the reconstruction of historical patterns and the prediction of future trends in surface water quality ( I ) . The recent “Interim Assessment of the Causes and Effects of Acid Deposition” report of the National Acid Precipitation Assessment Program (NAPAP) indicates that sulfur emissions in the United States peaked in the early 1970s and have declined to the present (1). Moreover, it is proposed that surface waters in the northeastern United States are at steady state with respect to current sulfur deposition, and that any future reductions in sulfur emissions will result in increases in pH and acid neutralizing capacity (ANC) (1). Efforts to predict past and future trends in the acidity of waters draining glaciated regions of North America are tenuous because of uncertainty in the res nse of basic cation (the sum of Car, Mg2+, K+, Na+) concentrations to corresponding changes in strong acid loading (2,3).An understanding of the facto& regulating concentrations of ba-
Bulk precipitation (rainfall) collector
sic cations coincident with changes in strong acid loadmg is critical if rates of surface water acidificatiodrecovery are to be @ i d . These factors are key to the development of sound legislation to curtail acidic precursor emissions. A 25-year, continuous record of the chemistry of bulk precipitation and headwater streams is available at the Hubhard B m k Experimental Forest (HBEF) in New Hampshire ( 4 , 5 ) .Precipitation data from the HBEF have been used extensively in national assessments (6. , . 71. but relativelv little attention has been given to the iong-term changes in streahwater chemisky. In I _
Study site In many respects the HBEF is an ideal site to examine the chemical r e sponse of streamwater to changes in atmospheric deposition. A number of assessments have classified waters draining the region around the HBEF as very sensitive to acidic deposition (8-11). Hubbard Brook watersheds are underlaid by pelitic schist, which is extensively intrnded by quartz monzonite (12). Soils are well-drained, highly acidic Spalosols (ppic Fragiorthods) developed from locally derived glacial till (13).The base saturation of the mineral soil is very low, ranging from 1 to 14% (14). The HBEF is largely a second-growth northern hardwood forest with coniferous stands at higher elevations, and is generally typical of remote forested catchments in the northeastern United States ( 4 , 5 , 1 4 , 1 5 ) . The Huhbard Brook Valley receives elevated loading of acidic deposition (4, 5, 16, I;?. Drainage water in soil solutions and headwater streams from small experimental watersheds is decidedly acidic, with sulfate &e predominant anion, negative values of ANC, and elevated concentrations of dissolved inorganic aluminum (4, 5, 12, 13, 18, 19). Our analysis of long-term chemical trends in first- and second-order streams draining undisturbed forest is made possible though the detailed
(4, 5, 16, 17). For example, decreases in volume-weighted concentrations of sulfate in precipitation (Figure la) appear to be closely coupled with a decline in sulfur dioxide emissions in the northeastern United States (6, 7, 16, 17). The decrease in precipitation sulfate has coincided with an increase in precipitation pH in recent years (Figure lb)(17). The decline in precipitation sulfate also is correlated with decreases in stream sulfate. For a 24-year period (1964-1987), annual values of sulfate inputs in bulk precipitation are linearly correlated to output values, which suggests that sulfate is relatively conservative within the system [stream S042- = 349 (SE 155) 0.98 (SE 0.21) *(precipitation S042-); in eqlha-yr; r2 = 0.50; p 0.1) (Figure 2c). In addition, dissolved silica is not correlated @>O. 1; no indication of curvilinear relationship) with annual, volume-weighted concentrations of individual basic cations or with the sum of basic cations in streamwater. Net calcium export (stream outputbulk deposition inputknet biomass storage) also has been used to quantify weathering at the HBEF (4). Values of stream output less bulk precipitation (5) and biomass storage (see following)are relatively constant over the record. If we assume that net storage on soil cation exchange sites is relatively constant over this period, then weathering based on net calcium loss shows no long-term trend. Neither of these analyses suggest any trend in Weathering rates during the past 24 years and thus we assume that the decline of basic cations in streamwater is not controlled directly by changes in weathering release. It might be hypothesized that longterm climatic changes have caused increased streamnow in recent years, resulting in an increased drainage through the upper, more acidic soil borizons (25-28), or decreased evapotranspiration (ET), both of which could lower concentrations of basic cations in streamwater. Regression analysis of annual, volume-weighted streamwater concentrations of individual basic cations and the sum of basic cations with
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annual precipitation showed no significant linear (p>O.l) or curvilinear (p>O. 1) relationships. Furthermore, the sum of basic cations was not positively correlated with ET, as would be expected if the long-term decline in basic cation concentrations were due to changes in evapotranspiration. Therefore long-term changes in
nage water cannot explain the decline in basic cation concentrations in streamwater. Long-term increases in the rate of cation storage by vegetation and forest floor could also cause declines in stream concentration and export. Element accumulation in biomass has been estimated in the HBEF reference waterEnviron. Sci. Technot.. Vol. 23.NO. 2,1989 139
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111 Hubbard Brook Experimenrul Forest, where treatments were conducted
shed (watershed 6) three times (in 1965, 1977, and 1982) since the beginning of the study (14,15). The rate of net basic cation accumulation by vegetation was 1350 eq/ha-yr for the period 1965 to 1977 and 1210 eq/ha-yr for 1977 to 1982. These results show a slight decrease in accumulation of basic cations by vegetation with time, and thus cannot explain the decline in stream losses. One unanticipated factor that appears to contribute to the decrease in stream concentrations of basic cations is atmospheric deposition of basic cations. The long-term record indicates a marked decline in bulk precipitation concentrations (Figure la) and deposition of calcium, magnesium, and sodium, as well as in the sum of basic cations with time. Although the precise consequences of this trend are unknown, the decline in precipitation inputs of basic cations has intriguing implications for nutrientpoor forest ecosystems in the northeastern United States. We used several approaches to calculate the Contribution of precipitation deposition of basic cations to the longterm decreases in stream basic cation concentrations (from 145 peq/L in 1963 to 104 p q / L today; Figure 2a). For a given year, we determined the expected stream concentration of basic cations derived from precipitation by concentrating volume-weighted precipitation inputs in accordance with annual amounts of ET [(element precipitation input) /(amount of precipitation-ET)]. Temporal trends in these expected input concentrations as well as measured stream concentrations were empirically fit by regression analysis to both linear (precipitation p
