Base Cation Export Research Articles

Sizeable net export of base cations from a Carpathian flysch catchment indicates their geogenic origin while the 26Mg/24Mg, 44Ca/40Ca and 87Sr/86Sr isotope ratios in runoff are indistinguishable from atmospheric input.

Nutrient imbalances may negatively affect the health status of forests exposed to multiple stress factors, including drought and bark beetle calamities. We studied the origin of base cations in runoff from a small Carpathian catchment underlain by base-poor flysch turbidites using magnesium (Mg), calcium (Ca) and strontium (Sr) isotope composition of 10 ecosystem compartments. Our objective was to constrain conclusions drawn from long-term hydrochemical monitoring of inputs and outputs. Annual export of Mg, Ca and Sr exceeds 5-to-15 times their atmospheric input. Mass budgets per se thus indicate sizeable net leaching of Mg, Ca and Sr from bedrock sandstones and claystones. Surprisingly, δ26Mg, δ44Ca and 87Sr/86Sr isotope ratios of runoff were practically identical to those of atmospheric deposition and soil water but significantly different from bedrock isotope ratios. We did not find any carbonates in the studied area as a hypothetical, easily dissolvable source of base cations whose isotope composition might corroborate the predominance of geogenic base cations in the runoff. Marine carbonates typically have lower δ26Mg and 87Sr/86Sr ratios, and silicate sediments often have higher δ26Mg and 87Sr/86Sr ratios than runoff at the study site. Mixing of these two sources, if confirmed, could reconcile the flux and isotope data.

Hydrological control of water quality – Modelling base cation weathering and dynamics across heterogeneous boreal catchments

Linking biogeochemical processes to water flow paths and solute travel times is important for understanding internal catchment functioning and control of water quality. Base cation weathering is a process closely linked to key factors affecting catchment functioning, including water pathways, soil contact time, and catchment characteristics, particularly in silicate-dominated areas. However, common process-based weathering models are often calibrated and applied for individual soil profiles, which can cause problems when trying to extrapolate results to catchment scale and assess consequences for stream water and groundwater quality. Therefore, in this work, base cation export was instead modelled using a fully calibrated 3D hydrological model (Mike SHE) of a boreal catchment, which was expanded by adding a relatively simple but still reasonably flexible and versatile weathering module including the base cations Na, K, Mg, and Ca. The results were evaluated using a comprehensive dataset of water chemistry from groundwater and stream water in 14 nested sub-catchments, representing different catchment sizes and catchment characteristics. The strongest correlations with annual and seasonal observations were found for Ca (r = 0.89-0.93, p < 0.05), Mg (r = 0.90-0.95, p < 0.05), and Na (r = 0.80-0.89, p < 0.05). These strong correlations suggest that catchment hydrology and landscape properties primarily control weathering rates and stream dynamics of these solutes. Furthermore, catchment export of Mg, Ca, and K was strongly connected to travel times of discharging stream water (r = 0.78-0.83). Conversely, increasing Na export was linked to a reduced areal proportion of mires (r = -0.79). The results suggest that a significant part (~45%) of the catchment stream export came from deep-soil weathering sources (>2.5 m). These results have implications for terrestrial and aquatic water quality assessments. If deep soils are present, focusing mainly on the shallow soil could lead to misrepresentation of base cation availability and the acidification sensitivity of groundwater and water recipients such as streams and lakes.

Catchment export of base cations: improved mineral dissolution kinetics influence the role of water transit time

Abstract. Soil mineral weathering is one of the major sources of base cations (BC), which play a dual role in forest ecosystems: they function as plant nutrients and buffer against the acidification of catchment runoff. On a long-term basis, soil weathering rates determine the highest sustainable forest productivity that does not cause acidification. It is believed that the hydrologic residence time plays a key role in determining the weathering rates at the landscape scale. The PROFILE weathering model has been used for almost 30 years to calculate weathering rates in the rooting zone of forest soils. However, the mineral dissolution equations in PROFILE are not adapted for the saturated zone, and employing these equations at the catchment scale results in a significant overprediction of base cation release rates to surface waters. In this study, we use a revised set of PROFILE equations which, among other features, include retardation due to silica concentrations. Relationships between the water transit time (WTT) and soil water concentrations were derived for each base cation, by simulating the soil water chemistry along a one-dimensional flow path, using the mineralogy from a glacial till soil. We show how the revised PROFILE equations are able to reproduce patterns in BC and silica concentrations as well as BC ratios (Ca2+/BC, Mg2+/BC and Na+/BC) that are observed in the soil water profiles and catchment runoff. In contrast to the original set of PROFILE equations, the revised set of equations could reproduce the fact that increasing WTT led to a decreasing Na+/BC ratio and increasing Ca2+/BC and Mg2+/BC ratios. Furthermore, the total release of base cations from a hillslope was calculated using a mixing model, where water with different WTTs was mixed according to an externally modeled WTT distribution. The revised set of equations gave a 50 % lower base cation release (0.23 eq m−2 yr−1) than the original PROFILE equations and are in better agreement with mass balance calculations of weathering rates. Thus, the results from this study demonstrate that the revised mineral dissolution equations for PROFILE are a major step forward in modeling weathering rates at the catchment scale.

Land-use and fire drive temporal patterns of soil solution chemistry and nutrient fluxes

Land-use type and ecosystem disturbances are important drivers for element cycling and bear the potential to modulate soil processes and hence ecosystem functions. To better understand the effect of such drivers on the magnitude and temporal patterns of organic matter (OM) and associated nutrient fluxes in soils, continuous flux monitoring is indispensable but insufficiently studied yet. We conducted a field study to elucidate the impact of land-use and surface fires on OM and nutrient fluxes with soil solution regarding seasonal and temporal patterns analyzing short (<3months) and medium-term (3–12months) effects. Control and prescribed fire-treated topsoil horizons in beech forests and pastures were monitored biweekly for dissolved and particulate OM (DOM, POM) and solution chemistry (pH value, elements: Ca, Mg, Na, K, Al, Fe, Mn, P, S, Si) over one post-fire year. Linear mixed model analyses exhibited that mean annual DOM and POM fluxes did not differ between the two land-use types, but were subjected to strong seasonal patterns. Fire disturbance significantly lowered the annual soil solution pH in both land-uses and increased water fluxes, while DOC fluxes remained unaffected. A positive response of POC and S to fire was limited to short-term effects, while amplified particulate and dissolved nitrogen fluxes were observed in the longer run and co-ocurred with accelerated Ca and Mg fluxes. In summary, surface fires generated stronger effects on element fluxes than the land-use. Fire-induced increases in POM fluxes suggest that the particulate fraction represent a major pathway of OM translocation into the subsoil and beyond. With regard to ecosystem functions, pasture ecosystems were less prone to the risk of nutrient losses following fire events than the forest. In pastures, fire-induced base cation export may accelerate soil acidification, consequently exhausting soil buffer systems and thus may reduce the resilience to acidic depositions and disturbances.

Proton production from nitrogen transformation drives stream export of base cations in acid-sensitive forested watersheds

The biogeochemical cycles of nitrogen (N) and base cations (BCs), (i.e., K+, Na+, Ca2+, and Mg2+), play critical roles in plant nutrition and ecosystem function. Empirical correlations between large experimental N fertilizer additions to forest ecosystems and increased BCs loss in stream water are well demonstrated, but the mechanisms driving this coupling remain poorly understood. We hypothesized that protons generated through N transformation (PPRN)—quantified as the balance of NH4+ (H+ source) and NO3− (H+ sink) in precipitation versus the stream output will impact BCs loss in acid-sensitive ecosystems. To test this hypothesis, we monitored precipitation input and stream export of inorganic N and BCs for three years in an acid-sensitive forested watershed in a granite area of subtropical China. We found the precipitation input of inorganic N (17.71kgNha−1year−1 with 54% as NH4+–N) was considerably higher than stream exported inorganic N (5.99kgNha−1year−1 with 83% as NO3−–N), making the watershed a net N sink. The stream export of BCs (151, 1518, 851, and 252molha−1year−1 for K+, Na+, Ca2+, and Mg2+, respectively) was positively correlated (r=0.80, 0.90, 0.84, and 0.84 for K+, Na+, Ca2+, and Mg2+ on a monthly scale, respectively, P<0.001, n=36) with PPRN (389molha−1year−1) over the three years, suggesting that PPRN drives loss of BCs in the acid-sensitive ecosystem. A global meta-analysis of 15 watershed studies from non-calcareous ecosystems further supports this hypothesis by showing a similarly strong correlation between ∑BCs output and PPRN (r=0.89, P<0.001, n=15), in spite of the pronounced differences in environmental settings. Collectively, our results suggest that N transformations rather than anions (NO3− and/or SO42−) leaching specifically, are an important mediator of BCs loss in acid-senstive ecosystems. Our study provides the first definitive evidence that the chronic N deposition and subsequent transformation within the watershed drive stream export of BCs through proton production in acid-sensitive ecosystems, irrespective of their current relatively high N retention. Our findings suggest the N-transformation-based proton production can be used as an indicator of watershed outflow quality in the acid-sensitive ecosystems.

Diatoms as indicators of long-term nutrient enrichment in metal-contaminated urban lakes from Sudbury, Ontario

The majority of the limnological research in Sudbury, Ontario, has focused on the anthropogenic impacts of industrial emissions (SO2 and metals), with the potential effects of cultural eutrophication largely being overlooked. However, the population of the City of Sudbury has grown with the prosperity of the mining sector, which poses a risk to the quality of freshwater resources. As with many environmental issues, there is often a lack of predisturbance data that can assist in gauging the full extent of environmental change. Therefore, paleolimnological approaches were used to track long-term biological changes in sedimentary diatom assemblages related to cultural eutrophication in 4 lakes from Sudbury. Diatom assemblages were primarily dominated by oligotrophic taxa prior to watershed development; however, with the onset of urban environmental stressors (e.g., septic systems, the application of lawn fertilizers and watershed development), there was a shift toward taxa that thrive in more productive systems. Diatom assemblages also seem to track an increase in lakewater pH through time, which is likely related to increased acid neutralizing capacity as a result of watershed disturbances, algal assimilation and bacterial reduction of NO− 3, and increased base cation export from the watershed due to acidic deposition. Insight into predisturbance conditions of the lakes should help lake managers set realistic biological targets for restoration and may be used to help gauge the response of these systems to future mitigation efforts.

Biogeochemical cycling and chemical fluxes in a managed northern forested wetland, Michigan, USA

Forest harvesting and subsequent regeneration treatments may cause changes in soil and solution chemistry that adversely affect forest productivity and environmental quality. The objective of this study was to assess soil carbon (C), nitrogen (N), and base cation pools and fluxes, and to construct a hydrogen ion (H+) mass balance to identify major processes controlling acidity production and consumption 14 years following whole-tree harvesting and regeneration in a northern forested wetland with underlying mineral soils derived from calcareous glacial drift. Results for soil elemental and nutrient pools in the harvested/regenerated stand were compared to an adjacent non-harvested stand and a riparian zone. The riparian zone had the highest soil total C, total N, and exchangeable calcium (Ca) and magnesium (Mg) pools; however, no difference in exchangeable potassium (K) was evident among stand types. Moreover, no differences between the harvested/regenerated and uncut stands were evident in any of the soil chemical pools.Net export of base cations was minimal and the H+ mass balance indicated that net cation exchange was not a significant process in H+ production or consumption in either the uncut or harvested/regenerated stands. The most striking differences in the H+ mass balance were (1) eight times the H+ consumption from sulfate (SO42−) reduction in the harvested/regenerated stand compared to that in the uncut condition and (2) nearly twice the H+ production due to N immobilization in the harvested/regenerated stand. However, both stand types were net H+ sinks and increases in H+ export due to whole-tree harvesting were not evident.The riparian zone was a net exporter of base cations. This finding was attributed to a combination of base cation exchange and carbonate mineral weathering; data suggested the importance of the latter. More research, however, is required to isolate the contributions of cation exchange and carbonate weathering on base cation export from the riparian zone. Stream chemistry was consistent with that of the riparian zone, indicating a strong linkage between the riparian zone stream chemistry, and whole-tree harvesting had no intermediate term (i.e., 14 years) effects on stream acidification in this managed northern wetland ecosystem.

Alkalinity and acidity cycling and fluxes in an intermediate fen peatland in northern Ontario

Peatlands are important to global carbon (C) sequestration and surface water acid–base status, both of which are affected by peatland alkalinity and acidity cycling. Relationships among sulfate (SO4 2−), nitrate (NO3 −), organic acids (OA−), base cations (i.e., Ca2+, Mg2+, K+, and Na+), proton (H+) acidity, and bicarbonate (HCO3 −) alkalinity were investigated in an intermediate fen peatland in northern Ontario during 2004 (an average precipitation year) and 2005 (a dry summer). Potential evapotranspiration was higher and the water table, groundwater input from the uplands, and runoff were lower during 2005. Net inputs of base cations, HCO3 −, SO4 2−, and OA−, and to a lesser degree NO3 −, were lower during the drier year, mainly due to lower groundwater transfer to the fen. Fen porewater HCO3 − concentration and net output were also lower in the drier year, whereas Ca2+, Mg2+, and SO4 2− concentrations and net output were higher. During the climatically average year, N immobilization, carbonic acid (H2CO3) dissociation, and OA dissociation were equally important H+-producing reactions. Peat cation exchange accounted for 50% of the H+ sink, while SO4 2− reduction and denitrification accounted for an additional 20 and 25% of the H+ sink, respectively. During the dry year, S oxidation accounted for 55% of the H+ net production, while that for H2CO3 dissociation was 70% lower than that during the climatically average year. Peat cation exchange consumed three times the acidity, and accounted for 92% of the H+ consumption during the dry year compared to the climatically average year. This was consistent with a three-fold higher net base cation export from the fen during the dry year. Based on the study results, a conceptual model was developed that describes the role of acidity formation and its implications to intermediate fen acidification.

Long-Term Effects of Acidic Deposition on Water Quality in a High-Elevation Great Smoky Mountains National Park Watershed: Use of an Ion Input–Output Budget

Impacts from acidic deposition on stream water quality in the Great Smoky Mountains National Park (GRSM) have long been reported; however, a better understanding of the biogeochemical processes that regulate stream acidification is needed for resource management. Water quality monitoring of Noland Divide Watershed (NDW), a high-elevation watershed in the GRSM, was used to generate an ion input–output budget in order to evaluate what processes have influenced stream pH and acid neutralizing capacity (ANC) over the long term. NDW was equipped with wet deposition, throughfall, soil lysimeters, and stream collection stations, and monitoring began in 1991 and continues to the present. Using data from 1991 to 2006, this study found annual deposition fluxes of SO42− and NO3− averaged 1,735 and 863 eq ha−1 year−1, respectively. Data indicated that 61% of the net SO42− entering the watershed was retained, suggesting soil adsorption dominates as a biogeochemical process. Although net SO42− retention was observed, SO42− appeared to move rapidly through NDW during large precipitation events causing stream acidification, as evidenced by significant inverse correlations between biweekly throughfall SO42− flux and stream event pH and ANC. Nitrogen uptake by forest vegetation and nitrification play key roles in regulating NO3− export to the stream as observed by 32% retention of net inorganic nitrogen, and 96% of NH4+ input was converted to NO3− in the uppermost soil horizon. Net export of base cations (Ca2+, Mg2+, Na+) is observed and apparently moderates stream acidification. In contrast, 71% of net K+ input was retained, which is likely due to forest vegetation uptake. Net export of Ca2+ was 867 eq ha−1 year−1 compared to net throughfall of 790 eq ha−1 year−1. Long-term cation depletion from the NDW soils could limit recovery potential in stream water quality. Findings from this NDW study suggest that future stream acidification conditions in high-elevation GRSM watersheds are dependent on interrelated biogeochemical processes and precipitation patterns, illustrating the need to better understanding potential impacts of climate variability on stream water quality.

Long-term trends in catchment export and lake concentrations of base cations in the Dorset study area, central Ontario

Decreases in exchangeable base cation reservoirs, base cation export, and lake concentrations have been reported for acid-sensitive regions in Europe and eastern North America. These decreases have produced smaller than anticipated increases in alkalinity in surface waters in response to reductions in sulphate emissions and may have ecological consequences. This study presents annual export rates of Ca, Mg, K, and Na from 20 forested catchments between 1978 and 1998 and concentrations in seven downstream lakes between 1975 and 2005 in an acid-sensitive region of central Ontario, Canada. In contrast to monovalent cation export, decreases in divalent cation export continued after an extended dry period ended. Lake concentration trends showed three distinct periods. The first period (1975–1976 to 1982–1983) coincided with high runoff and was characterized by relatively high Ca, Mg, and K concentrations. This was followed by a 10-year period of fluctuations around the long-term mean. Concentrations then dropped below the long-term means and remained 5%–20% below the means until 2004–2005. The latter trend in lake concentrations suggests that export likely remained low but stable after May 1998 (the last month for which export data are available). Na increased between 250% and 350% in Dickie and Harp lakes as a result of road salt contamination.

Modelling the effects of climate change on an acidic upland stream

Most modelling studies of soil and surface water recovery from acidification assume a constant influence of climate over the simulation period. Given the likelihood of future climate change, and recent identification of links between climatic fluctuations and surface water trends on decadal time scales, an attempt is made to simulate a number of climate-related effects on the recovery of an acidified stream in mid-Wales. Empirical relationships were derived to model past and future variations in (i) runoff, as a function of rainfall and temperature; (ii) stream chloride concentration and sea-salt deposition, as a function of the North Atlantic Oscillation Index (NAOI); and (iii) stream dissolved organic carbon (DOC) concentration, as a function of summer temperatures and sulphur deposition. Incorporating these relationships in the MAGIC dynamic model reproduced a significant part of the observed variation in streamwater chemistry. Current scenarios of rising UK temperature and NAOI, and falling rainfall and acid deposition, were used to forecast effects of climate change on soil and water recovery from acidification. Results suggest that increasing DOC would have a significant negative impact on soil recovery, with elevated organic acidity lowering soil water pH and severely depleting soil base saturation. The resulting increase in soil base cation export would, however, have a positive impact on streamwater recovery. For increased sea-salt deposition, effects are essentially the opposite; increased base cation inputs enhance the recovery of soil base saturation, but displace acidity to the stream. However, simulated effects of raising DOC considerably outweighed those of increased sea-salt deposition. It is argued that greater consideration of these, and other, climatic effects on biogeochemical cycles needs to be taken when predicting the recovery of soils and surface waters from acidification.

Monitoring the results of Canada/U.S.A. acid rain control programs: some lake responses.

Aquatic acidification by deposition of airborne pollutants emerged as an environmental issue in southeastern Canada during the 1970s. Drawing information from the extensive research and monitoring programs, a sequence of issue assessments demonstrated the necessity of reducing the anthropogenic emissions of acidifying pollutants, particularly sulphur dioxide (SO2). The 1991 Canada-U.S. Air Quality Agreement (AQA) was negotiated to reduce North American SO2 emissions by approximately 40% relative to 1980 levels by 2010, and at present, both countries have reduced emissions beyond their AQA commitment. In response to reduced SO2 emissions, atmospheric deposition of sulphate (SO4(2-)) and SO4(2-) concentrations in many lakes have declined, particularly in south-central Ontario and southern Québec. Sulphate deposition still exceeds aquatic critical loads throughout southeastern Canada however. Increasing pH or alkalinity (commonly deemed 'recovery') has been observed in only some lakes. Several biogeochemical factors have intervened to modify the lake chemistry response to reduced SO4(2-) input, notably release of stored SO4(2-) from wetlands following periods of drought and reduction in the export of base cations from terrestrial soils. Three examples from Ontario are presented to illustrate these responses. Significant increases in pH and alkalinity have been observed in many lakes in the Sudbury area of Ontario due to the large reductions in local SO2 emissions; 'early-stage' biological recovery is evident in these lakes. An integrated assessment model predicts that AQA emission reductions will not be sufficient to promote widespread chemical or biological recovery of Canadian lakes. Monitoring and modeling are mutually supporting assessment activities and both must continue.

Base cation and nitrogen budgets for seven forested catchments in central Ontario, 1983–1999

There is widespread concern that the combination of acid (S and N) deposition and tree harvesting may deplete available base cation pools in soils to the extent that forest health and productivity may be impaired. Here we present mass balance budgets for Ca, Mg, K and N for seven forested catchments in Muskoka–Haliburton, Ontario over a 17-year period, during which time SO 4 deposition decreased by ∼40%, but N deposition was unchanged. Mass balance budgets were calculated as the difference between inputs from deposition and mineral weathering (base cations only) and export in stream water plus uptake in biomass. There were significant decreases in annual SO 4 export from all catchments, although temporal trends in base cation export were varied. Export of Ca, Mg and SO 4 exceeded inputs in deposition, but concentrations in stream water were highly variable from year-to-year. Temporal patterns in Ca, Mg and SO 4 concentration in stream water were synchronous among all seven catchments and appear related to climate variations over the 17-year period. Most of the N exported from each catchment was organic N, although export of NO 3 varied considerably (10-fold), both between catchments and between years. Differences in NO 3 export appear to be due to differences in catchment characteristics and also climate variations. Nevertheless, net N accumulation at the seven catchments was between 92.6 and 128.1 kg/ha N over the 17-year period. All catchments experienced a net loss of Ca, which varied between 33 and 287 kg/ha over the 17-year period. Net losses of Mg were lower, and in general, mass balance estimates indicate that most catchments are in balance with respect to K. Losses of Ca over this 17-year period, represent up to 60% of the current exchangeable Ca soil pool. If Ca losses, due to acid deposition and harvesting continue at the present rate, forest health and productivity may be impaired within just a few decades.

Processes regulating the solute concentrations of snowmelt runoff in two subalpine catchments of the Sierra Nevada, California

Geochemical processes regulating solute concentrations in snowmelt runoff were investigated for the snowmelt periods of 1992 and 1993 in two subalpine catchments in Sequoia National Park, California. The catchments, 0.5 and 0.2 ha, have 10 and 25% soil coverage, respectively; the remaining area is exposed granodiorite bedrock. Mineral weathering was the major source of solutes in runoff. Calcium export in excess of stoichiometric plagioclase weathering was attributed to dry deposition and the weathering of mafic minerals and disseminated calcite. Cation exchange was important in the regulation of K and increased base cation export in response to rain‐on‐snow events and LiBr tracer. Sulfate fluxes were attributed to snowpack elution and SO4 desorption after the beginning of melt. The selective retention of Li during tracer experiments suggests that acid‐neutralizing capacity is linked to soil stocks along convoluted flow paths and is not necessarily compromised in areas of sparse soil cover.

Catchment-scale coupling between pyrite oxidation and calcite weathering

Integrated time series of major element concentrations and flow rates from a small watershed (White Rock Creek, Dallas, TX) are used to examine the general hydrogeochemical controls on calcite weathering and the specific role of trace pyrite oxidation in base cation export. White Rock Creek is a perennial gaining stream occupying a catchment underlain by expanding clay soils, and an uppermost fractured bedrock weathering zone (which acts as a surface aquifer) below which there is low permeability Austin Chalk. Seasonal variations in cation export are controlled predominantly by temperature effects on carbonate solubility, dilution by variable amounts of rainfall, and possibly organic activity. Creek water P CO 2 is 7 to 10 times atmospheric P CO 2 throughout the year, and does not correlate with surface temperature. Non-pyrite sulfate inputs are well-constrained; moreover, integrated runoff and solute export from the basin are similar to global mean values. Pyrite, although present only as a trace component, accounts for a disproportionate amount of bedrock weathering (∼40%) when oxidized to produce sulfuric acid. Irreversible dissolution of pyrite is insensitive to seasonal changes in temperature, runoff, and organic activity and probably reflects the slow movement of a weathering front at depth through the aquifer.

Primary mineral weathering in the central Appalachians: a mass balance approach

We use a mass balance approach to calculate long-term weathering rates for three forested watersheds in the southeastern USA. One watershed (Shaver Hollow) is underlain by Precambrian granodiorite, and two (White Oak Run, Deep Run) are underlain by Cambrian metasedimentary units. Each of the study areas receives high levels of acidic precipitation, and each watershed was almost completely defoliated by gypsy moth larvae for at least two consecutive years during the study period. Our analysis uses stream and precipitation chemistry obtained weekly from 1987 to 1993 for Shaver Hollow and from 1980 to 1993 for White Oak Run and Deep Run. Mass balance calculations for the granodiorite watershed indicate that plagioclase feldspar is the dominant reactant and that basalt dikes which comprise a small fraction of the catchment make a significant contribution to the base cation budget. Corresponding calculations for the metasedimentary bedrock watersheds indicate contributions from plagioclase feldspar, muscovite, and biotite. At best, the mass balance models provide an approximation of weathering processes because the soils are immature and contain few stoichiometric clay minerals such as kaolinite. Annual cation release rates from Shaver Hollow suggest soil profile ages of ∼4 ka, consistent with the observed surficial geology and the geological history of the area. Base cation efflux patterns during the period of severe defoliation differ markedly between the three watersheds and reflect a continuum of ecosystem sensitivities. The Pedlar granodiorite system is relatively robust to the effects of disturbance: calculated weathering rates and proportions change by <10% during defoliation. The Chilhowee Formation watershed White Oak Run is moderately acid-sensitive and is unstable with respect to defoliation: annual base cation export by roughly 30% during each year of larval infestation. The Deep Run watershed is superficially robust to defoliation, as base cation efflux increased by only 5% during the disturbance. However, seasonal patterns of base cation uptake and release both prior to and during defoliation indicate this system is severely stressed, presumably as a result of chronic acid deposition. Mass balance calculations suggest that base cation availability in this watershed is low enough to inhibit normal biomass aggradation even in the absence of extreme defoliation.

CAN SITE-SPECIFIC TRENDS BE EXTRAPOLATED TO A REGION? AN ACIDIFICATION EXAMPLE FOR THE NORTHEAST

In the absence of true regional data on changes in the acid/base status of lakes in the northeastern United States, we explore the possibility of using site-specific trends information from a judgment sample of lakes to assess the efficacy of the Clean Air Act Amendments. A meta-analytical technique is used to combine trends results from 44 Long-Term Monitoring (LTM) lakes in the Northeast for the period 1982–1994, with the goal of producing estimates of overall trends in the region. The lakes are subdivided into subpopulations (High ANC, Intermediate Till Drainage, Thin Till Drainage and Perched Seepage lakes) on the basis of their expected response to changes in acidic deposition, and they appear to represent the most acid-sensitive of these lake classes well. While the overall tendencies in the trends are as expected (e.g., most of the recovery is observed in the most sensitive subpopulations), there is significant trend heterogeneity among the lakes within most of the subpopulations; this heterogeneity prohibits the summarizing of trends at the regional level (i.e., for all of the Northeast). This heterogeneity is explained by differences in the responses of lakes in two subregions (Adirondacks vs. New England), and we present trends results separately for each subpopulation within these two subregions. All subpopulations in both subregions showed decreasing trends in sulfate concentrations, probably a reflection of decreasing trends in sulfur deposition in the region. Few trends in nitrate concentrations were observed. Recovery (as evidenced by increasing trends in acid-neutralizing capacity) was evident in Thin Till and Intermediate Till Drainage lakes in New England, but not in the Adirondacks. Most groups of lakes exhibited downward trends in base cations (Σ[Ca2+ + Mg2+ + Na+ + K+]); the magnitudes of these trends were always greater in Adirondack lakes than in similar New England lakes. This suggests that the depletion of soil cation pools in the Adirondacks may be responsible for some of the differences in recovery between Adirondack and New England lakes. While export of base cations may be the key difference producing different trends results in the two subregions, the site-specific nature of the trends, and their possible lack of regional representation, should be considered in interpreting the overall results.

Export of Nutrients and Major Ions from Caribbean Catchments

Export of N, P, major ions, and SiO 2 was measured for streams draining tropical rain forest catchments on three islands of volcanic origin in the Lesser Antilles. Structure of the native forests was similar in each island, but land use varied in each. Patterns of inorganic nitrogen export followed land use patterns, with greatest export (7 kg ha -1 yr -1 ) from the most densely populated catchment. Phosphorus followed a different pattern, with greatest export (0.5 kg ha -1 yr -1 ) from the catchment with the lowest population density and least agriculture. Export of base cations and silica varied primarily with runoff. Weathering rates were high in these humid tropical basins, with SiO 2 export of 400-800 kg ha -1 yr -1 . These data provide some of the first estimates of the impact of land use on nutrient export from small tropical streams, and point to the possible impact of soil chemistry and weathering in regulating phosphoru...

Response of the West Bear Brook Watershed, Maine, USA, to the addition of (NH 4) 2SO 4: 3-year results

Addition of 1800 eq (NH 4) 2SO 4 ha −1 year −1 for 3 years to a catchment that originally produced low acid neutralizing capacity (ANC) runoff has resulted in significant episodic acidification during high discharge and slight chronic acidification during base flow. These relationships imply that the response to chemical amendments is occurring largely in upper soil horizons and shallow flow paths. The principal responses in stream water chemistry include: elevated volume-weighted concentrations of NO 3, SO 4, base cations, Al, and H, coupled with decreased ANC. The export of dissolved organic carbon may have decreased slightly. Export of SiO 2 has not changed significantly; thus the increase in base cations is interpreted to be largely from desorption from soil. Effects of the manipulation increase from Year 1 to Year 2 to Year 3. Percentage retention of the added SO 4 is declining; retention of NH 4 remains at 100% while export of NO 3 is increasing with time, implying nitrogen saturation and an important change in N dynamics. The export of base cations and Al continues to increase.

Export of carbon, nitrogen, and major ions from three tropical montane watersheds

Annual exports of suspended sediment, dissolved and particulate C and N, dissolved N and P, and major cations and anions were measured in three montane tropical rain forest watersheds in Puerto Rico during 1983–1986. Organic C was primarily exported in the form of DOC, and DOC export (33–94 kg ha−1 yr−1) was similar to values in larger tropical watersheds with similar runoff. Particulate and dissolved organic N accounted for 60–70% of the 4–9 kg ha−1 yr−1 of total N exported. Export of base cations and rates of weathering varied with bedrock geology in the three watersheds. Concentrations of suspended sediment, particulate C and N, and DOC increased as a function of discharge in all three streams. NH4, NO3−, SO42−, and K+ concentrations showed little or no response to variations in discharge; Na+, Ca2+, Mg2, Cl −, HCO3−, and SiO2 all decreased at high flows. After removing the effects of discharge, residual NO3− concentrations in each stream were inversely related to estimated rates of leaf fall. On a watershed basis, export and accumulation of N in biomass were greater than precipitation inputs, suggesting that unmeasured inputs (8–16 kg ha−1 yr−1) were large.